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Hearing aid

A hearing aid is a device designed to improve hearing by making sound audible to a person with hearing loss. Hearing aids are classified as medical devices in most countries, and regulated by the respective regulations. Small audio amplifiers such as personal sound amplification products (PSAPs) or other plain sound reinforcing systems cannot be sold as "hearing aids".

Early devices, such as ear trumpets or ear horns, were passive amplification cones designed to gather sound energy and direct it into the ear canal. Modern devices are computerised electroacoustic systems that transform environmental sound to make it audible, according to audiometrical and cognitive rules. Modern devices also utilize sophisticated digital signal processing, aiming to improve speech intelligibility and comfort for the user. Such signal processing includes feedback management, wide dynamic range compression, directionality, frequency lowering, and noise reduction.

Modern hearing aids require configuration to match the hearing loss, physical features, and lifestyle of the wearer. The hearing aid is fitted to the most recent audiogram and is programmed by frequency. This process called "fitting" can be performed by the user in simple cases, by a Doctor of Audiology, also called an audiologist (AuD), or by a Hearing Instrument Specialist (HIS) or audioprosthologist. The amount of benefit a hearing aid delivers depends in large part on the quality of its fitting. Almost all hearing aids in use in the US are digital hearing aids, as analog aids are phased out. Devices similar to hearing aids include the osseointegrated auditory prosthesis (formerly called the bone-anchored hearing aid) and cochlear implant.

Uses
Hearing aids are used for a variety of pathologies including sensorineural hearing loss, conductive hearing loss, and single-sided deafness. Hearing aid candidacy was traditionally determined by a Doctor of Audiology, or a certified hearing specialist, who will also fit the device based on the nature and degree of the hearing loss being treated. The amount of benefit experienced by the user of the hearing aid is multi-factorial, depending on the type, severity, and etiology of the hearing loss, the technology and fitting of the device, and on the motivation, personality, lifestyle, and overall health of the user. Over-the-counter hearing aids, which address mild- to moderate- hearing loss, are designed to be adjusted by the user.

Hearing aids are incapable of truly correcting a hearing loss; they are an aid to make sounds more audible. The most common form of hearing loss for which hearing aids are sought is sensorineural, resulting from damage to the hair cells and synapses of the cochlea and auditory nerve. Sensorineural hearing loss reduces the sensitivity to sound, which a hearing aid can partially accommodate by making sound louder. Other decrements in auditory perception caused by sensorineural hearing loss, such as abnormal spectral and temporal processing, and which may negatively affect speech perception, are more difficult to compensate for using digital signal processing and in some cases may be exacerbated by the use of amplification. Conductive hearing losses, which do not involve damage to the cochlea, tend to be better treated by hearing aids; the hearing aid is able to sufficiently amplify sound to account for the attenuation caused by the conductive component. Once the sound is able to reach the cochlea at normal or near-normal levels, the cochlea and auditory nerve are able to transmit signals to the brain normally.

Common issues with hearing aid fitting and use are the occlusion effect, loudness recruitment, and understanding speech in noise. Once a common problem, feedback is generally now well-controlled through the use of feedback management algorithms.

Candidacy and acquisition
There are several ways of evaluating how well a hearing aid compensates for hearing loss. One approach is audiometry which measures a subject's hearing levels in laboratory conditions. The threshold of audibility for various sounds and intensities is measured in a variety of conditions. Although audiometric tests may attempt to mimic real-world conditions, the patient's own every day experiences may differ. An alternative approach is self-report assessment, where the patient reports their experience with the hearing aid.

Hearing aid outcome can be represented by three dimensions:
 * 1) hearing aid usage
 * 2) aided speech recognition
 * 3) benefit/satisfaction

The most reliable method for assessing the correct adjustment of a hearing aid is through real ear measurement. Real ear measurements (or probe microphone measurements) are an assessment of the characteristics of hearing aid amplification near the ear drum using a silicone probe tube microphone.

Current research is also pointing towards hearing aids and proper amplification as a treatment for tinnitus, a medical condition which manifests itself as a ringing or buzzing in the ears.

Types
There are many types of hearing aids (also known as hearing instruments), which vary in size, power and circuitry. Among the different sizes and models are:

Body-worn
Body worn aids were the first portable electronic hearing aids, and were invented by Harvey Fletcher while working at Bell Laboratories. Body aids consist of a case and an earmold, attached by a wire. The case contains the electronic amplifier components, controls and battery, while the earmold typically contains a miniature loudspeaker. The case is typically about the size of a pack of playing cards and is carried in a pocket or on a belt. Without the size constraints of smaller hearing devices, body worn aid designs can provide large amplification and long battery life at a lower cost. Body aids are still used in emerging markets because of their relatively low cost.

Behind the ear
Behind the ear hearing aids are one of two major classes of hearing aids – behind the ear (BTE) and in the ear (ITE). These two classes are distinguished by where the hearing aid is worn. BTE hearing aids consist of a case which hangs behind the pinna. The case is attached to an earmold or dome tip by a traditional tube, slim tube, or wire. The tube or wire courses from the superior-ventral portion of the pinna to the concha, where the ear mold or dome tip inserts into the external auditory canal. The case contains the electronics, controls, battery, and microphone(s).The loudspeaker, or receiver, may be housed in the case (traditional BTE) or in the earmold or dome tip (receiver-in-the-canal, or RIC). The RIC style of BTE hearing aid is often smaller than a traditional BTE and more commonly used in more active populations.

BTEs are generally capable of providing more output and may therefore be indicated for more severe degrees of hearing loss. However, BTEs are very versatile and can be used for nearly any kind of hearing loss. BTEs come in a variety of sizes, ranging from a small, "mini BTE", to larger, ultra-power devices. Size typically depends on the output level needed, the location of the receiver, and the presence or absence of a telecoil. BTEs are durable, easy to repair, and often have controls and battery doors that are easier to manipulate. BTEs are also easily connected to assistive listening devices, such as FM systems and induction loops. BTEs are commonly worn by children who need a durable type of hearing aid.

In the ear
In the ear aids (ITE) devices fit in the outer ear bowl (called the concha). Being larger, these are easier to insert and can hold extra features. They are sometimes visible when standing face to face with someone. ITE hearing aids are custom made to fit each individual's ear. They can be used in mild to some severe hearing losses. Feedback, a squealing/whistling caused by sound (particularly high frequency sound) leaking and being amplified again, may be a problem for severe hearing losses. Some modern circuits are able to provide feedback regulation or cancellation to assist with this. Venting may also cause feedback. A vent is a tube primarily placed to offer pressure equalization. However, different vent styles and sizes can be used to influence and prevent feedback. Traditionally, ITEs have not been recommended for young children because their fit could not be as easily modified as the earmold for a BTE, and thus the aid had to be replaced frequently as the child grew. However, there are new ITEs made from a silicone type material that mitigates the need for costly replacements. ITE hearing aids can be connected wirelessly to FM systems, for instance with a body-worn FM receiver with induction neck-loop which transmits the audio signal from the FM transmitter inductively to the telecoil inside the hearing instrument.

Mini in canal (MIC) or completely in canal (CIC) aids are generally not visible unless the viewer looks directly into the wearer's ear. These aids are intended for mild to moderately severe losses. CICs are usually not recommended for people with good low-frequency hearing, as the occlusion effect is much more noticeable. Completely-in-the-canal hearing aids fit tightly deep in the ear. It is barely visible. Being small, it will not have a directional microphone, and its small batteries will have a short life, and the batteries and controls may be difficult to manage. Its position in the ear prevents wind noise and makes it easier to use phones without feedback. In-the-canal hearing aids are placed deep in the ear canal. They are barely visible. Larger versions of these can have directional microphones. Being in the canal, they are less likely to cause a plugged feeling. These models are easier to manipulate than the smaller completely in-the-canal models but still have the drawbacks of being rather small.

In-the-ear hearing aids are typically more expensive than behind-the-ear counterparts of equal functionality, because they are custom fitted to the patient's ear. In fitting, the audiologist takes a physical impression (mold) of the ear. The mold is scanned by a specialized CAD system, resulting in a 3D model of the outer ear. During modeling, the venting tube is inserted. The digitally modeled shell is printed using a rapid prototyping technique such as stereolithography. Finally, the aid is assembled and shipped to the audiologist after a quality check.

Invisible-in-canal hearing aids
Invisible-in-canal hearing aids (IIC) style of hearing aids fits inside the ear canal completely, leaving little to no trace of an installed hearing aid visible. This is because it fits deeper in the canal than other types, so that it is out of view even when looking directly into the ear bowl (concha). A comfortable fit is achieved because the shell of the aid is custom-made to the individual ear canal after taking a mold. Invisible hearing aid types use venting and their deep placement in the ear canal to give a more natural experience of hearing. Unlike other hearing aid types, with the IIC aid the majority of the ear is not blocked (occluded) by a large plastic shell. This means that sound can be collected more naturally by the shape of the ear, and can travel down into the ear canal as it would with unassisted hearing. Depending on their size, some models allow the wearer to use a mobile phone as a remote control to alter memory and volume settings, instead of taking the IIC out to do this. IIC types are most suitable for users up to middle age, but are not suitable for elderly people with unsteady hands.

Extended wear hearing aids
Extended wear hearing aids are hearing devices that are non-surgically placed in the ear canal by a hearing professional. The extended wear hearing aid represents the first "invisible" hearing device. These devices are worn for 1–3 months at a time without removal. They are made of soft material designed to contour to each user and can be used by people with mild to moderately severe hearing loss. Their close proximity to the ear drum results in improved sound directionality and localization, reduced feedback, and improved high frequency gain. While traditional BTE or ITC hearing aids require daily insertion and removal, extended wear hearing aids are worn continuously and then replaced with a new device. Users can change volume and settings without the aid of a hearing professional. The devices are very useful for active individuals because their design protects against moisture and earwax and can be worn while exercising, showering, etc. Because the device's placement within the ear canal makes them invisible to observers, extended wear hearing aids are popular with those who are self-conscious about the aesthetics of BTE or ITC hearing aid models. As with other hearing devices, compatibility is based on an individual's hearing loss, ear size and shape, medical conditions, and lifestyle. The disadvantages include regular removal and reinsertion of the device when the battery dies, inability to go underwater, earplugs when showering, and for some discomfort with the fit since it is inserted deeply in the ear canal, the only part of the body where skin rests directly on top of bone.

CROS hearing aid
A CROS hearing aid is a hearing aid that transmits auditory information from one side of the head to the other side of the head. Candidates include people who have poor word understanding on one side, no hearing on one side, or who are not benefiting from a hearing aid on one side. CROS hearing aids can appear very similar to behind the ear hearing aids. The CROS system can assist the patient in sound localization and understanding auditory information on their poor side. While CROS hearing aids can be quite effective, the long-term solution for those with hearing issues on one side is to use a BiCROS system. This creates more of a balance for wearers.

Bone-anchored
A bone anchored hearing aid (BAHA) is a surgically implanted auditory prosthetic based on bone conduction. It is an option for patients without external ear canals, when conventional hearing aids with a mold in the ear cannot be used. The BAHA uses the skull as a pathway for sound to travel to the inner ear. For people with conductive hearing loss, the BAHA bypasses the external auditory canal and middle ear, stimulating the functioning cochlea. For people with unilateral hearing loss, the BAHA uses the skull to conduct the sound from the deaf side to the side with the functioning cochlea.

Individuals under the age of two (five in the USA) typically wear the BAHA device on a Softband. This can be worn from the age of one month as babies tend to tolerate this arrangement very well. When the child's skull bone is sufficiently thick, a titanium "post" can be surgically embedded into the skull with a small abutment exposed outside the skin. The BAHA sound processor sits on this abutment and transmits sound vibrations to the external abutment of the titanium implant. The implant vibrates the skull and inner ear, which stimulate the nerve fibers of the inner ear, allowing hearing.

The surgical procedure is simple both for the surgeon, involving very few risks for the experienced ear surgeon. For the patient, minimal discomfort and pain is reported. Patients may experience numbness of the area around the implant as small superficial nerves in the skin are sectioned during the procedure. This often disappears after some time. There is no risk of further hearing loss due to the surgery. One important feature of the BAHA is that, if a patient for whatever reason does not want to continue with the arrangement, it takes the surgeon less than a minute to remove it. The BAHA does not restrict the wearer from any activities such as outdoor life, sporting activities etc.

A BAHA can be connected to an FM system by attaching a miniaturized FM receiver to it.

Two main brands manufacture BAHAs today – the original inventors Cochlear, and the hearing aid company Oticon.

Eyeglass aids
During the late 1950s through 1970s, before in-the-ear aids became common (and in an era when thick-rimmed eyeglasses were popular), people who wore both glasses and hearing aids frequently chose a type of hearing aid that was built into the temple pieces of the spectacles. However, the combination of glasses and hearing aids was inflexible: the range of frame styles was limited, and the user had to wear both hearing aids and glasses at once or wear neither. Today, people who use both glasses and hearing aids can use in-the-ear types, or rest a BTE neatly alongside the arm of the glasses. There are still some specialized situations where hearing aids built into the frame of eyeglasses can be useful, such as when a person has hearing loss mainly in one ear: sound from a microphone on the "bad" side can be sent through the frame to the side with better hearing.

This can also be achieved by using CROS or bi-CROS style hearing aids, which are now wireless in sending sound to the better side.

Spectacle hearing aids
These are generally worn by people with a hearing loss who either prefer a more cosmetic appeal of their hearing aids by being attached to their glasses or where sound cannot be passed in the normal way, via a hearing aids, perhaps due to a blockage in the ear canal. pathway or if the client experiences continual infections in the ear. Spectacle aids come in two forms, bone conduction spectacles and air conduction spectacles.

Bone conduction spectacles
Sounds are transmitted via a receiver attached from the arm of the spectacles which are fitted firmly behind the boney portion of the skull at the back of the ear, (mastoid process) by means of pressure, applied on the arm of the spectacles. The sound is passed from the receiver on the arm of the spectacles to the inner ear (cochlea), via the bony portion. The process of transmitting the sound through the bone requires a great amount of power. Bone conduction aids generally have a poorer high pitch response and are therefore best used for conductive hearing losses or where it is impractical to fit standard hearing aids.

Air conduction spectacles
Unlike the bone conduction spectacles the sound is transmitted via hearing aids which are attached to the arm or arms of the spectacles. When removing your glasses for cleaning, the hearing aids are detached at the same time. Whilst there are genuine instances where spectacle aids are a preferred choice, they may not always be the most practical option.

Directional spectacles
These 'hearing glasses' incorporate a directional microphone capability: four microphones on each side of the frame effectively work as two directional microphones, which are able to discern between sound coming from the front and sound coming from the sides or back of the user. This improves the signal-to-noise ratio by allowing for amplification of the sound coming from the front, the direction in which the user is looking, and active noise control for sounds coming from the sides or behind. Only very recently has the technology required become small enough to be fitted in the frame of the glasses. As a recent addition to the market, this new hearing aid is currently available only in the Netherlands and Belgium.

Stethoscope
These hearing aids are designed for medical practitioners with hearing loss who use stethoscopes. The hearing aid is built into the speaker of the stethoscope, which amplifies the sound.

Hearing aid applications
Hearing aid applications (HAA) are software which, when installed on mobile computational platforms, transforms them into hearing aids.

The principle of HAA operation corresponds to the basic principles of operation of traditional hearing aids: the microphone receives an acoustic signal and converts it into a digital form. Sound amplification is achieved by the means of a mobile computational platform, in accordance with the degree and type of the user's hearing loss. The processed audio signal is transformed into an audio signal and output to the user into the headphones/headset. Signal processing is implemented in real time.

Constructional features of mobile computational platforms imply preferred use of stereo headsets with two speakers, which allows carrying out binaural hearing correction for the left and right ear separately. HAAs can work with both wired and wireless headsets and headphones.

As a rule, HAAs have two operation modes: setup mode and hearing aid mode. Setup mode involves the user passing an in situ-audiometry procedure, which determines the user's hearing characteristics. Hearing aid mode is a hearing correction system that corrects the user's hearing in accordance with the user's hearing thresholds. HAAs also incorporate background noise suppression and acoustic feedback suppression.

The user can independently choose a formula to enhance the sound, as well as adjust the level of the desired amplification to their wishes.

HAAs have several advantages (compared to traditional hearing aids):
 * HAAs do not cause any psychological inconvenience;
 * it is possible to achieve the highest sound pressure level and get high sound quality (due to large speakers and a long battery life);
 * it is possible to use more complex audio signal processing algorithms and a higher sampling rate (because of capacious battery);
 * the possibility to implement more convenient application control functions for people with poor motor skills;
 * resistance to ingress of earwax and moisture;
 * software flexibility;
 * the large distance between the microphone and the speaker prevents the occurrence of acoustic feedback;
 * the set up of HAAs in simple cases does not require special equipment and qualifications;
 * the user does not need to purchase and carry any separate device;
 * various types of headphones and headsets can be used.

HAAs also have some disadvantages (compared to traditional hearing aids):
 * because the microphone is not located in the ear, it does not use the functional advantages of the auricle and the natural acoustics of the outer ear.
 * they are more noticeable and less comfortable to wear.

Technology
The first electrical hearing aid used the carbon microphone of the telephone and was introduced in 1896. The vacuum tube made electronic amplification possible, but early versions of amplified hearing aids were too heavy to carry around. Miniaturization of vacuum tubes lead to portable models, and after World War II, wearable models using miniature tubes. The transistor invented in 1948 was well suited to the hearing aid application due to low power and small size; hearing aids were an early adopter of transistors. The development of integrated circuits allowed further improvement of the capabilities of wearable aids, including implementation of digital signal processing techniques and programmability for the individual user's needs.

Compatibility with telephones
A hearing aid and a telephone are "compatible" when they can connect to each other in a way that produces clear, easily understood sound. The term "compatibility" is applied to all three types of telephones (wired, cordless, and mobile). There are two ways telephones and hearing aids can connect with each other:
 * Acoustically: the sound from the phone's speaker is picked up by the hearing aid's microphone.
 * Electromagnetically: the signal inside the phone's speaker is picked up by the hearing aid's "telecoil" or "T-coil", a special loop of wire inside the hearing aid.

Note that telecoil coupling has nothing to do with the radio signal in a cellular or cordless phone: the audio signal picked up by the telecoil is the weak electromagnetic field that is generated by the voice coil in the phone's speaker as it pushes the speaker cone back and forth.

The electromagnetic (telecoil) mode is usually more effective than the acoustic method. This is mainly because the microphone is often automatically switched off when the hearing aid is operating in telecoil mode, so background noise is not amplified. Since there is an electronic connection to the phone, the sound is clearer and distortion is less likely. But in order for this to work, the phone has to be hearing-aid compatible. More technically, the phone's speaker has to have a voice coil that generates a relatively strong electromagnetic field. Speakers with strong voice coils are more expensive and require more energy than the tiny ones used in many modern telephones; phones with the small low-power speakers cannot couple electromagnetically with the telecoil in the hearing aid, so the hearing aid must then switch to acoustic mode. Also, many mobile phones emit high levels of electromagnetic noise that creates audible static in the hearing aid when the telecoil is used. A workaround that resolves this issue on many mobile phones is to plug a wired (not Bluetooth) headset into the mobile phone; with the headset placed near the hearing aid the phone can be held far enough away to attenuate the static. Another method is to use a "neckloop" (which is like a portable, around-the-neck induction loop), and plug the neckloop directly into the standard audio jack (headphones jack) of a smartphone (or laptop, or stereo, etc.). Then, with the hearing aids' telecoil turned on (usually a button to press), the sound will travel directly from the phone, through the neckloop and into the hearing aids' telecoils.

On 21 March 2007, the Telecommunications Industry Association issued the TIA-1083 standard, which gives manufacturers of cordless telephones the ability to test their products for compatibility with most hearing aids that have a T-Coil magnetic coupling mode. With this testing, digital cordless phone manufacturers will be able to inform consumers about which products will work with their hearing aids.

The American National Standards Institute (ANSI) has a ratings scale for compatibility between hearing aids and phones:
 * When operating in acoustic (Microphone) mode, the ratings are from M1 (worst) to M4 (best).
 * When operating in electromagnetic (Telecoil) mode, the ratings are from T1 (worst) to T4 (best).

The best possible rating is M4/T4 meaning that the phone works well in both modes. Devices rated below M3 are unsatisfactory for people with hearing aids.

Computer programs that allow the creation of a hearing aid using a PC, tablet or smartphone are currently gaining in popularity. Modern mobile devices have all the necessary components to implement this: hardware (an ordinary microphone and headphones may be used) and a high-performance microprocessor that carries digital sound processing according to a given algorithm. Application configuration is carried out by the user themselves in accordance with the individual features of their hearing ability. The computational power of modern mobile devices is sufficient to produce the best sound quality. This, coupled with software application settings (for example, profile selection according to a sound environment) provides for high comfort and convenience of use. In comparison with the digital hearing aid, mobile applications have the following advantages: It should be clearly understood that "hearing aid" application for smartphone / tablet cannot be considered a complete substitution of a digital hearing aid, since the latter: Functionality of hearing aid applications may involve a hearing test (in situ audiometry) too. However, the results of the test are used only to adjust the device for comfortable working with the application. The procedure of hearing testing in any way cannot claim to replace an audiometry test carried out by a medical specialist, so cannot be a basis for diagnosis.
 * acoustic gain is up to 30 dB (with a standard headset);
 * complete invisibility (smartphone is not associated with a hearing aid);
 * ease of use (no need to use additional devices, batteries and so on.);
 * Fast switching between the external headset and phone microphone;
 * free distribution of applications.
 * High duration of the battery;
 * high sampling frequency (44.1 kHz) providing for excellent sound quality;
 * high wearing comfort;
 * low delay in audio processing (from 6,3 to 15,7 ms – depending on the mobile device model);
 * No loss of settings when switching from one gadget to another and back again;
 * No need to get used to it, when changing mobile devices;
 * user-friendly interface of software settings;
 * is a medical device (exposed to the relevant procedures of testing and certification);
 * is adjusted using audiometry procedures.
 * is designed for use by doctor's prescription;
 * Apps such as Oticon ON for certain iOS (Apple) and Android devices can assist in locating a lost/misplaced hearing aid.

Wireless
Recent hearing aids include wireless hearing aids. One hearing aid can transmit to the other side so that pressing one aid's program button simultaneously changes the other aid, so that both aids change background settings simultaneously. FM listening systems are now emerging with wireless receivers integrated with the use of hearing aids. A separate wireless microphone can be given to a partner to wear in a restaurant, in the car, during leisure time, in the shopping mall, at lectures, or during religious services. The voice is transmitted wirelessly to the hearing aids eliminating the effects of distance and background noise. FM systems have shown to give the best speech understanding in noise of all available technologies. FM systems can also be hooked up to a TV or a stereo.

2.4 gigahertz Bluetooth connectivity is the most recent innovation in wireless interfacing for hearing instruments to audio sources such as TV streamers or Bluetooth enabled mobile phones. Current hearing aids generally do not stream directly via Bluetooth but rather do so through a secondary streaming device (usually worn around the neck or in a pocket), this bluetooth enabled secondary device then streams wirelessly to the hearing aid but can only do so over a short distance. This technology can be applied to ready-to-wear devices (BTE, Mini BTE, RIE, etc.) or to custom made devices that fit directly into the ear.

In developed countries FM systems are considered a cornerstone in the treatment of hearing loss in children. More and more adults discover the benefits of wireless FM systems as well, especially since transmitters with different microphone settings and Bluetooth for wireless cell phone communication have become available.

Many theatres and lecture halls are now equipped with assistive listening systems that transmit the sound directly from the stage; audience members can borrow suitable receivers and hear the program without background noise. In some theatres and churches FM transmitters are available that work with the personal FM receivers of hearing instruments.

Directional microphone
Most older hearing aids have only an omnidirectional microphone. An omnidirectional microphone amplifies sounds equally from all directions. In contrast, a directional microphone amplifies sounds from one direction more than sounds from other directions. This means that sounds originating from the direction the system is steered toward are amplified more than sounds coming from other directions. If the desired speech arrives from the direction of steering and the noise is from a different direction, then compared to an omnidirectional microphone, a directional microphone provides a better signal-to-noise ratio. Improving the signal-to-noise ratio improves speech understanding in noise. Directional microphones have been found to be the second best method to improve the signal-to-noise ratio (the best method was an FM system, which locates the microphone near the mouth of the desired talker).

Many hearing aids have both an omnidirectional and a directional microphone mode. This is because the wearer may not need or desire the noise-reducing properties of the directional microphone in a given situation. Typically, the omnidirectional microphone mode is used in quiet listening situations (e.g. living room) whereas the directional microphone is used in noisy listening situations (e.g. restaurant). The microphone mode is typically selected manually by the wearer. Some hearing aids automatically switch the microphone mode.

Adaptive directional microphones automatically vary the direction of maximum amplification or rejection (to reduce an interfering directional sound source). The direction of amplification or rejection is varied by the hearing aid processor. The processor attempts to provide maximum amplification in the direction of the desired speech signal source or rejection in the direction of the interfering signal source. Unless the user manually temporarily switches to a "restaurant program, forward only mode" adaptive directional microphones frequently amplify the speech of other talkers in a cocktail party type environments, such as restaurants or coffee shops; this can also be helpful during business meetings. The presence of multiple speech signals makes it difficult for the processor to correctly select the desired speech signal. Another disadvantage is that some noises often contain characteristics similar to speech, making it difficult for the hearing aid processor to distinguish the speech from the noise. Despite the disadvantages, adaptive directional microphones can provide improved speech recognition in noise.

FM systems have been found to provide a better signal-to-noise ratio even at larger speaker-to-talker distances in simulated testing conditions.

Telecoil
Telecoils or T-coils (from "Telephone Coils") are small devices installed in hearing aids or cochlear implants. An audio induction loop generates an electromagnetic field that can be detected by T-coils, allowing audio sources to be directly connected to a hearing aid. The T-coil is intended to help the wearer filter out background noise. They can be used with telephones, FM systems (with neck loops), and induction loop systems (also called "hearing loops") that transmit sound to hearing aids from public address systems and TVs. In the UK and the Nordic countries, hearing loops are widely used in churches, shops, railway stations, and other public places. In the US, telecoils and hearing loops are gradually becoming more common. Audio induction loops, telecoils and hearing loops are gradually becoming more common also in Slovenia.

A T-coil consists of a metal core (or rod) around which ultra-fine wire is coiled. T-coils are also called induction coils because when the coil is placed in a magnetic field, an alternating electric current is induced in the wire (Ross, 2002b; Ross, 2004). The T-coil detects magnetic energy and transduces (converts) it to electrical energy. In the United States, the Telecommunications Industry Association's TIA-1083 standard, specifies how analog handsets can interact with telecoil devices, to ensure the optimal performance.

Although T-coils are effectively a wide-band receiver, interference is unusual in most hearing loop situations. Interference can manifest as a buzzing sound, which varies in volume depending on the distance the wearer is from the source. Sources are electromagnetic fields, such as CRT computer monitors, older fluorescent lighting, some dimmer switches, many household electrical appliances and airplanes.

The states of Florida and Arizona have passed legislation that requires hearing professionals to inform patients about the usefulness of telecoils.

Legislation affecting use
In the United States, the Hearing Aid Compatibility Act of 1988 requires that the Federal Communications Commission (FCC) ensure that all telephones manufactured or imported for use in the United States after August 1989, and all "essential" telephones, be hearing aid-compatible (through the use of a telecoil).

"Essential" phones are defined as "coin-operated telephones, telephones provided for emergency use, and other telephones frequently needed for use by persons using such hearing aids." These might include workplace telephones, telephones in confined settings (like hospitals and nursing homes), and telephones in hotel and motel rooms. Secure telephones, as well as telephones used with public mobile and private radio services, are exempt from the HAC Act. "Secure" phones are defined as "telephones that are approved by the U.S. Government for the transmission of classified or sensitive voice communications."

In 2003, the FCC adopted rules to make digital wireless telephones compatible with hearing aids and cochlear implants. Although analog wireless phones do not usually cause interference with hearing aids or cochlear implants, digital wireless phones often do because of electromagnetic energy emitted by the phone's antenna, backlight, or other components. The FCC has set a timetable for the development and sale of digital wireless telephones that are compatible with hearing aids. This effort promises to increase the number of digital wireless telephones that are hearing aid-compatible. Older generations of both cordless and mobile phones used analog technology.

Audio boot
An audio boot or audio shoe is an electronic device used with hearing aids; hearing aids often come with a special set of metal contacts for audio input. Typically the audio boot will fit around the end of the hearing aid (a behind-the-ear model, as in-the-ear do not afford any purchase for the connection) to link it with another device, like an FM system or a cellphone or even a digital audio player.

Direct audio input
Direct audio input (DAI) allows the hearing aid to be directly connected to an external audio source like a CD player or an assistive listening device (ALD). By its very nature, DAI is susceptible to far less electromagnetic interference, and yields a better quality audio signal as opposed to using a T-coil with standard headphones. An audio boot is a type of device that may be used to facilitate DAI.

Processing
Every electronic hearing aid has at minimum a microphone, a loudspeaker (commonly called a receiver), a battery, and electronic circuitry. The electronic circuitry varies among devices, even if they are the same style. The circuitry falls into three categories based on the type of audio processing (analog or digital) and the type of control circuitry (adjustable or programmable). Hearing aid devices generally do not contain processors strong enough to process complex signal algorithms for sound source localization.

Analog
Analog audio may have:
 * Adjustable control: The audio circuit is analog with electronic components that can be adjusted. The hearing professional determines the gain and other specifications required for the wearer, and then adjusts the analog components either with small controls on the hearing aid itself or by having a laboratory build the hearing aid to meet those specifications. After the adjustment the resulting audio does not change any further, other than overall loudness that the wearer adjusts with a volume control. This type of circuitry is generally the least flexible. The first practical electronic hearing aid with adjustable analog audio circuitry was based on US Patent 2,017,358, "Hearing Aid Apparatus and Amplifier" by Samual Gordon Taylor, filed in 1932.
 * Programmable control: The audio circuit is analog but with additional electronic control circuitry that can be programmed by an audiologist, often with more than one program. The electronic control circuitry can be fixed during manufacturing or in some cases, the hearing professional can use an external computer temporarily connected to the hearing aid to program the additional control circuitry. The wearer can change the program for different listening environments by pressing buttons either on the device itself or on a remote control or in some cases the additional control circuitry operates automatically. This type of circuitry is generally more flexible than simple adjustable controls. The first hearing aid with analog audio circuitry and automatic digital electronic control circuitry was based on US Patent 4,025,721, "Method of and means for adaptively filtering near-stationary noise from speech" by D Graupe, GD Causey, filed in 1975. This digital electronic control circuitry was used to identify and automatically reduce noise in individual frequency channels of the analog audio circuits and was known as the Zeta Noise Blocker.

Digital
Digital audio, programmable control: Both the audio circuit and the additional control circuits are fully digital. The hearing professional programs the hearing aid with an external computer temporarily connected to the device and can adjust all processing characteristics on an individual basis. Fully digital circuitry allows implementation of many additional features not possible with analog circuitry, can be used in all styles of hearing aids and is the most flexible; for example, digital hearing aids can be programmed to amplify certain frequencies more than others, and can provide better sound quality than analog hearing aids. Fully digital hearing aids can be programmed with multiple programs that can be invoked by the wearer, or that operate automatically and adaptively. These programs reduce acoustic feedback (whistling), reduce background noise, detect and automatically accommodate different listening environments (loud vs. soft, speech vs. music, quiet vs. noisy, etc.), control additional components such as multiple microphones to improve spatial hearing, transpose frequencies (shift high frequencies that a wearer may not hear to lower frequency regions where hearing may be better), and implement many other features. Fully digital circuitry also allows control over wireless transmission capability for both the audio and the control circuitry. Control signals in a hearing aid on one ear can be sent wirelessly to the control circuitry in the hearing aid on the opposite ear to ensure that the audio in both ears is either matched directly or that the audio contains intentional differences that mimic the differences in normal binaural hearing to preserve spatial hearing ability. Audio signals can be sent wirelessly to and from external devices through a separate module, often a small device worn like a pendant and commonly called a "streamer", that allows wireless connection to yet other external devices. This capability allows optimal use of mobile telephones, personal music players, remote microphones and other devices. With the addition of speech recognition and internet capability in the mobile phone, the wearer has optimal communication ability in many more situations than with hearing aids alone. This growing list includes voice activated dialing, voice activated software applications either on the phone or on the internet, receipt of audio signals from databases on the phone or on internet, or audio signals from television sets or from global positioning systems. The first practical, wearable, fully digital hearing aid was invented by Maynard Engebretson, Robert E Morley Jr. and Gerald R Popelka. Their work resulted in US Patent 4,548,082, "Hearing aids, signal supplying apparatus, systems for compensating hearing deficiencies, and methods" by A Maynard Engebretson, Robert E Morley Jr. and Gerald R Popelka, filed in 1984. This patent formed the basis of all subsequent fully digital hearing aids from all manufacturers, including those produced currently.

The signal processing is performed by the microprocessor in real time and taking into account the individual preferences of the user (for example, increasing bass for better speech perception in noisy environments, or selective amplification of high frequencies for people with reduced sensibility to this range). The microprocessor automatically analyzes the nature of the external background noise and adapts the signal processing to the specific conditions (as well as to its change, for example, when the user goes outside from the building).

In speech enhancement, for example using neural networks, finds application in hearing aids. Problems may arise if these methods filter out emergency sounds such as fire alarms and car horns.

Difference between digital and analog hearing aids
Analogue hearing aids make all the sounds picked up by the microphone louder. For example, speech and ambient noise will be made louder together. On the other hand, digital hearing aid (DHA) technology processes the sound using digital technology. Before transmitting the sound to the speaker, the DHA microprocessor processes the digital signal received by the microphone according to an algorithm. This allows certain-frequency sounds to be made louder according to the individual user's settings (personal audiogram), and the DHA can automatically adjust to various environments (noisy streets, quiet room, concert hall, etc.).

For users with varying degrees of hearing loss, it is difficult to perceive the entire frequency range of external sounds. DHAs with multi-channel digital processing allow a user to "compose" the output sound by fitting a whole spectrum of the input signal into it. This gives users with limited hearing abilities the opportunity to perceive the whole range of ambient sounds, despite the personal difficulties of perception of certain frequencies. Moreover, even in this "narrow" range the DHA microprocessor is able to emphasize desired sounds (e.g. speech), lowering unwanted loud, high, etc., sounds at the same time.

According to research DHAs have a number of significant advantages compared to analogue hearing aids:
 * "Self-learning" and adaptive adjustment. They can implement adaptive selection of amplification parameters and processing.
 * Effective acoustic feedback reduction. The acoustic whistling common to all hearing aids can be adaptively controlled.
 * Effective use of directional microphones. Directional microphones can be adaptively controlled.
 * Extended frequency range. A larger range of frequencies can be implemented with frequency shifting.
 * Flexibility in selective amplification. They can provide more flexibility in frequency specific amplification to match the individual hearing characteristics of the user.
 * Improved connection to other devices. Connection to other devices such as smartphones and televisions is possible.
 * Noise reduction. They can reduce the background noise level to increase user comfort in noisy environments.
 * Speech recognition. They can distinguish the speech signal from the overall spectrum of sounds, which facilitates speech perception.

These advantages of DHAs were confirmed by a number of studies  relating to the comparative analysis of digital hearing aids of second and first generations and analog hearing aids.

Difference between digital hearing aids and hearing aid applications
Smartphones have all the necessary hardware to perform the functions of a digital hearing aid: microphone, AD converter, digital processor, DA converter, amplifier, and speakers. External microphone and speakers can also be connected as a special headset.

The operational principles of hearing aid applications correspond to general operational principles of digital hearing aids: the microphone perceives an acoustic signal and converts it to digital form. Sound amplification is achieved through hardware and software in accordance with the user's hearing characteristics. Then, the signal is converted to analog form and received in the headphones by the user. The signal is processed in real time.

Stereo headsets with two speakers can be used, which allows separate binaural hearing correction for the left and right ear.

Unlike digital hearing aids, the adjustment of hearing aid applications is an integral part of the application itself. Hearing aid applications are adjusted in accordance with the user's audiogram. The whole adjustment process is automated so that the user can perform audiometry on their own.

The hearing correction application has two modes: audiometry and correction. In the audiometry mode, hearing thresholds are measured. In the correction mode, the signal is processed with respect to the obtained thresholds.

Hearing aid applications also provide for different computational formulas for the calculation of sound amplification based on the audiometry data. These formulas are intended for maximum comfortable speech amplification and best sound intelligibility.

Hearing aid applications allow the user to save different user profiles for different acoustic environments. Thus, in contrast to the static settings of digital hearing aids, the user can quickly switch between the profiles depending on the acoustic environment.

One of the most important characteristics of the hearing aid is acoustic feedback. In hearing aid applications, there is a significant hardware delay, so hearing aid applications use a signal processing scheme with the minimum possible algorithmic delay to make it as short as possible.

Difference between PSAP and digital hearing aids
Personal sound amplification products (PSAP) are classified by the FDA as "personal sound amplification devices". These compact electronic devices are designed for people without hearing loss. Unlike hearing aids (which the FDA classifies as devices to compensate for hearing impairment), the use of PSAP does not require a medical prescription. Such devices are used by hunters, naturalists (for audio observation of animals or birds), ordinary people (for example, to increase the volume of the TV in a quiet room), etc. PSAP models differ significantly in price and functionality. Some devices simply amplify sound. Others contain directional microphones, equalizers to adjust the audio signal gain and filter noise. In modern days, some people refer to these devices as OTC hearing aids.

Evolution of hearing aid applications
There are audio players designed specifically for the hard-of-hearing. These applications amplify the volume of the reproduced audio signal in accordance with the user's hearing characteristics and act as a music volume amplifier and assistive hearing aid. The amplification algorithm works on the frequencies that the user hears worse, thus restoring natural hearing perception of the sound of music.

Just as in hearing aid applications, the player adjustment is based on the user's audiogram.

There are also applications that not only adapt the sound of music but also include some hearing aid functions. Such applications include a sound amplification mode in accordance with the user's hearing characteristics as well as a noise suppression mode and a mode allowing the user to hear ambient sound without pausing the music.

Also, some applications allow the hard-of-hearing to watch video and listen to the radio with comfort. The operational principles of these applications are similar to those of hearing aid applications: the audio signal is amplified on the frequencies that the user hears worse.

Hearing aid adaptation
A person using a hearing aid for the first time often cannot make use of all its advantages quickly. The structure and characteristics of hearing aids are thoroughly devised by specialists in order to make the adjustment period as simple and quick as possible. However, despite this, a beginning hearing aid user certainly needs time to get used to it.

The process of adjusting to hearing prostheses consists of the following steps:
 * Initial adjustment of the device
 * Fine adjustments
 * Adaptation to the new sound

Due to the plasticity of the central nervous system, inactive hearing centers in the brain's cortex switch over to processing auditory stimuli in another frequency and intensity. The brain starts to perceive sounds amplified by the hearing aid immediately after the initial adjustment; however, it may not process them correctly right away.

Feeling the hearing aid in the ear may seem unusual. It also takes time to adapt to a new way of hearing. The ear has to be gradually adjusted to the new sound. The sound may seem unnatural, metallic, too loud or too quiet. A whistling sound may also appear, which can be unpleasant.

Hearing aids do not provide immediate improvement. The adjustment period can last from several hours to several months.

Patients are offered an initial schedule to wear their hearing aid, ensuring gradual adaptation to it. If the patient wears the hearing aid continually from the beginning, the unfamiliar sound may cause a headache, and as a result, the user may refuse to wear a hearing aid despite the fact that it helps. Audiologists often run a quick preparation course for the patients. As a rule, users have inflated expectations of hearing aids. They expect that hearing aids will help them to hear in the same way as before hearing loss, but it is not the case. Training sessions help hearing aid users to get accustomed to the feeling of new sounds. Users are strongly recommended to regularly visit an audiologist, including for the purposes of additional hearing aid adjustment.

Hearing aid applications, in contrast to traditional hearing aids, allow the implementation of options such as a built-in adaptation course.

The functions of the course may include:
 * control of the amount of time spent on learning;
 * control over the sequence of exercises;
 * daily reminders to do the exercises.

The goal of the course is to help a user adapt to using a hearing aid application.

The adaptation course includes a certain number of stages, starting from listening to a set of low everyday sounds in a quiet environment, getting accustomed to one's own speech and other people's speech, getting accustomed to speech among background noise, etc.

History
The first hearing aids were ear trumpets, and were created in the 17th century. Some of the first hearing aids were external hearing aids. External hearing aids directed sounds in front of the ear and blocked all other noises. The apparatus would fit behind or in the ear.

The movement toward modern hearing aids began with the creation of the telephone, and the first electric hearing aid, the "akouphone", was created about 1895 by Miller Reese Hutchison. By the late 20th century, digital hearing aids were commercially available.

The invention of the carbon microphone, transmitters, digital signal processing chip or DSP, and the development of computer technology helped transform the hearing aid to its present form.

History of digital aids
The history of DHA can be divided into three stages. The first stage began in the 1960s with the widespread use of digital computers for simulation of audio processing and for the analysis of systems and algorithms. The work was conducted with the help of the very large digital computers of that era. These efforts were not actual digital hearing aids because the computers were not fast enough for audio processing in real time and their size prevented them from being described as wearable, but they allowed successful studies of the various hardware circuits and algorithms for digital processing of audio signals. The software package Block of Compiled Diagrams (BLODI) developed by Kelly, Lockbaum and Vysotskiy in 1961 allowed simulation of any sound system that could be characterized in the form of a block diagram. A special phone was created so that a person with a hearing impairment could listen to the digitally processed signals, but not in real time. In 1967, Harry Levitt used BLODI to simulate a hearing aid on a digital computer.

Almost ten years later the second stage began with the creation of the hybrid hearing aid, in which the analog components of a conventional hearing aid consisting of amplifiers, filters and signal limiting were combined with a separate digital programmable component in a conventional hearing aid case. The audio processing remained analog but it was controlled by the digital programmable component. The digital component could be programmed by connecting the device to an external computer in the laboratory then disconnected to allow the hybrid device to function as a conventional wearable hearing aid.

The hybrid device was effective from a practical point of view because of the low power consumption and compact size. At that time, low-power analog amplifier technology was well developed in contrast to the available semiconductor chips able to process digital audio in real time. The combination of high performance analog components for real time audio processing and a separate low power digital programmable component only for controlling the analog signal led to the creation of several low power digital programmable components able to implement different types of control.

A hybrid hearing aid was developed by Etymotic Design. A little later, Mangold and Lane created a programmable multi-channel hybrid hearing aid. Graupe with co-authors developed a digital programmable component that implemented an adaptive noise filter.

The third stage began in the early 1980s by a research group at Central Institute for the Deaf headed up by faculty members at Washington University in St. Louis MO. This group created the first fully digital wearable hearing aid. They first conceived a complete, comprehensive full digital hearing aid, then designed and fabricated, miniaturized full digital computer chips using custom digital signal processing chips with low power and very large scale integrated (VLSI) chip technology able to process both the audio signal in real time and the control signals, yet able to be powered by a battery and be fully wearable as a full digital wearable hearing aid able to be actually used by individuals with hearing loss in real-world environments. Engebretson, Morley and Popelka were the inventors of the first full digital hearing aid. Their work resulted in US Patent 4,548,082, "Hearing aids, signal supplying apparatus, systems for compensating hearing deficiencies, and methods" by A Maynard Engebretson, Robert E Morley Jr. and Gerald R Popelka, filed in 1984 and issued in 1985. This full digital wearable hearing aid also included many additional features now used in all contemporary full digital hearing aids including a bidirectional interface with an external computer, self-calibration, self-adjustment, wide bandwidth, digital programmability, a fitting algorithm based on audibility, internal storage of digital programs, and fully digital multichannel amplitude compression and output limiting. This group created several of these full digital hearing aids and used them for research on hearing impaired people as they wore them in the same manner as conventional hearing aids in real-world situations. In this first full DHA all stages of sound processing and control were carried out in binary form. The external sound was picked up by a microphone positioned in an ITE ear module to take advantage of the acoustic effects of the pinna, then converted into binary code, digitally processed and digitally controlled in real time, then converted back to an analog signal sent to two miniature loudspeakers positioned in the same ITE ear module. The ITE module also contained an inward facing microphone to measure the sound actually generated in the ear canal, a precursor to separate probe tube measures now routinely used for hearing aid fitting. The necessary electronic components, including batteries, to support this arrangement were situated in a BTE module that could be supplemented with a body worn module. These specialized hearing aid chips continued to become smaller, increase in computational ability and require even less power. Now, virtually all commercial hearing aids are fully digital and their digital signal processing capability has significantly increased. Very small and very low power specialized digital hearing aid chips are now used in all hearing aids manufactured worldwide. Many additional new features also have been added with various on-board advanced wireless technology.

Canada
Hearing aids are Class II regulated medical devices under Canada's Food and Drugs Act.

Under Health Canada, the Medical Devices Directorate (MDD) regulates the safety, quality, and effectiveness of hearing aids. All hearing aids imported and sold in Canada are subject to a pre-market review. Post-market, Health Canada monitors the performance of the hearing aid and any consumer complaints.

Hearing aid financial assistance is available at both the federal and provincial level. Provincial hearing aid assistance and coverage can vary widely depending on the province and territory.

In Canada, a prescription is required to purchase hearing aids. Only licensed audiologists, Ear, Nose and Throat (ENT) doctors, hearing instrument practitioners (where the profession exists), and audioprothésistes (in Quebec) can prescribe hearing aids. Over-the-counter (OTC) hearing aids are currently not available for sale in Canada.

Canadian taxpayers can claim tax relief for hearing aids as a medical expense.

Ireland
Like much of the Irish health care system, hearing aid provision is a mixture of public and private.

Hearing aids are provided by the state to children, OAPs and to people whose income is at or below that of the state pension. The Irish state hearing aid provision is extremely poor; people often have to wait for two years for an appointment.

It is estimated that the total cost to the state of supplying one hearing aid exceeds €2,000.

Hearing aids are also available privately, and there is grant assistance available for insured workers. For the fiscal year ending 2016, the grant stands at a maximum of €500 per ear.

Irish taxpayers can also claim tax relief at the standard rate as hearing aids are recognised as a medical device.

Hearing aids in the Republic of Ireland are exempt from VAT.

Hearing aid providers in Ireland mostly belong to the Irish Society of Hearing Aid Audiologists.

United States
Ordinary hearing aids are Class I regulated medical devices under Federal Food and Drug Administration (FDA) rules. A 1976 statute explicitly prohibits any state requirement that is "different from, or in addition to, any requirement applicable" to regulated medical devices (which includes hearing aids) which relates "to the safety and effectiveness of the device". Inconsistent state regulation is preempted under the federal law. In the late 1970s, the FDA established federal rules governing hearing aid sales, and addressed various requests by state authorities for exemptions from federal preemption, granting some and denying others. The Over-the-Counter Hearing Aid Act (OTC Act) was passed under the FDA Reauthorization Act of 2017, creating a class of hearing aids regulated by the FDA available directly to consumers without involvement from a licensed professional. This law's provisions are expected to go into effect in 2020.

In August 2022, the FDA issued a final rule to improve access to hearing aids. The action establishes a new category of over-the-counter (OTC) hearing aids, enabling consumers with perceived mild to moderate hearing impairment to purchase hearing aids directly from stores or online retailers without the need for a medical exam, prescription or a fitting adjustment by an audiologist. The FDA action amends existing rules that apply to prescription hearing aids for consistency with the new OTC category, it repeals the conditions for sale for hearing aids, and it includes provisions that address some of the effects of the FDA OTC hearing aid regulations on state regulation of hearing aids. The FDA also issued the final guidance, Regulatory Requirements for Hearing Aid Devices and Personal Sound Amplification Products (PSAPs), to clarify the differences between hearing aids, which are medical devices, and PSAPs, consumer products that help people with normal hearing amplify sounds.

Cost

Several industrialized countries supply free or heavily discounted hearing aids through their publicly funded health care system.

Australia
The Australian Department of Health and Ageing provides eligible Australian citizens and residents with a basic hearing aid free-of-charge, though recipients can pay a "top up" charge if they wish to upgrade to a hearing aid with more or better features. Maintenance of these hearing aids and a regular supply of batteries is also provided, on payment of a small annual maintenance fee.

Canada
In Canada, health care is a responsibility of the provinces. In the province of Ontario, the price of hearing aids is partially reimbursed through the Assistive Devices Program of the Ministry of Health and Long-Term care, up to $500 for each hearing aid. Like eye appointments, audiological appointments are no longer covered through the provincial public health plan. Audiometric testing can still easily be obtained, often free of charge, in private sector hearing aid clinics and some ear, nose and throat doctors offices. Hearing aids may be covered to some extent by private insurance or in some cases through government programs such as Veterans Affairs Canada or Workplace Safety & Insurance Board.

Iceland
Social Insurance pays a one time fee of ISK 30,000 for any kind of hearing aid. However, the rules are complicated and require that both ears have significant hearing loss in order to qualify for reimbursement. BTE hearing aids range from ISK 60,000 to ISK 300,000.

India
In India hearing aids of all kinds are easily available. Under central and state government health services, the poor can often avail themselves of free hearing devices. However, market prices vary for others and can range from Rs 10,000 to Rs 275,000 per ear.

United Kingdom
From 2000 to 2005 the Department of Health worked with Action on Hearing Loss (then called RNID) to improve the quality of NHS hearing aids so every NHS audiology department in England was fitting digital hearing aids by March 2005. By 2003 over 175,000 NHS digital hearing aids had been fitted to 125,000 people. Private companies were recruited to enhance the capacity, and two were appointed – David Ormerod Hearing Centres, partly owned by Alliance Boots and Ultravox Group, a subsidiary of Amplifon.

Within the UK, the NHS provides digital BTE hearing aids to NHS patients, on long-term loan, free of charge. Other than BAHAs (bone anchored hearing aid) or cochlear implants, where specifically required, BTEs are usually the only style available. Private purchases may be necessary if a user desires a different style. Batteries are free.

In 2014 the Clinical Commissioning Group in North Staffordshire considered proposals to end provision of free hearing aids for adults with mild to moderate age related hearing loss, which currently cost them £1.2m a year. Action on Hearing Loss mobilised a campaign against the proposal.

In June 2018 the National Institute for Health and Care Excellence produced new guidance saying that hearing aids should be offered at the first opportunity when hearing loss affects the individual's ability to hear and communicate, rather than waiting for arbitrary thresholds of hearing loss to be reached.

United States
Most private healthcare providers in the United States do not provide coverage for hearing aids, so all costs are usually borne by the recipient. The cost for a single hearing aid can vary between $500 and $6,000 or more, depending on the level of technology and whether the clinician bundles fitting fees into the cost of the hearing aid. Though if an adult has hearing loss which substantially limits major life activities, some state-run vocational rehabilitation programs can provide upwards of full financial assistance. Severe and profound hearing loss often falls within the "substantially limiting" category. Less expensive hearing aids can be found on the internet or mail order catalogs, but most in the under-$200 range tend to amplify the low frequencies of background noise, making it harder to hear the human voice.

Military veterans receiving VA medical care are eligible for hearing aids based on medical need. The Veterans Administration pays the full cost of testing and hearing aids to qualified military veterans. Major VA medical facilities provide complete diagnostic and audiology services.

The cost of hearing aids is a tax-deductible medical expense for those who itemize medical deductions.

Research involving more than 40,000 US households showed a convincing correlation between the degree of hearing loss and the reduction of personal income. According to the same research, hearing aids were shown to mitigate the impact of income loss by 90%–100% for those with milder hearing losses and from 65%–77% for those with severe to moderate hearing loss.

Batteries
While there are some instances that a hearing aid uses a rechargeable battery or a long-life disposable battery, the majority of modern hearing aids use one of five standard button cell zinc–air batteries. (Older hearing aids often used mercury battery cells, but these cells have become banned in most countries today.) Modern hearing aid button cell types are typically referred to by their common number name or the color of their packaging.

They are typically loaded into the hearing aid via a rotating battery door, with the flat side (case) as the positive terminal (cathode) and the rounded side as the negative terminal (anode).

These batteries all operate from 1.35 to 1.45 volts.

The type of battery a specific hearing aid utilizes depends on the physical size allowable and the desired lifetime of the battery, which is in turn determined by the power draw of the hearing aid device. Typical battery lifetimes run between 1 and 14 days (assuming 16-hour days).

title: Hearing aid


text:
Two-color system

The two-color system of projection is a name given to a variety of methods of projecting a full-color image using (only) two different single-color projectors. James Clerk Maxwell first suggested he had discovered such a projection system, but it was not reproduced until the 1950s, when Edwin Land accidentally noticed a similar effect while working on his three-color system of projection.

Despite Land's later work on the subject, the physics behind the success of this system of projection (and similar methods of apparent full-color projection involving only one color of light, sometimes in different polarizations) is not clearly understood since it involves not only the projected light but also the human visual system's response to it.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Two-color system


text:
Certified information systems security professional

CISSP (Certified Information Systems Security Professional) is an independent information security certification granted by the International Information System Security Certification Consortium, also known as ISC2.

As of July, 2022, there were 156,054 ISC2 members holding the CISSP certification worldwide.

In June 2004, the CISSP designation was accredited under the ANSI ISO/IEC Standard 17024:2003. It is also formally approved by the U.S. Department of Defense (DoD) in their Information Assurance Technical (IAT), Managerial (IAM), and System Architect and Engineer (IASAE) categories for their DoDD 8570 certification requirement.

In May 2020, The UK National Academic Recognition Information Centre assessed the CISSP qualification as a Level 7 award, the same level as a Masters degree. The change enables cyber security professionals to use the CISSP certification towards further higher education course credits and also opens up opportunities for roles that require or recognize master's degrees.

History
In the mid-1980s, a need arose for a standardized, vendor-neutral certification program that provided structure and demonstrated competence. In November 1988, the Special Interest Group for Computer Security (SIG-CS), a member of the Data Processing Management Association (DPMA), brought together several organizations interested in this goal. The International Information Systems Security Certification Consortium or "ISC2" formed in mid-1989 as a non-profit organization.

By 1990, the first working committee to establish a Common Body of Knowledge (CBK) had been formed. The first version of the CBK was finalized by 1992, and the CISSP credential was launched by 1994.

In 2003 the CISSP was adopted as a baseline for the U.S. National Security Agency's ISSEP program.

Certification subject matter
The CISSP curriculum breaks the subject matter down into a variety of Information Security topics referred to as domains. The CISSP examination is based on what ISC2 terms the Common Body of Knowledge (or CBK). According to ISC2, "the CISSP CBK is a taxonomy – a collection of topics relevant to information security professionals around the world. The CISSP CBK establishes a common framework of information security terms and principles that allow information security professionals worldwide to discuss, debate and resolve matters pertaining to the profession with a common understanding."

On April 15, 2024, a refreshed exam outline applies. The updates are the result of the Job Task Analysis (JTA), which is an analysis of the current content of the credential evaluated by ISC2 members on a triennial cycle. The impact of the change is limited to the weighting of the domains; the domains themselves did not change.

On 1st May 2021 there was a domain refresh that impacted the weighting of the domains; the domains themselves did not change.

From 15 April 2018, the eight domains covered are :
 * 1) Security and risk management
 * 2) Asset security
 * 3) Security architecture and engineering
 * 4) Communication and network security
 * 5) Identity and access management (IAM)
 * 6) Security assessment and testing
 * 7) Security operations
 * 8) Software development security

From 2015 to early 2018, the CISSP curriculum was divided into eight domains similar to the latest curriculum above. The only domain to have changed its name was "Security Engineering," which in the 2018 revision was expanded to "Security Architecture and Engineering."

Before 2015, it covered ten domains:

 * 1) Operations security
 * 2) Telecommunications and network security
 * 3) Information security governance and risk management
 * 4) Software development security
 * 5) Cryptography
 * 6) Security architecture and design
 * 7) Access control
 * 8) Business continuity and disaster recovery planning
 * 9) Legal, regulations, investigations and compliance
 * 10) Physical (environmental) security

Requirements

 * Possess a minimum of five years of direct full-time security work experience in two or more of the ISC2 information security domains (CBK). One year may be waived for having either a four-year college degree, a master's degree in Information Security, or for possessing one of a number of other certifications. A candidate without the five years of experience may earn the Associate of ISC2 designation by passing the required CISSP examination, valid for a maximum of six years. During those six years a candidate will need to obtain the required experience and submit the required endorsement form for certification as a CISSP. Upon completion of the professional experience requirements the certification will be converted to CISSP status.
 * Attest to the truth of their assertions regarding professional experience and accept the CISSP Code of Ethics.
 * Answer questions regarding criminal history and related background.
 * Pass the multiple choice CISSP exam (four hours, up to 175 questions, in an adaptive exam) with a scaled score of 700 points or greater out of 1000 possible points, you must achieve a pass in all eight domains.
 * Have their qualifications endorsed by another ISC2 certification holder in good standing.

Member counts
Number of CISSP members as of July, 2022 is 156,054.

Concentrations
Holders of CISSP certifications can earn additional certifications in areas of speciality. There are three possibilities of the concentrations as listed below.

Information Systems Security Architecture Professional (CISSP-ISSAP)
It is an advanced information security certification issued by (ISC)² that focuses on the architecture aspects of information security. The certification exam consists of 125 questions covering six domain areas:
 * 1) Identity and Access Management Architecture
 * 2) Security Operations Architecture
 * 3) Infrastructure Security
 * 4) Architect for Governance, Compliance, and Risk Management
 * 5) Security Architecture Modeling
 * 6) Architect for Application Security

As of July, 2022, there were 2,307 ISC2 members holding the CISSP-ISSAP certification worldwide.

Information Systems Security Engineering Professional (CISSP-ISSEP)
It is an advanced information security certification issued by (ISC)² that focuses on the engineering aspects of information security across the systems development life cycle. In October 2014 it was announced that some of its curricula would be made available to the public by the United States Department of Homeland Security through its National Initiative for Cybersecurity Careers and Studies program. Both ZDNet and Network World have named ISSEP one of tech’s most valuable certifications. The certification exam consists of 125 questions covering 5 domain area:
 * 1) Security Engineering Principles
 * 2) Risk Management
 * 3) Security Planning, Design, and Implementation
 * 4) Secure Operations, Maintenance, and Disposal
 * 5) Secure Engineering Technical Management

As of July, 2022, there were 1,382 ISC2 members holding the CISSP-ISSEP certification worldwide.

Information Systems Security Management Professional (CISSP-ISSMP)
It is an advanced information security certification issued by (ISC)² that focuses on the management aspects of information security. In September 2014, Computerworld rated ISSMP one of the top ten most valuable certifications in all of tech. The certification exam consists of 125 questions covering 6 domain areas:
 * 1) Leadership and Business Management
 * 2) Systems Lifecycle Management
 * 3) Risk Management
 * 4) Threat Intelligence and Incident Management
 * 5) Contingency Management
 * 6) Law, Ethics, and Security Compliance Management

As of July, 2022, there were 1,458 ISC2 members holding the CISSP-ISSMP certification worldwide.

Fees and ongoing certification
The standard exam costs $749 US as of 2021. On completion of the exam, to gain certification you need to complete an endorsement process to evidence at least five years experience within a mix of the domains. A dispensation can be claimed for one year with the relevant academic qualification. The final step is payment of the annual maintenance fee of $135 (as of 2024).

The CISSP credential is valid for three years; holders renew either by submitting 40 Continuing Professional Education (CPE) credits per year over three years or re-taking the exam.

CPE credits are gained by completing relevant professional education.

Value
In 2005, Certification Magazine surveyed 35,167 IT professionals in 170 countries on compensation and found that CISSPs led their list of certificates ranked by salary. A 2006 Certification Magazine salary survey also ranked the CISSP credential highly, and ranked CISSP concentration certifications as the top best-paid credentials in IT.

In 2008, another study came to the conclusion that IT professionals in the Americas holding the CISSP (or other major security certifications) and at least 5 years of experience had salaries of up to 26% higher than IT professionals with similar experience levels who did not have such certificates. Note that any actual cause-and-effect relationship between the certificate and salaries remains unproven.

As of 2017, a study by CyberSecurityDegrees.com surveyed some 10,000 current and historical cyber security job listings that preferred candidates holding CISSP certifications. CyberSecurityDegrees found that these job openings offered an average salary of more than the average cyber security salary.

ANSI certifies that CISSP meets the requirements of ANSI/ISO/IEC Standard 17024, a personnel certification accreditation program.


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Spray painting

Spray painting is a painting technique in which a device sprays coating material (paint, ink, varnish, etc.) through the air onto a surface. The most common types employ compressed gas—usually air—to atomize and direct the paint particles.

Spray guns evolved from airbrushes, and the two are usually distinguished by their size and the size of the spray pattern they produce. Airbrushes are hand-held and used instead of a brush for detailed work such as photo retouching, painting nails, or fine art. Air gun spraying uses generally larger equipment. It is typically used for covering large surfaces with an even coating of liquid. Spray guns can be either automated or hand-held and have interchangeable heads to allow for different spray patterns.

Single color aerosol paint cans are portable and easy to store.

History
Spraying paint with compressed air can be traced back to its use on the Southern Pacific Railroad in the early 1880s In 1887 Joseph Binks, the maintenance supervisor at Chicago's Marshall Field's Wholesale Store developed a hand-pumped cold-water paint spraying machine to apply whitewash to the subbasement walls of the store. Francis Davis Millet, the decorations director for the World's Columbian Exposition in Chicago in 1893, used Binks and his spray painting system to apply whitewash consisting of a mix of oil and white lead to the buildings at the Exposition, taking considerably less time than traditional brush painting and turning it into what has been called the White City. In 1949, Edward Seymour developed a type of spray painting, aerosol paint, that could be delivered via a compressed aerosol in a can.

Air gun spraying
This process occurs when the paint is applied to an object through the use of an air-pressurized spray gun. The air gun has a nozzle, paint basin, and air compressor. When the trigger is pressed the paint mixes with the compressed air stream and is released in a fine spray.

Due to a wide range of nozzle shapes and sizes, the consistency of the paint can be varied. The shape of the workpiece and the desired paint consistency and pattern are important factors when choosing a nozzle. The three most common nozzles are the full cone, hollow cone, and flat stream. There are two types of air-gun spraying processes. In a manual operation method the air-gun sprayer is held by a skilled operator, about 6 to(-) from the object, and moved back and forth over the surface, each stroke overlapping the previous to ensure a continuous coat. In an automatic process the gun head is attached to a mounting block and delivers the stream of paint from that position. The object being painted is usually placed on rollers or a turntable to ensure overall equal coverage of all sides.

High volume low pressure
High volume low pressure (HVLP) is similar to a conventional spray gun using a compressor to supply the air, but the spray gun itself requires a lower pressure (LP). A higher volume (HV) of air is used to aerosolize and propel the paint at lower air pressure. The result is a higher proportion of paint reaching the target surface with reduced overspray, materials consumption, and air pollution.

A regulator is often required so that the air pressure from a conventional compressor can be lowered for the HVLP spray gun. Alternatively, a turbine unit (commonly containing a vacuum cleaner type motor, reverse mounted) can be used to propel the air without the need for an airline running to the compressor.

A rule of thumb puts two-thirds of the coating on the substrate and one third in the air. True HVLP guns use 8–20 cfm (13.6–34 m3/h), and an industrial compressor with a minimum of 5 hp output is required. HVLP spray systems are used in the automotive, decorative, marine, architectural coating, furniture finishing, scenic painting, and cosmetic industries.

Low volume low pressure
Like HVLP, low volume low pressure (LVLP) spray guns also operate at a lower pressure (LP), but they use a low volume (LV) of air when compared to conventional and HVLP equipment. This is a further effort at increasing the transfer efficiency (amount of coating that ends up on the target surface) of spray guns while decreasing the amount of compressed air consumption.

Electrostatic spray painting
The electrostatic painting was first patented in the U.S. by Harold Ransburg in the late 1940s. Harold Ransburg founded Ransburg Electrostatic Equipment and discovered that electrostatic spray painting was an immediate success as manufacturers quickly perceived the substantial materials savings that could be achieved. In electrostatic spray painting or powder coating, the atomized particles are made to be electrically charged, thereby repelling each other and spreading themselves evenly as they exit the spray nozzle. The object being painted is charged oppositely or grounded. The paint is then attracted to the object giving a more even coat than wet spray painting, and also greatly increasing the percentage of paint that sticks to the object. This method also means that paint covers hard to reach areas. The whole may then be baked to properly attach the paint: the powder turns into a type of plastic. Car body panels and bike frames are two examples where electrostatic spray painting is often used.

There are three main technologies for charging the fluid (liquid or powders):
 * Direct charging: An electrode is immersed in the paint supply reservoir or the paint supply conduit.
 * Tribo charging: This uses the friction of the fluid which is forced through the barrel of the paint gun. It rubs against the side of the barrel and builds up an electrostatic charge.
 * Post-atomization charging: The atomized fluid comes into contact with an electrostatic field downstream of the outlet nozzle. The electrostatic field may be created by electrostatic induction or corona, or by one or more electrodes (electrode ring, mesh, or grid).

Rotational bell
With this method the paint is flung into the air by a spinning metal disc ("bell"). The metal disc also imparts an electrical charge to the coating particle.

Electric fan
There is a variety of hand-held paint sprayers that either combine the paint with air or convert the paint to tiny droplets and accelerate these out of a nozzle.

Hot spray
By heating the full-bodied paint to 60-80 °C, it is possible to apply a thicker coat. Originally the paint was recirculated, but as this caused bodying up, the system was changed to direct heating on line. Hot spraying was also used with Airless and Electrostatic Airless to decrease bounce-back. Two-pack materials usually had premix before tip systems using dual pumps.

Air assisted airless spray guns
These use air pressure and fluid pressure 300 to(-) to achieve atomization of the coating. This equipment provides high transfer and increased application speed and is most often used with flat-line applications in factory finish shops.

The fluid pressure is provided by an airless pump, which allows much heavier materials to be sprayed than is possible with an air spray gun. Compressed air is introduced into the spray via an air nozzle (sometimes called air cap) similar to a standard conventional spray gun. The addition of compressed air improves the fineness of atomization. Additionally, unlike a pure airless spray gun, an AA gun has some control over fan spray to round spray. Some electric airless sprayers (Wagner and Graco) are fitted with a compressor to allow the use of an air-assisted airless gun in situations where portability is important.

Airless spray guns
These operate connected to a high-pressure pump commonly found using 300 to(-) pressure to atomize the coating, using different tip sizes to achieve the desired atomization and spray pattern size. This type of system is used by contract painters to paint heavy duty industrial, chemical, and marine coatings and linings.

Advantages of airless spray are:
 * The coating penetrates better into pits and crevices.
 * A uniform thick coating is produced, reducing the number of coats required.
 * A very "wet" coating is applied, ensuring good adhesion and flow-out.

Most coatings can be sprayed with very little thinner added, thereby reducing drying time and decreasing the release of solvent into the environment.

Care must be used when operating, as airless spray guns can cause serious injury, such as injection injuries, due to the paint ejecting from the nozzle at high pressure.

Airless pumps can be powered by different types of motor: electric, compressed air (pneumatic), or hydraulic. Most have a paint pump (also called a lower) that is a double-acting piston, in which the piston pumps the paint in both the down and the upstroke. Some airless pumps have a diaphragm instead of a piston, but both types have inlet and outlet valves.

Most electric-powered airless pumps have an electric motor connected through a gear train to the paint piston pump. The pressure is achieved by stopping and starting the motor via a pressure sensor (also called a transducer); in more advanced units, this is done by digital control in which the speed of the motor varies with the demand and the difference from the pressure set-point, resulting in very good pressure control. Some direct drive piston pumps are driven by a gasoline engine with pressure control via an electric clutch. In electric diaphragm pumps, the motor drives a hydraulic piston pump that transmits the oil displaced by the piston, to move the diaphragm.

Hydraulic and air-powered airless pumps have linear motors that require a hydraulic pump or an air compressor, which can be electric or gasoline-powered, although an air compressor is usually diesel-powered for mobile use or electric for fixed installations. Some airless units have the hydraulic pump and its motor, built onto the same chassis as the paint pump.

Hydraulic or air-powered airless provide a more uniform pressure control since the paint piston moves at a constant speed except when it changes direction. In most direct drive piston pumps, the piston is crankshaft driven in which the piston will be constantly changing speed. The linear motors of hydraulic or compressed air drive pumps are more efficient in converting engine power to material power than crankshaft-driven units. All types of paint can be painted using an airless method.

Automated linear spray systems
Manufacturers who mass-produce wood products use automated spray systems, allowing them to paint materials at a very high rate with a minimum of personnel. Automated spray systems usually incorporate a paint-saving system that recovers paint not applied to the products. Commonly, linear spray systems are for products which are lying flat on a conveyor belt and then fed into a linear spray system, where automated spray guns are stationed above. When the material is directly below the guns, the guns begin to paint the material. Materials consist of lineal parts usually less than 12 in wide, such as window frames, wood molding, baseboard, casing, trim stock, and any other material that is simple in design. These machines are commonly used to apply the stain, sealer, and lacquer. They can apply water- or solvent-based coatings. In recent years ultraviolet-cured coatings have become commonplace in profile finishing, and there are machines particularly suited to this type of coating.

Automated flatline spray systems
Mass-produced material is loaded on a conveyor belt where it is fed into one of these flatline machines. Flatline machines are designed to specifically paint material that is less than 4 in thick and complex in shape, for example, a kitchen cabinet door or drawer front. Spray guns are aligned above the material and the guns are in motion to hit all the grooves of the material. The guns can be moved in a cycle, circle, or can be moved back and forth to apply the paint evenly across the material. Flatline systems are typically large and can paint doors, kitchen cabinets, and other plastic or wooden products.

Spray booth
A spray booth is a pressure-controlled closed environment, originally used to paint vehicles in a body shop. Its effective design promotes efficient paint application, minimizing contamination and maximizing the quality of the finished product. To ensure the ideal working conditions (temperature, airflow, and humidity), these environments are equipped with ventilation, consisting of mechanical fans driven by electric motors, and optionally burners to heat the air to speed paint drying. Toxic solvents and paint particles are exhausted outside, possibly after filtering and treatment to reduce air pollution. Prevention of fires and dust explosions is also a high priority. To assist in the removal of the over sprayed paint from the air and to provide efficient operation of the down-draft, water-washed paint spray booths utilize paint detackifying chemical agents.

Artists may also make use of spray booth facilities to enable them to use spray paints (including automotive finishes) efficiently and safely. They may rent space and time in auto body shops or set up their facilities in association with schools or artist cooperatives.

Safety
Spray painting poses health hazards that affect the respiratory, nervous, and circulatory systems. Similarly, using solvents to clean one's hands of paint marks and residue may cause skin irritation or even more serious issues since many are carcinogenic or neurotoxic. There are risks involved in working with substances such as paint and thinner, which contain compounds that are potentially harmful to health, or even fatal.

Appropriate training for personnel who are responsible for conducting the painting procedures is important, which may be from a professional training provider or the product supplier. There are also hazards related to the disposal of wastes and materials that are contaminated with potentially harmful chemicals. Decontamination procedures and Material Safety Data Sheets for various products are important. Safety is improved through:

 * Personal protective equipment (PPE) use: PPE must be used when handling spray paint materials, particularly PPE that offers protection to the skin. Some of the essential personal protective equipment are overalls with a hood, protective goggles for the eyes, half-mask respirators, and single-use nitrile gloves. One of the most essential types of PPE is respiratory protective equipment (RPE). Nevertheless, basic RPE does not offer ample protection from the negative effects of isocyanates in human tissue. Paint products containing isocyanates must be handled while donning an air-fed RPE that has a 20 or higher APF (assigned protection factor). Air-fed respiratory protective equipment needs extra attention since they provide breathable air to the user. When in use, measures must be implemented to prevent contamination of the air supply since there is a risk of harmful substances entering the intake valve if it is not positioned outside of the spray area.
 * Health monitoring: To avoid the development of illnesses associated with exposure to isocyanates, health authorities recommend that people who use spray paint products that contain the substance provide a urine sample after a work shift at least once a year, with high frequencies in first few months on the job. A urine sample with ascertain levels of exposure, not the presence of disease associated with harmful chemicals.
 * Proper storage: Since paints and thinners are fire hazards, extra care must be taken not only while they are in use. Fire safety should also be considered when storing paint supplies. In the United States, the Occupational Safety and Health Administration (OSHA) provides guidelines for the proper storage of flammable materials. Many products used in spray paint are flammable such that fire risk is likely within a distance of 15 cm from the nozzle. As such, ignition sources must be placed at a safe distance. Also, there is a risk of dust explosions when finely-divided paint particles become airborne.
 * Proper recordkeeping: One of the basic tenets of risk control is the maintenance of updated health records of personnel handling spray paint products. Confidential data on biological monitoring results must be appropriately kept. Records of the schedule and result of testing procedures should also be kept. Some of the most important tests to be conducted regularly are air quality testing, testing of pressure systems and electrical systems, and testing of compressor reservoir air filters.

Defects

 * Orange peel, an undesirable rippled texture
 * Fisheye, blemishes caused by contamination such as oil or water

Other applications
One application of spray painting is graffiti. The introduction of inexpensive and portable aerosol paint has been a boon to this art form, which has spread all over the world. Spray painting has also been used in fine art. Jules Olitski, Dan Christensen, Peter Reginato, Sir Anthony Caro, and Jean-Michel Basquiat have used airbrushes, for both painting and sculpture.

More spray painting resources

 * "How to Buy a Paint Sprayer and Make it Pay." Popular Science, September 1966, pp. 160–163, article on house paint sprayers.
 * "How Long Does Spray Paint Take To Dry On All Surfaces." Paint Hack, September 2022, article on how long does spray paint takes to dry.

title: Spray painting


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The Dancing Wu Li Masters

The Dancing Wu Li Masters is a 1979 book by Gary Zukav, a popular science work exploring modern physics, and quantum phenomena in particular. It was awarded a 1980 U.S. National Book Award in category of Science. Although it explores empirical topics in modern physics research, The Dancing Wu Li Masters gained attention for leveraging metaphors taken from eastern spiritual movements, in particular the Huayen school of Buddhism with the monk Fazang's treatise on the Golden Lion, to explain quantum phenomena and has been regarded by some reviewers as a New Age work, although the book is mostly concerned with the work of pioneers in western physics down through the ages.

The toneless pinyin phrase Wu Li in the title is most accurately rendered in Chinese characters, one Chinese translation of the word "physics" in the light of the book's subject matter. This becomes somewhat of a pun as there are many other Chinese characters that could be rendered as "wu li" in atonal pinyin, and chapters of the book are each titled with alternative translations of Wu Li, such as "Nonsense", "My Way" and "I Clutch My Ideas". Zukav participated as a journalist in a 1976 physics conference of eastern and western scientists at Esalen Institute, California; and he used the occasion as material for his book. At the conference, it was said that the Chinese term for physics is 'Wu Li', or "patterns of organic energy." Zukav, among others, conceptualized 'physics' as the dance of the Wu Li Masters – teachers of physical essence. Zukav explains the concept further:

 * The Wu Li Master dances with his student. The Wu Li Master does not teach, but the student learns. The Wu Li Master always begins at the center, the heart of the matter...

Editions

 * The Dancing Wu Li Masters: An Overview of the New Physics (1979). New York: William Morrow and Company, hardcover: ISBN 0-688-03402-0, paperback: ISBN 0-688-08402-8, 352 p.
 * (1984) Bantam mass market paperback: ISBN 0-553-26382-X, 337 p.
 * (1990) Audio Renaissance audiocassette: ISBN 1-55927-058-6 (abridged)
 * (2001) Harper Perennial paperback: ISBN 0-06-095968-1, 416 p.
 * (2001) Audio Renaissance CD: ISBN 1-55927-643-6 (abridged)

title: The Dancing Wu Li Masters