text:
A Loucif

B Duffet

A L Helbert

L Prévond

F Brisset

T Baudin

Characteristics of thin Al/steel/Al sandwich produced via accumulative roll bonding technique for electromagnetic shielding applications

Keywords: Al/steel/Al composites, accumulative roll bonding (ARB), magnetic shielding

Tri-layered Al/steel/Al composites with 0.2 mm thickness were produced by accumulative roll bonding (ARB) process at room temperature using Al-1050 sheets and a commercial low carbon steel DC01 up to four cycles. The effect of ARB cycles on the microstructure, mechanical properties and magnetic shielding of the ARBed sandwiches is investigated. The microstructural results obtained using SEM and EBSD techniques showed that the steel layers were necked and fractured as the strain increases and a small part of steel fragments was observed in some points of Al matrix. Further, the outer Al layers tend to disappear and to be replaced by steel layers, which may be due to the softening of the Al alloy resulting from the intermediate annealing performed at 400°C for 10 min between each ARB cycle. During ARB cycles a pronounced decrease of the grain size is observed in Al layers and the HAGB of both Al and steel layers showed a little increase with respect to primary sandwiches. The tensile tests revealed a significant reduce in tensile properties of the sandwich after 2 passes due to the fragmentation of steel inside the Al matrix. The shielding effectiveness (SE) of multi-layered Al/steel/Al composites is evaluated from 20 Hz to 20 KHz versus the cycle number. The SE of ARB processed sandwich showed a similar behavior as steel shielding at low frequency and gradually increases with increasing frequency.

Introduction

Nowadays, electronic devices such as computers, smart phones and Wi-Fi systems are extensively used in our daily lives. Such devices emit electromagnetic waves (EMW) which may cause disturbance, noise and malfunctions for other devices. Thus, it is crucial to use shielding materials that act as obstacles against occurrence of electro-magnetic interference (EMI) and suitable shielding for specific frequency ranges. Light weight, less thickness and good shielding effectiveness (SE) are prerequested for shielding materials over a wide range of frequencies [START_REF] Oh | Effect of various seed metals on uniformity of Ag layer formed by atmospheric plasma reduction on polyethylene terephthalate substrate: An application to electromagnetic interference shielding effectiveness[END_REF]. In this regard, research to develop EMI shielding material increased promptly during the last two decades [START_REF] Chung | Materials for electromagnetic interference shielding[END_REF]. Multilayer composites have been of great interest to use as adquate shielding materials against EMI at low frequencies owing to their new magnetic, optical and electronic property [3, 4]. Aluminum matrix composites (AMCs) have received an attractive attention thanks to their low density, high solidity, low rate of erosion, and good corrosion [START_REF] Mansouri | Producing multi-layer composite of stainless steel/aluminum/copper by accumulative roll bonding (ARB) process[END_REF].

Accumulative roll bonding (ARB) process as a severe plastic deformation SPD method has been regarded as an effective tool for the production of diverse multi-layered metal composites in the forms of sheets [START_REF] Ghalandari | Production of Al/Sn multilayer composite by accumulative roll bonding (ARB): A study of microstructure and mechanical properties[END_REF]. As respect to the other SPD techniques, ARB takes more advantagres process for its simple equipment and accessible to process the huge volume of piece [START_REF] Mansouri | Producing multi-layer composite of stainless steel/aluminum/copper by accumulative roll bonding (ARB) process[END_REF]. The principal purpose of ARB process is to impose a very large plastic strain within the material, resulting in structural refinement and increase its strength without changing sample dimensions. Therfore, numerous studies have been done in this field and several AMCs have been successfully fabricated through ARB process including Al/Sn To date, very few works have been done to estimate the SE for the Al/steel/Al sandwiches produced using ARB [START_REF] Verstraete | Magnetic shielding at low frequencies: application for an aluminum/steel composite elaborated by accumulative roll bonding[END_REF]. In this study, the thickness of the prepared sandwiches was 1mm. In the curent study, shielding efficiency is evaluated for a vey thin tri-layered Al/steel/Al with 0.2 mm in thickness fabricated using ARB process up to 4 cycles at room temperature. The SE values were taken at low frequencies between 20 Hz to 20 kHz; that are generally emitted by power supplies, switches and motors that enable cause distortions in the form of noise and disruption to the sensitive devices. The evolution of microstructure and related mechanical properties during ARB cycles of the produced sandwiches were also investigated by means of optical microscopy, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) techniques and tensile tests.

Experimental Section

Materials and processing

The raw materials used in this study were commercial Al-1050 (state H18) and low carbon steel DC04 sheets with 0.1 mm in thickness. The chemical composition of these sheets is given in Table 1. Fig. 1 shows the schematic diagram of ARB process. The multilayer composites were made in two steps: first, a primary Al/steel/Al sandwich structure was formed by bonding two Al sheets (0.1mm thick) with a steel sheet of 0.1mm thickness between them. The three strips were degreased with acetone and wire-brushed with a circular steel brush. In order to avoid sliding on each other, the sheets were fixed together by iron wires from the edges and then conducted through a 33% reduction in thickness (0.2 mm) at room temperature (Fig. 1a). In the second step, the Al/steel/Al sandwich structure was cut into two halves, fastened together and then roll-bonded with reduction in thickness of 50% (Fig. 1b). The process was carried out at room temperature and repeated up to four cycles without using any lubricant. An intermediate annealing for 10 min at 400°C was carried out at each cycle before cold-rolling. Finally, the sheet surfaces were degreased and wire-brushed before stacking among ARB cycles to achieve good bonding.

Characterization techniques

The microstructure of the composites during the ARB process was examined using a scanning electron microscope (ZEISS SUPRA 55 VP) operating at 20 kV with a TSL orientation imaging system, OIM TM software. The samples for EBSD analysis were first mechanically ground and then electrolytically polished with an A2 Struers solution. The tensile tests were conducted at room temperature using Zwick Roell Z010 with the initial strain rate of 6.7 × 10 -7 𝑠 -1 . The tests were collected along the rolling direction on the samples with the gauge dimensions as shown in Fig. 2. The magnetic shielding is the ability of a given material or composite to reduce the amount of emission of electromagnetic waves transmitted through it and generally given in dB [START_REF] Wanasinghe | A review on recent advancement of electromagnetic interference shielding novel metallic materials and processes[END_REF]. To measure the SE of Al/steel/Al multilayered composites at low frequencies (20 Hz -20 kHz), an adhoc device was elaborated represented in Fig. 3a with the principle of shielding measurement (Fig. 3b). A 100 turn copper wire coil of 90 mm diameter aligned by a low frequency generator of 400 mA and generate the magnetic field 𝑩 ⃗⃗ which is proportional to the applied voltage V. The samples with a parallelepiped shape of 30 mm side length and 1 mm thick were placed above the coil (10 mm) and centered on its revolution axis (Fig. 3a). By measuring the applied voltage above the sample (𝑽 𝒔 ) and the applied voltage of the same position without sample (𝑽 𝒂𝒊𝒓 ), the SE can be evaluated using the following formula:

𝑺𝑬 (𝒅𝒃) = 𝟐𝟎 𝒍𝒐𝒈( 𝑽 𝒂𝒊𝒓 𝑽 𝒔 ) (1)
The values obtained are proportional to the tensions measured by a Hall probe which was positioned above the sample (Fig. 3b).

Results

Effect of ARB process on microstructure

Fig. 4 illustrates the optical micrographs of composites in the RD-ND plane of the primary sandwich (Fig. 4a) and the samples processed through 1, 2, 3 and 4 cycles (Fig. 4b-e) correspond to total strains of 0.8, 1.6, 2.4 and 4, respectively. According to these images, the superposition of 3 layers is observed in the primary sandwich (Fig. 4a). After the first cycle, the layers keep their continuity and aligned in straight line along the rolling direction. In the composite processed through the second cycle, the steel layers forfeiture their straightness and undergo local necking. As the ARB cycle number increases up to 4 the steel is completely fractured and the fragments act as reinforcements in the Al matrix (Fig. 4d). As ARB cycle number increases, the Al alloy is removed from the outer layers (up and down), probably due to its low yield stress compared to the steel alloy. Fig. 5 shows the SEM micrographs of the interface of Al/steel/Al composites taken after 0, 2 and 4 ARB cycles. It is clearly observed in Fig. 5a that the Al and steel layers were well bonded after primary sandwich preparation. These observations are the same showing in optical microscopy (Fig. 4a). As the ARB cycle increases, the layer dislocates and became refine without showing any significant interface degradation. Moreover, both fragmentation and refinement of the layers continue with increasing the ARB cycle up to 4 and small parts of steel with the length of about 12 μm were observed in some points of the Al matrix (Fig. 5c). Similar observations were reported for multilayered Al/Ni composites processed up to 2 ARB cycles at room temperature [START_REF] Mozaffari | Evaluation of mechanical properties and structure of multilayered Al/Ni composites produced by accumulative roll bonding (ARB) process[END_REF]. Fig. 6 exhibits the EBSD micrographs of Al/steel/Al multilayered composite in 0, 2, and 4 cycles together with phase mapping. Without ARB processing, the microstructure is composed of large elongated grains in steel layer and a clear interface between Al and steel phases were observed with medium adhesion (Fig. 6a) due to the fact that the processing was conducted at room temperature and the apparition of porosities resulting from the defects generated by brushing. As illustrated in (Fig. 6b), the steel reinforcement has been formed in Al matrix up to two ARB cycles. Further, Al/Al interfaces are clearly visible on the phase mapping in some areas (Fig. 6c). Variations of the average grain size and high angle grain boundaries (HAGB) fraction, having disorientation greater than 15°, of Al and steel sheets as function of number of ARB cycles were measured and the plotted curve is illustrated in Fig. 7. A significant decrease in grain size of Al sheet from 20 μm (for the primary sandwich) to ~ 2μm (after 2 cycles) is observed. The size remains then constant up to 4 cycles (Fig. 7a). This may be due to the fact that between each cycle, the composite was annealed at 400°C for 10 min, allowing a recovery of the Al phase, preventing the grain size decrease by cold-rolling. For Al alloy, the grain size decreases between 0 and 2 cycles is accompanied with an increase of HAGB as expected (Fig. 7b). For the hard phase, the microstructure (main grain size and HAGB) is approximately the same during ARB processing since the steel mechanical strength is higher than the Al one. For both alloys, a very weak change in HAGB fraction is observed between 2 and 4 cycles, probably due to the fact that the an annealing is

Steel

Efect of ARB process on mechanical properties

The mechanical properties such as tensile strength (Rm), yield strength (Re) and elongation to failure of Al/steel/Al multilayered composites were evaluated before and after two ARB cycles and the engineering stress-strain curves are displayed in Fig. 8. The calculated tensile properties extracted from the stress-strain curves of samples are summarized in Fig. 9. It can be seen that the elongation to failure of the primary sandwich (zero cycle) is similar to the less ductile constituent material: the steel alloy. Although its low elongation to fracture (0.14%), the primary sandwich exhibits a good mechanical resistance (Re = 220 MPa and Rm = 250 MPa), greater than Al alloy (Re = 130 MPa and Rm = 154 MPa). After the second cycle of the ARB process, mechanical properties showed a decrease with respect to the primary sandwich. It is noteworthy that, the yield strength of the composite reached 100 MPa after the second cycle, which is quite the same as the starting Al (130 MPa) and almost 6 times lower than that of steel (560 MPa). This loss of strength between zero and 2 cycles may be due to steel fragmentation. Despite the low values of mechanical properties obtained when ARB is conducted up to 2 cycles, it is interesting to measure the associated magnetic performances considering the steel fragmentation could help the magnetic shielding [START_REF] Verstraete | Magnetic shielding at low frequencies: application for an aluminum/steel composite elaborated by accumulative roll bonding[END_REF].

Effect of ARB process on magnetic shielding

The effect of ARB cycle number on the magnetic shielding was investigated by estimating the shielding effectiveness of the Al/steel/Al sandwich for up to 4 ARB cycles. The experiments were repeated several times to ensure the reproducibility of data. The data obtained are plotted versus frequencies (20 Hz -20 KHz) in Fig. 10. The Al sheet starts to shield at 200 Hz and SE increases gradually with frequency due to its higher electrical conductivity. In fact, the steel sheet shields at low frequency (~1.98dB) due to its higher magnetic permeability. The magnetic shielding of the primary and ARB sandwiches is a compromise between the steel and Al properties. The sandwiches are able to shield for low frequencies (but less efficiently than steel itself) and their SE increases with increasing frequency, such as the Al alloy.

Discussion

According to the literature, the strengthening of the severely deformed composites, composed of hard and soft phases together) is not only affected by strain hardening (in the initial cycles) and grain refinement (in the subsequent cycles) but also by hard layers reinforcing the matrix [7, 16-19]. Nevertheless, several reports find that the necking and fracture of the hard layers inside the matrix have an undesirable effect on the composite strengthening [6, 14, 20]. The raison of necking and fracture in hard layer is due to the difference in mechanical properties of dissimilar metal composites which leads to plastic instabilities [START_REF] Reihanian | An analytical approach for necking and fracture of hard layer during accumulative roll bonding (ARB) of metallic multilayer[END_REF][START_REF] Quadir | Shear banding and recrystallization texture development in a multilayered Al alloy sheet produced by accumulative roll bonding[END_REF][START_REF] Hwang | Analysis of plastic instability during sandwich sheet rolling[END_REF][START_REF] Nowicke | The effect of roll and clad sheet geometry on the necking instability during rolling of clad sheet metals[END_REF]. In this research, after 2 ARB cycles necking and steel fragments are observed (Fig. 5b) with an abrupt decrease of tensile properties of the Al/steel/Al multilayered composite (Fig. 9). Besides, the ARB process has only a pronounce effect on the grain size of Al layers whereas the misorientation of both dissimilar layers showed a weak increase with respect to primary sandwich. As mentioned in the experimental procedure, an intermediate annealing for 10 min at 400°C is performed for ARBed composites, conducting to a significant decrease in dislocation density and residual stresses which are important for the subgrain nucleation surrounded by HAGBs (i.e recovery process). Our results are in align with previous literature where, the tensile properties of Al/steel composite were dramatically decreased after 2 ARB cycles [20]. This was attributed to the steel layers rupture inside the Al matrix. The mechanical strength after 2 cycles is similar to the Al one, since the steel is fragmented and is no longer useful to support the tensile strength. Besides, the loss of elongation to fracture compared to the Al sheet may be due to the imperfect interfaces created between Al layers. This lack of bounding can generate cracks that will propagate and lead to an early failure. The ARB process at higher temperature (300°C for example) could be benefit to the bounding and thus to the sandwich ductility. More, an annealing in a controlled Ar-H2 atmosphere between each cycle, instead of air, could prevent corrosion at the Al/steel interfaces in the ARB sandwiches and thus may improve the mechanical properties. It is worth mentioning that during ARB cycles the outer Al layers tend to disappear and be replaced by steel layers as it is clearly apparent in the SEM image (Fig. 5c) and more conspicuous in EBSD phase mapping image after 4 cycles (Fig. 6c). This is probably as a result of the increase in steel strength and the increase in ductility of the soft layers (Al) after annealing. In addition, the recovery process is slow in the steel layers because of a lower stacking fault energy of steel. Consequently, in this study, a sudden decrease of strength and elongation to failure of the Al/steel/Al multilayered composite after the second ARB cycle is attributed to the necking and rupture which takes place in the hard phase.

It has been reported that the composite thickness as well as the hard phase fragmentation in the matrix influenced the shielding effectiveness [14, 21]. The decrease in thickness led to the decrease in SE [14, 21] because the EMW absorption loss of the shielding material is proportional to its thickness [START_REF] Wanasinghe | A review on recent advancement of electromagnetic interference shielding novel metallic materials and processes[END_REF]. Recently, Clerico et al.

[21] showed that the steel fragmentation in Al matrix of Al/Steel/Al processed via cold roll bonding (CRB) led to the decrease in shielding effectiveness and a more negative impact of steel fragmentation is often observed at higher frequency (10 kHz). On the contrary, in the work of Verstraete et al. [14] after the third ARB cycle of Al/Steel/Al, fragments of the steel in the Al matrix led to an improve in shielding which may be probably due to the difference in elaboration processes. In the present study, it can be observed that SE is improved from 0 to 2 cycles as suggested by Verstraete et al. associated to steel fragmentation. As mentioned above, the microstructure of the ARB sandwiches is stable after 2 cycles. As a consequence, the magnetic properties remain constant between 2 and 4 cycles.

Conclusion

The tri-layered Al/steel/Al composites with 0.2 mm in thickness have been successfully fabricated by ARB up to 4 cycles at room temperature. An intermediate annealing between each cycle is performed at 400°C for 10 min in the air. The influence of the cycle number on the microstructural, mechanical, magnetic shielding properties of the prepared sandwiches was studied. The conclusions drawn from the results obtained can be summarized in the following points:

1. The microstructure of the Al/steel/Al composite showed a necking and fragmentation of steel in Al matrix during ARB process. Further, the outer of Al layers tends to disappear and be replaced by steel layers in the composite proceed for 4 cycles. 2. The grain size of Al layers gradually decreased from 20 μm in the primary sandwich to ~ 2 μm after 2 cycles and remains constant as ARB cycle increases up to 4 cycles. Steel grain size is not greatly impacted by the ARB process up to 4 cycles. 3. After 2 ARB cycles, the microstructure of the sandwiches remains stable. 4. The tensile properties for the sandwich reduced after 2 cycles due to the fragmentation of steel inside the Al matrix. 5. The optimal ARB cycle number is 2 for these thin sandwiches. Increasing the cycle number does not permit to improve nor the mechanical nor the magnetic properties. 6. The 2 cycles ARB processed sandwich is able to shield at low frequency and a gradually increase in SE with increased frequency is achieved. It combines the advantages of steel and aluminum magnetic abilities.

[ 6 ]

 6 , Al/Zn [7], Al/Ni [8], stainless steel/Al/Cu [9], Al/Ni/Cu [10], Al/Cu/Mn [11], Al/Mg [12], and Al/Sc [13]; acceptable results have been achieved.

Fig. 1 Fig. 2

 12 Fig. 1 Schematic diagram illustration of (a) primary sandwich preparation and (b) ARB processes

Fig. 3

 3 Fig. 3 (a) Equipment set-up and (b) principle of shielding measurement

Fig. 4

 4 Fig. 4 Optical migrographs of ARB-processed Al/steel/Al composites (thickness of 200 µm) after : (a) zero (primary sandwich), (b) one, (c) two, (d) three and (e) four cycles.

Fig. 5

 5 Fig. 5 SEM images of ARB-processed Al/steel/Al interfaces after: (a) zero, (b) two, and (c) four cycles.

Fig. 6

 6 Fig. 6 EBSD images of the RD-ND plane showing the microstructure of ARB-processed Al/steel/Al composites after :(a) zero, (b) two, and (c) four cycles. The images in the right present the phase mapping (green for Al and red for steel)

  performed at each cycle, which prevents the creation of a very fine microstructure with a large fraction of HAGB. The microstructure has reached a saturate state after 2 ARB cycles.

Fig. 7

 7 Fig. 7 (a) Average grain size and (b) HAGB fraction vs. ARB cycle number of Al and steel sheets

Fig. 8 Fig. 9

 89 Fig. 8 Engineering stress-strain curves of the starting materials and ARBed Al/steel/Al composites

Fig. 10

 10 Fig. 10 Change of SE with respect to the frequency for starting steel and Al/steel/Al composites processed at different number of ARB cycles

Table 1

 1 Chemical composition (wt. %) of the materials used in this study.

	Chemical composition (wt. %)


Simon Benacchio

Olivier Doutres

Arthur Varoquaux

Eric Wagnac

Arnaud Le Troter

Virginie Callot

Franck Sgard

Use of magnetic resonance image registration to estimate displacement in the human earcanal due to the insertion of in-ear devices

In-ear devices are used in a wide range of applications for which the device's usability and/or efficiency is strongly related to comfort aspects that are influenced by the mechanical interaction between the device and the walls of the earcanal. Although the displacement of the earcanal walls due to the insertion of the device is an important characteristic of this interaction, existing studies on this subject are very limited. This paper proposes a method to estimate this displacement in vivo using a registration technique on magnetic resonance images. The amplitude, the location and the direction of the earcanal wall displacement are computed for four types of earplugs used by one participant. These displacements give indications on how each earplug deforms the earcanal for one specific earcanal geometry and one specific earplug insertion. Although the displacement due to a specific earplug family cannot be generalized using the results of this paper, the latter help to understand where, how much, and how each studied earplug deforms the earcanal of the participant. This method is revealed as a promising tool to investigate further acoustical and physical comfort aspects of in-ear devices.

I. INTRODUCTION

In-ear devices (IEDs) such as hearing protectors, hear-ing aids, or earbuds are used in a wide range of applications. IEDs exert a mechanical static pressure (SMP) on the earca-nal walls that both acoustically seal the earcanal from exter-nal noise and ensure its stability inside the earcanal. This SMP is pointed out regularly as one of the most direct causes of physical discomfort due to IED insertion [START_REF] Wheeler | The industrial hygienist and ear protection[END_REF][START_REF] Brown-Rothwell | The comfort of earplugs: Basis for a descriptive model[END_REF][START_REF] Casali | Rating and ranking meth-ods for hearing protector wearability[END_REF][START_REF] Kochkin | MarkeTrak V: 'Why my hearing aids are in the drawer': The consumers' perspective[END_REF][START_REF] Gerges | Hearing protectors[END_REF][START_REF] Baker | Evaluating hearing pro-tection comfort through computer modeling[END_REF][START_REF] Terroir | Toward a 'global' definition of the comfort of earplugs[END_REF][START_REF] Doutres | A critical review of the literature on comfort of hear-ing protection devices: Definition of comfort and identification of its main attributes for earplug types[END_REF], which hinders a correct and consistent use of the device. It is necessary to assess the SMP in order to control it during the IED design phase: SMP should not be too low to not induce acoustic leaks and it should not be too high to not ignite physical dis-comfort. Unfortunately, SMP has never been assessed in vivo either by direct or indirect techniques according to the authors' knowledge. Direct methods would require mechanically transparent pressure sensors to be placed between the IED and the earcanal skin. Some indirect techni-ques based on numerical tools would require the knowledge of the displacement exerted by the IED on the earcanal walls, in addition to the properties of the tissues surrounding the earcanal. This work focuses on the estimation of the dis-placement exerted by IEDs on the earcanal, which could be used as an indirect technique for assessing SMP. The displacement of the earcanal walls corresponding to the difference of geometry between the open and occluded ear is difficult to measure quantitatively even though it has already been studied qualitatively [START_REF] Wheeler | The industrial hygienist and ear protection[END_REF]. Indeed, when the ear is occluded, the presence of the IED prevents the use of classic techniques usually employed to assess the geometry of the open earcanal [START_REF] Zemplenyi | Optical method for measurement of earcanal length[END_REF][START_REF] Rasetshwane | Inverse solution of ear-canal area function from reflectance[END_REF]. For example, optical scanners used to obtain the open earcanal shape cannot be used when the latter is occluded. The well-known ear impres-sion technique is also not suitable to evaluate the shape of the earcanal occluded by an IED. Although this technique is used to manufacture custom molded earplugs and hearing aids [START_REF] Pirzanski | Attenuation variables in earmolds for hearing protection devices[END_REF][START_REF] Pirzanski | If you blame buffing for bad fittings, you've probably given the wrong impression[END_REF] or to evalu-ate morphological earcanal differences in intra and inter-subject studies [START_REF] Oliveira | The active earcanal[END_REF][START_REF] Grenness | Mapping earcanal movement using area-based surface matching[END_REF][START_REF] Paulsen | Building and testing a statistical shape model of the human earcanal[END_REF][START_REF] Darkner | Analysis of deformation of the human ear and canal caused by mandibular movement[END_REF][START_REF] Delnavaz | Ear canal dynamic motion as a source of power for in-ear devices[END_REF], it is known to give only an approximate shape of the open earcanal. Indeed, this shape might be deformed by the silicone used to make the impression depending on its viscosity and by motions of the mouth when the user is speaking or chewing [START_REF] Pirzanski | If you blame buffing for bad fittings, you've probably given the wrong impression[END_REF][START_REF] Oliveira | The dynamic earcanal and its implications[END_REF]. One way to overcome these issues is to use medical imaging techniques [START_REF] Oliveira | A look at earcanal changes with jaw motion[END_REF][START_REF] Oliveira | The active earcanal[END_REF][START_REF] Inoue | Earplug fitting and the computed tomography[END_REF][START_REF] Darkner | In vivo study of the human earcanal using contrastenhanced MRI[END_REF] whose main advantages are the possibilities of obtaining (i) images of both an open and occluded earcanal without applying additional displacements on it and (ii) images of the tissues surrounding the earcanal such as soft tissues (i.e., skin, fat, muscles, etc.), cartilage, or bones, depending on the imaging technique modality. Several studies have already used medical imaging techniques to investigate the earcanal anthropometry [START_REF] Egolf | Quantifying ear-canal geometry with multiple computer-assisted tomo-graphic scans[END_REF][START_REF] Yu | Anthropometry of external auditory canal by non-contactable measurement[END_REF][START_REF] Darkner | In vivo study of the human earcanal using contrastenhanced MRI[END_REF] or to study the influence of the jaw motion on the canal shape [START_REF] Oliveira | The dynamic earcanal and its implications[END_REF]. However, none of them have investigated how IEDs affect the earcanal geometry. [START_REF] Benacchio | Estimation of the earcanal displacement field due to in-ear device insertion using a registration method on a human-like artificial ear[END_REF] recently proposed a method based on a common image processing tool to assess the earcanal displacement field from medical images of both open and occluded earcanals. The aforementioned tool, called the image registration method, had already been used in the past to compute the deformation of the earcanal due to the opening of the mouth [START_REF] Darkner | Analysis of deformation of the human ear and canal caused by mandibular movement[END_REF][START_REF] Darkner | Analysis of surfaces using constrained regression models[END_REF] and to obtain an average earcanal geometry extracted from 45 human earcanal magnetic resonance (MR) images [START_REF] Darkner | In vivo study of the human earcanal using contrastenhanced MRI[END_REF]. Contrary to previous studies [START_REF] Oliveira | A look at earcanal changes with jaw motion[END_REF][START_REF] Pirzanski | Earmolds and hearing aid shells: A tutorial Part 2: Impression-taking techniques that result in fewer remakes[END_REF] where the magnitude of the dis-placement was determined only at some specific anatomical landmarks or in planes corresponding to cross sections of the earcanal, the method proposed by [START_REF] Benacchio | Estimation of the earcanal displacement field due to in-ear device insertion using a registration method on a human-like artificial ear[END_REF] pro-vides the displacement vector field (both magnitude and direc-tion) of the entire earcanal wall. In this later work, it was shown that the method enables the estimation of the amplitude of the displacement and of its location with an accuracy of þ/0.2 mm and þ/0.4 mm, respectively, when the displace-ment is larger than þ/0.3 mm. This study also demonstrates that the inter-operator variability inherent to the method affects the results only slightly. However, the proposed method was validated on an artificial ear using micro-computed tomography images. Among imaging technique modalities, the magnetic resonance imaging (MRI) seems more interesting to study the earcanal displacement of a human subject since (i) it is a non-invasive method, (ii) it is able to give a good contrast between tissues (depending on the parameters of the sequence), and (iii) it now offers an acceptable image quality and resolution as compared to earlier studies [START_REF] Oliveira | A look at earcanal changes with jaw motion[END_REF][START_REF] Oliveira | The active earcanal[END_REF]. Although a pre-liminary investigation on MRI-like images has given satisfac-tory results [START_REF] Benacchio | Estimation of the earcanal displacement field due to in-ear device insertion using a registration method on a human-like artificial ear[END_REF], the proposed method must still be investigated in the case of MR images of the human earcanal collected in vivo. This paper aims to investigate the possibilities offered by the method previously proposed by the authors [START_REF] Benacchio | Estimation of the earcanal displacement field due to in-ear device insertion using a registration method on a human-like artificial ear[END_REF] for computing the displacement vector field (called displacement in this paper for the sake of simplicity) applied to the earcanal of a human subject. An adapted MRI sequence was performed to obtain the images of both the open and occluded ears of the participant wearing different types of IEDs (custom molded, roll-down foam, push-to-fit, and premolded earplugs). Results showing the displacement computed using rigid and deformable registra-tions are presented for each occlusion case. The paper is organized as follows. In Sec. II, informa-tion about the participant and about the four earplug types used in this study are given. Then, details about the imaging and registration steps are presented. Graphical representa-tions are proposed to facilitate the interpretation of the results. In Sec. III, the displacements obtained for the four occlusion cases are presented. Similarities and differences observed between the left and the right ear of the participant are described. These results are discussed and analyzed in Sec. IV and the abilities offered by the proposed method and its limits are finally pointed out.

II. MATERIALS AND METHODS

A. Participant Since the aim of this work was to demonstrate that the proposed method can be applied to images collected in vivo, a single participant was considered in this study. The partici-pant is a healthy male volunteer, 29 years old. He was eligi-ble according to the criteria of the ethics protocol number H20161101 (ethic comity of the Ecole de technologie superieure). The inclusion criteria defined in this protocol included an otoscopic examination to ensure that the partici-pant's earcanals were not damaged or occluded by cerumen.

B. Earplugs

In order to compute the displacement applied by differ-ent kinds of in-ear devices, four types of earplugs selected from four earplugs families [custom molded, roll-down foam, push-to-fit, and premolded; see Fig. 1(a)] were consid-ered to occlude the participant's left [see Fig. 1(b)] and right [see Fig. 1(c)] ear. Note that pictures of the right ear have been horizontally flipped to have the same orientation as the left ear. Although different kinds of earplugs are tested here, it should be recalled that this study is not intended to give general results on the displacement due to a specific earplug family or another but to investigate the abilities of the pro-posed method used in vivo. The custom molded earplugs tested in this study are depicted in Fig. 1(a.1). They were molded from ear impres-sions of the participant using biocompatible silicone. It should be noted that the earplug manufacturer was asked to manufacture "large" and "long" custom molded earplugs to ensure acoustic sealing and hold in place, thus leaving aside aspects of physical comfort. The resulting earplugs are not necessarily representative of custom molded earplugs. A rough approximation of the length of the custom earplugs is 30mm and their insertion is considered as standard since their shape fits the geometry of the ears. Roll-down foam earplugs are made of soft closed-cell foam and have a cylindrical initial shape that does not corre-spond to the earcanal geometry before being inserted. They must be rolled between fingers and compressed into a tight cylinder before insertion. The slow-recovery foam then expands in the earcanal. The roll-down foam earplugs tested in this study are the foam E-A-R Classic earplugs from 3 M (Maplewood, MN) and are shown in Fig. 1(a.2). The length of these earplugs is approximately 19 mm and their insertion depth is considered as standard [see Figs. 1(b.2) and 1(c.2)] according to [START_REF] Berger | Calibrating' the insertion depth of roll-down foam ear-plugs[END_REF], which means that approximately 50%-60% of the earplug is in contact with the earcanal walls. Push-to-fit earplugs are made of a plastic rigid stem terminated with a spherical foam (polyurethane) tip. This tip is deformed when inserted in the earcanal and is used to create the acoustic seal between the eardrum and the out-side of the ear. The push-to-fit earplugs tested in this study are the E-A-R Push-Ins earplugs from 3 M and are shown in Fig. 1(a.3). The length and diameter of the foam tip are approximately 9 and 12mm, respectively. The insertion depth is considered deep since the entire tip of the earplug is inside the earcanal and is not visible in Figs. 1(b.3) and 1(c.3). The premolded earplugs are made of silicone (elastomer polymer) with a 30-mm long cylinder and three flanges at one end, whose diameter progressively increases in size from the one situated at the tip to the one more distal from the tip. Those tested in this study are the E-A-R UltraFit ear-plugs from 3 M and are shown in Fig. 1(a.4). The length of the earplug section occupied by the three flanges and used to produce the acoustic seal in the earcanal is approximately 16 mm. The smallest flange has a diameter of 8 mm and the largest one has a diameter of 13 mm. The insertion depth has been chosen so that the first and second flanges are completely inside the earcanal [see Figs. 1(b.4) and 1(c.4)]. These earplugs were inserted by the participant and were visually controlled by an otolaryngologist before start-ing the measurements. It should be mentioned that earplugs can be inserted in several ways. In the present study, only one insertion for each earplug is investigated and may be dif-ferent for the left and right ear. Thus, the results presented in Sec. III are inherently dependent on the way the earplugs were inserted despite the visual control of the otolaryngologist.

C. Imaging 1. MR acquisition Acquisitions of the whole head of the participant were per-formed on a Siemens 3 Tesla (Verio) MR system using a 32-channel head radio-frequency coil. A Space three-dimensional (3 D) T1-weighted sequence (Sampling Perfection with Application optimized Contrasts using different flip angle Evolution) has been adapted to obtain a contrast and a resolu-tion allowing to observe the deformation of earcanals due to the insertion of earplugs. Details of this sequence are given in Table I and resulting images are presented in Fig. 2. The right half of the axial view of the participant's head is shown in Fig. 2 Unfortunately, the axial cutting plane chosen to reveal the end of the earcanal does not make it possible to observe simultaneously this zone and an entire section of the earplug because of the curved shape of the earcanal. Axial slices using another cutting plane and showing the subject's earcanal occluded by other earplugs are presented in the Appendix.

Noise measurement in the MR environment

Since the non occluded ear images are used as reference in the methodology for estimating earcanal displacement induced by the insertion of earplugs, it was also necessary to image the participant's open ears. However, MR systems are known to be noisy environments and it is often necessary to wear hearing protectors during MR imaging [START_REF] Salvi | Is noise in the MR imager a significant risk factor for hearing loss?[END_REF]. To ensure that the noise level inside the MR system was not too high for the participant during open ear measurement, the sequence was used in a "quiet" opera-tion mode (increased pulse timing and reduced gradient amplitudes and slew rates as compared to normal mode to minimize acoustic noise). Noise level measurements inside the MR system were performed using a remote acquisition system made up of a 1 =4 in. Br€uel & Kjaer microphone type 4961 inserted in a flexible plastic tube (length and diameter, respectively, equal to 3 and 0.01 meters) whose free end was positioned at the center of the MR coil. This remote system was used to avoid any effect of the high magnetic field on the ferromagnetic parts of the microphone. The measured noise levels were corrected using the transfer function of the flexible tube. These corrected measurements give the actual noise levels that would have been measured directly by a microphone at the center of the MR coil. The noise level inside the coil was 90.9 dBA for the "quiet" sequence and 96.1 dBA for the sequence used in "normal" mode. According to the European regulation (equivalent exposition of 85 dBA for an 8 h work day), the time allowed for open ear measurements was 2 h and 2 min for the "quiet" sequence and 36min for the sequence used in "normal" mode. The time allowed for the "quiet" sequence was therefore long enough to image the open ears of the participant without any risk to his health. Note that the "quiet" sequence was also used to image the occluded ears to ensure consistency of measurements.

Measurement procedure

The MR acquisitions of the participant's right and left earcanals were performed at the same time. First, the partici-pant's non occluded ears were imaged, then the ears were successively imaged when occluded with custom molded, roll-down foam, push-to-fit and premolded earplugs. The participant was asked not to move during each acquisition.

D. Registration method

A registration method was used to estimate the defor-mation of the earcanal induced by the earplug. This method, previously validated for the estimation of an artificial earca-nal deformation caused by the insertion of in-ear devices [START_REF] Benacchio | Estimation of the earcanal displacement field due to in-ear device insertion using a registration method on a human-like artificial ear[END_REF], consists in applying rigid and deformable registrations on medical images using the ANTs library [START_REF] Avants | A reproducible evaluation of ANTs similarity metric perfor-mance in brain image registration[END_REF] details of which can be found in the documentation. 1 The main steps of the approach are as follows: 1. Image denoising A non-local mean filter is applied to the images using the DenoiseImage function from ANTs. This step improves the discrimination of the tissues surrounding the earcanal (bone and soft tissue). 2. Over-sampling Since a higher image resolution allows a smoother seg-mentation of the earcanal [see Sec. II D 6)], the images col-lected here are then over sampled. The resolution of images is empirically multiplied by ten to obtain a voxel size equal to 0.06 mm isotropic. 3. Rigid registration Rigid registration is used to orient and align images with and without earplugs in the same way. Image of the whole head with open earcanal is used as the target image for the rigid registration. Images of the whole head with occluded earcanals are used as source images. Parameters of the rigid registration step are given in Table II.

Extraction of the ROI

In order to focus the computation efforts of the deform-able registration on the earcanal displacement estimation, a parallelepipedic area surrounding the earcanal (41.446.2 54.2mm, CoronalAxialSagittal) is chosen as region of interest for each ear (see Fig. 2 for example).

Re-orientation of the left ear

Steps (1) to ( 4) are performed for images of the entire head of the participant. In order to be able to compare the results from the right and left ears, the two earcanals must have the same orientation. Thus, the extracted ROI of the left ear is reoriented to match that of the right earcanal. First, the images are flipped in the medial/lateral direction (see Fig. 3) and then a rigid registration is applied using the open left earcanal as the source image and the open right earcanal as the target image. Finally, the resulting transformation is applied to the images of the occluded left earcanal. The segmentations include the air inside the earcanal for both open and occluded ear images, in addition to the earplug for occluded ear images. The precision and accuracy of this segmentation step has already been tested in [START_REF] Benacchio | Estimation of the earcanal displacement field due to in-ear device insertion using a registration method on a human-like artificial ear[END_REF] and has been shown to be precise and accurate enough to enable the estimation of the displace-ment of the earcanal walls.

Deformable registration

A deformable registration is then applied to the seg-mented images of the occluded ear to estimate the displace-ment of the earcanal due to the insertion of the earplug. For the right and the left ears, the image of the open ear is used as the source image and the occluded ear is used as the target image. The parameters of the deformable registration step are given in Table III.

E. Accuracy of the computed displacement

The displacement of the earcanal walls is finally calcu-lated using the transformation computed during the deform-able registration step. The accuracy of the estimated displacement is limited by the quality of the images (spatial resolution, contrast-to-noise ratio). Under a certain thresh-old, referred to as the minimum accuracy level in this paper, the computed estimation of the displacement is no more reli-able. This minimum accuracy level was estimated by apply-ing the registration method to a region considered as non deformed and was evaluated to 0.5 mm for the registration parameters that were utilized and the actual image quality.

F. Graphical representations Finally, two complementary graphical representations of the earcanal deformation are proposed. For the first one, only the magnitude of the displacement is displayed. The second representation displays the components of the dis-placement in the directions presented in Fig. 3. From there, it is therefore possible to determine in which direction the maximum displacement occurs, which is not possible by using only the magnitude of the displacement. For these components and for both ears, displacements along the superior, anterior, and lateral directions, called upward, forward, and outward, respectively, are considered positive and represented in red in Fig. 3. Displacements along the inferior, posterior and medial directions, referred to as downward, backward and inward respectively, are con-sidered negative and represented in blue in Fig. 3. The mag-nitude together with the component values of the displacement are plotted using a polar representation similar to the one introduced in [START_REF] Benacchio | Estimation of the earcanal displacement field due to in-ear device insertion using a registration method on a human-like artificial ear[END_REF], which gives not only the amplitude of the displacement but also its com-plete cartography inside the earcanal. This representation is explained in Fig. 4(b). In Fig. 4, the earcanal is separated in several parts dis-played alternatively in white and grey and roughly corre-sponding to (1) the concha, (2) the first and (3) the second bends, (4) the cartilage/bone junction, and (5) the bony part of the earcanal. Vertical lines in Fig. 4(a) correspond to cir-cular lines in Fig. 4(b) and constitute important landmarks along the earcanal. In Fig. 4(a), transparent red rectangles correspond to areas of the earcanal non-displayed in the polar representation of Fig. 4(b). This latter representation is used to display the earcanal displacement magnitude [see Fig. 5(a.1) for example] and its component values [see Figs. 5(a.2), 5(a.3), and 5(a.4) for example] from 10 mm from the eardrum [second solid vertical line (from left to right) in Fig. 4(a)] to the entrance of the concha located at approximately 35 mm from the eardrum [last solid vertical line in Fig. 4(a)]. The concha entrance corresponds to the largest circle of the polar plot and the plane located at 10 mm from the earcanal tip is the center of the polar plot in Fig. 4(b). It is worth men-tioning that each colored circle of the polar representation [see Fig. 5(a.1) for example] corresponds to the cross-section between the displacement projected on the earcanal walls and sagittal planes. Compared to the previous study [START_REF] Benacchio | Estimation of the earcanal displacement field due to in-ear device insertion using a registration method on a human-like artificial ear[END_REF] the concha is included in the polar representation in addition to the earcanal. Thus, it is also possible to estimate the displacement due to the earplugs in the concha prior to the actual entrance of the earcanal. In order to help readers to evaluate the amplitude of the maximum displacement magnitude of the earcanal walls, line plots are also displayed. These plots show both the val-ues of the maximum displacement magnitude [see Fig. 5(b.1) for example] and the "maximum" values (both posi-tive and negative) of each component [see Fig. 5(b.2) for example] along the earcanal. These maximum displacement curves are plotted for the sections of the earcanal comprised between the two red rectangles using the same delimitations as those presented in Fig. 4(a).

III. RESULTS

In the first part of this section, similar displacement trends computed for left and right ear are described using only the left ear results for illustration. In the second part, the differences observed between the two ears are reported and analyzed using the results of both the left and right ears. A. Displacement in the left ear Results are presented using the two graphical represen-tations described in Sec. II F. Figures 5,6,7, and 8 show the magnitude of the displacement of the left ear together with the value of its components in each anatomical direction (see Fig. 3) for each considered earplug. 1. Custom molded earplugs According to Fig. 5(a), the custom molded earplug applies deformations to a long portion of the earcanal. It also pulls the earcanal walls outward overall since the displace-ment in the medial/lateral direction is positive almost every-where on the polar plot of Fig. 5(a.2). From the entrance of the concha to the entrance of the earcanal, the magnitude of the maximum displacement is approximately 2.5mm [see Fig. 5(b.1)]. Two parts are mainly deformed in this region [see Fig. 5(a.1)] that is (i) the anterior part of the concha corresponding to the tragus [see Fig. 1(b.1)], which is mainly deformed forward [see Fig. 5(a.3)] and (ii) a thin region just before the entrance of the earcanal covering its superior/posterior part for which the displacement has an outward [see Fig. 5(a.2)] and downward [see Fig. 5 From the second bend to the plane located at 10 mm from the tip of the eardrum, the earcanal walls are less deformed. From the beginning of the second bend, the displacement magnitude drastically decreases until the entrance of the carti-lage/bone junction [see Fig. 5(b.1)] to reach the minimum accu-racy level described in Sec. II E. Thus, the custom earplug does not seem to induce deformation of the earcanal in the inner tip region.

Push-to-fit earplugs

The push-to-fit earplug mainly deforms the earcanal walls in one restricted location [see Figs. 7(a.1) and 7(b.1)]. This local deformation can be explained by the shape of the earplug itself, which is a thin rigid plastic stem ended by a spherical foam head. From the entrance of the concha to the entrance of the earcanal, the plastic stem of the earplug can interact with the concha but deforms only slightly this region in the case of the left ear [see Fig.

7(a.1)].

Between the two bends, the regions of maximum dis-placement magnitude are located just before the second bend in the superior/posterior and inferior/anterior part of the earcanal [see Fig. 7(a.1)]. The main directions of these displacements are backward/upward and forward/downward, respectively [see Figs. 7(a.3) and 7(a.4)] and the maximum displacement magnitude in these regions is equal to 1.8mm [see Fig. 7(b.1)]. It can also be noticed that the superior/posterior part of the earcanal is pulled outward and the inferior/anterior one is pushed inwards [see Fig. 7(a.2)]. From the second bend to the plane located at 10mm from the tip of the eardrum, the push-to-fit earplug does not deform the earcanal walls in its current insertion depth since the maximum displacement modulus is close to the mini-mum accuracy level defined in Sec. II E [see Fig. 7(b.1)].

Premolded earplugs

The premolded earplug locally deforms the earcanal in several locations due to its outer tip and its three flanges. From the entrance of the concha to the entrance of the earcanal, the largest flange stays outside of the earcanal [see Fig. 1(b.4)] and does not deform the base of the concha [see Fig. 8(a (a.3) and 8(a.4)]. The maximum displacement of this second region is smaller than the displacement of the superior/posterior region [see Fig. 8(a.1)] and its amplitude is about 1.6 mm. From the second bend to the plane located at 10 mm from the tip of the eardrum, the third flange of the premolded earplug deforms the superior part of the entrance of the cartilage/bone junction [see Figs. 8(a.1) and 8(b.1)]. In this region, the maximum displacement magnitude is about 1 mm. The direction of this displacement is difficult to evaluate since its amplitude is close to the minimum accuracy level defined in Sec. II E. B. Differences observed on the right ear As mentioned previously, similar trends are observed between the displacement computed for the left and right ear. However, some differences occur between the two occluded ears in some specific regions. These differences are presented in this section and discussed in Sec. IV using the magnitude of the displacement along the right earcanal and the amplitude of its components in each anatomical direc-tion, presented in Figs. 9, 10, 11, and 12. 1. Custom molded earplug For the custom molded earplug, only small differences can be observed between the left (see Fig. 5) and right ear (see Fig. 9). The main difference is that the maximum dis-placement magnitude is smaller for the right ear than for the left ear. inferior/superior (downward/upward) components of the displacement.

Roll-down foam earplug

Unlike the left ear, the tragus of the right ear is slightly deformed whereas the posterior part of the base of the con-cha is largely deformed [see Fig. 10(a.1)]. The correspond-ing displacement occurs inward [see Fig. 10(a.2)] and backward [see Fig. 10(a.3)]. Another difference compared to the left ear is that the roll-down foam earplug globally pushes the right earcanal walls inward since almost all the earcanal displacement is negative in Fig. 10(a.2). Unlike the left ear, the roll-down foam earplug does not seem to deform the cartilage/bone junction of the right earcanal since the magnitude of the maximum displacement in this region [see Fig. 10(b.1)] is close to the minimum accuracy level described in Sec. II E.

Push-to-fit earplug

The main difference between the displacement com-puted for both ears occluded by the push-to-fit earplug is that the tip of the rigid plastic stem of the right earplug deforms the superior/posterior part of the base of the concha [see Fig. 11(a.1)] outward [see Fig. 11(a.2)] unlike the left ear. The stem of the earplug also deforms the posterior part of the ear-canal entrance [see Fig. 11( (b.4) and 1(c.4) that the insertion orienta-tion of these two earplugs is different. The magnitude of the maximum displacement due to the outer tip of the premolded earplug inserted in the right ear is equal to 1.2 mm [see Fig. 12(b.1)]. After the second bend, the right ear displacement is also different from the left one since not only the superior part of the earcanal is deformed but also its inferior/posterior part [see Fig. 12(a.1)].

IV. DISCUSSION

The results presented in Sec. III are discussed here. First, the influence of the earcanal geometry and earplug shape and insertion on the computed displacement is pointed out. Then, the trends observed for both the left and right ears are discussed. Finally, overall remarks on the method are given. A. Effect of the earcanal geometry It should be noted that the geometry of the participant's ears has important effects on the results obtained for the computed displacement. In this study, the magnitude of the displacement is globally smaller for the right ear than for the left ear of the participant (see Figs. 5,6,7,8 and 9,10,11,12). This is explained by the fact that the left earcanal has an overall diameter smaller than the right one (data not shown). Thus, small differences between the left and right ear might be explained by the geometry of each ear.

However, the difference between the two earcanals of the participant is also an advantage for the validation of the proposed method. Since the two earcanals of the participant are different and four different earplugs were investigated in this study, an equivalent number of eight configurations were used to assess the displacement of the earcanal result-ing in the observation of various kinds of displacements with various amplitudes and in various locations of the earcanal. This number can be considered as large enough to have con-fidence in the proposed method to estimate the earcanal dis-placement in most of the human earcanal. B. Effect of the earplug shape and insertion The shape and the insertion of earplugs are also two important parameters which can greatly affect the displace-ment of the earcanal walls. This can be illustrated consider-ing the left and right ear of the participant occluded by the custom molded earplugs. As mentioned in Sec. III B 1, small differences are observed in the results between the left (see Fig. 5) and right ear (see Fig. 9) in this occlusion case. These differences can be explained by the dissimilarity between geometries of the left and right earcanal, but also by the fact that the earplug shapes were manually carved and smoothed out by the manufacturer during the manufacturing finishing stage. Thus, the final shapes of the custom earplugs may differ from the geometry of the earca-nals and might deform them in a different manner for the left and right ears. The insertion of the custom molded ear-plugs is determined by their shapes. Since the length of the left and right custom molded earplugs is similar, the part of the earcanal which is deformed is almost the same for the left and right ear. The initial shape of the other earplugs (roll-down foam, push-to-fit, and premolded) does not change between left and right ear. However, their insertion may be different. For example, Figs. 1(b.2) and 1(c.2) show that the right roll-down foam earplug was not inserted exactly in the same way as the left earplug. Thus, the displacement of the earca-nal walls is not the same for each ear [see Figs. 6(b.1) and 10(b.1)]. Clear-sighted readers would have noticed that neither hearing aids nor earbuds are tested in this study. However, the shapes of these in-ear device types can be compared to the shapes of the earplugs tested. For example, hearing aids and custom molded earplugs have similar shapes since their manufacturing process both uses ear impressions. Some ear-buds have similar shapes as the tested premolded earplugs since flanges are commonly used on earbuds. Finally, the relevance of measuring the displacement of the earcanal due to the insertion of earplugs for a single par-ticipant is discussed here. It is clear that the results presented in this paper cannot be generalized or used to conclude on the earcanal displacement due to the insertion of a given earplug in the human earcanal. If some trends can be observed here for the different types of earplugs, the estimated dis-placement is always closely related to the geometry of the user's earcanal, to the characteristics of the tissues surround-ing it and to the shape and insertion of earplugs. Thus, even for a specific earplug, the displacement can be different from one subject to the other since all human earcanals are differ-ent. In order to identify effective trends about the displace-ment due to in-ear devices, future works should investigate this method with a broader range of participants. C. Noticeable deformed regions of the earcanal From the entrance of the concha to the entrance of the earcanal, the anterior region of the outer part, corresponding to the tragus, is often deformed by earplugs with an outward/ forward displacement. This displacement is due to the outer tip of earplugs, which is either large for custom molded and roll-down foam earplugs or oriented in the tragus direction for other earplugs. The posterior part of the junction between the base of the concha and the entrance of the earcanal is also frequently deformed. But the corresponding displace-ment seems to be more dependent on how earplugs are inserted. Between the two bends, and for roll-down foam and push-to-fit earplugs, the regions of maximum displace-ments occur in the superior/posterior and inferior/anterior parts of the earcanal and the displacement of these two regions is respectively upward/backward and downward/ forward. These preferred directions may be explained by the fact that, between the two bends, the cross section of the participant's earcanal is not circular but ellipsoidal with its minor axis in the superior/posterior-inferior/anterior direction (see Fig. 13). Thus, the roll-down foam earplugs and the foam part of push-to-fit earplugs can easily expand in the direction of the major axis of the ellipsoidal cross section of the earcanal leading to a larger deformation in the direction of the minor axis of the ellipse.

D. Representation of the displacement

The relevance of displaying the maximum amplitudes (both positive and negative) of the displacement components along the earcanal can be difficult to grasp. However, these representations offer useful information to better understand how earplugs deform the earcanal. For example, maximum amplitudes obtained simultaneously for opposite directions [see dasheddiamond lines in Fig. 7(b.2) for example] indi-cate that the earplug pushes on each side of the earcanal walls as if it was expanding in the earcanal [see Fig. 14(a)]. When the displacement amplitude is maximum in one direc-tion and equal to zero in the opposite direction [see dasheddiamond lines in Fig. 5(b.2) for example], the earplug deforms the earcanal in only one direction in the corresponding cross section [see Fig. 14(b)]. These two examples are illustrated in Fig. 14 using three-dimensional projections of the displacement in the posterior/anterior direction on the walls of the left earcanal flipped in the medial/lateral direction. Figure 14 shows that if the displacement induced by the push-to-fit earplug seems intuitive [see Fig. 14(a)], the dis-placement due to the custom molded earplug is not [see Fig. 14(b)]. Indeed, as the custom molded earplug is considered as rigid, its motion in the axial plane is a rotation around an axis located in the middle of the first bend of the earcanal leading to an inward and backward displacement of its outer and inner tips, respectively. This information could be useful to earplug manufacturers to improve the fitting characteris-tics of earplugs. Another example of unexpected direction of the displacement is given by Figs. 5(a.2) and 5(a.4), where the superior/posterior part of the base of the concha moves outward/downward. This displacement might be due to a complex interaction between the concha and the earplug and cannot be observed using only the maximum amplitude of the displacement presented in Fig. 5(a.1). In order to improve the relevance of the proposed repre-sentations (polar plots and maximum displacement along the earcanal), the displacement of the earcanal walls due to the insertion of earplug could be plotted along the center axis of the earcanal and not along a straight line normal to sagittal planes [START_REF] Stinson | Specification of the geometry of the human earcanal for the prediction of sound-pressure level distribution[END_REF]. Using the center axis of the earcanal would allow a better localization of the first and second bends of the earcanal using, for example, the curva-ture or the torsion of the canal [START_REF] Delnavaz | Ear canal dynamic motion as a source of power for in-ear devices[END_REF]. V. CONCLUSION This paper proposes to investigate the possibilities offered by the method described in [START_REF] Benacchio | Estimation of the earcanal displacement field due to in-ear device insertion using a registration method on a human-like artificial ear[END_REF] to estimate the earcanal displacement due to the insertion of in-ear devices using magnetic resonance images taken in vivo on a human subject. First, the earcanal open and occluded by an in-ear device is imaged using an adapted MRI sequence. Then, the method relies on rigid and deform-able registrations of the MR images obtained to compute the displacement of the earcanal walls. In this study, the dis-placement induced by four different types of in-ear devices (custom molded, roll-down foam, push-to-fit, and premolded earplugs) were investigated for each ear of a participant. Results were displayed using polar plots of the displacement magnitude and of its components in each direction or line plots of their maximum value along the earcanal (perpendicularly to sagittal planes). Thanks to these graphi-cal representations, it was possible to observe the location and the amplitude of maximum displacement along the ear-canal. Polar representations of the displacement components provided the direction in which these regions move and allow for identifying how earplugs deform the earcanal (expansion, rigid motion in one direction). In spite of small differences due to the geometry of the left and right earca-nals and the shape and insertion of the earplugs, the results show a reasonable similarity between the displacements computed for the participant's left and right ears when they are occluded by the same earplug type. Thus, the precision of the proposed method allows for investigating quantitatively the effects of geometrical or physical features of the earcanal or of in-ear devices on comfort aspects whereas these features were only qualitatively studied in the past. However, the results cannot be generalized since they were obtained from only a single participant. But the diver-sity of results obtained for the eight different configurations (two earcanals and four different earplugs) investigated in this study suggests that the proposed method is relevant to estimate the displacement in most human earcanals. Future stud-ies must investigate displacements due to other in-ear devices and be computed for a large panel of participants to obtain trends on how a specific in-ear device family deforms the ear-canal. These results could be used by hearing protector or hearing aid manufacturers to improve the comfort aspects of their devices. Displacements could also be used in numerical simulations to understand how the deformation of the earcanal affect acoustical comfort aspects of hearing protectors and hearing aids such as sound attenuation or occlusion effect.

This figure shows an axial view of the earcanal of the sub-ject with an orientation different from that presented in Fig. 2. Figure 15 It can be seen that the custom molded earplug does not make a full contact with all portions of the earcanal wall. This does not mean that this earplug does not create an effi-cient acoustic seal since it might be ensured at other loca-tions of the earcanal. Moreover, it is recalled here that the custom molded earplug manufacturer has been asked to manufacture a "large" and "long" earplug. The underlying shape modifications may have created some unexpected large dimensions of the earplug leading to unusual displace-ment of the earplug in the earcanal and maybe some abnor-mal loss of contact between the earplug and the earcanal wall. Since they are made with foam and/or plastic, the roll-down foam and the push-to-fit earplugs are not visible on the MR images [see Figs. 15(c) and 15(d)]. However, the dis-placement resulting from their insertion is still clearly observable. As the premolded earplug is made with silicone, it is observable in Fig. 15(e), but not very clearly due to its complex shape compressed in the subject's earcanal.

  FIG. 1. (Color online) Pictures of (a) earplugs and (b) left and (c) right ears occluded by the (1) custom molded, (2) roll-down foam, (3) push-to-fit, and (4) premolded earplugs. Pictures of the right ear are horizontally flipped to have the same orientation as the left ear.

  (a). In this figure, the ears of the participant are occluded with the silicone custom molded earplugs. The red rectangle shows the region of interest (ROI) used later to perform the deformable registration and presented in Fig.2(b). The corresponding ROI for the open ear is given in Fig. 2(c). Numbers on Figs. 2(b) and 2(c) correspond respectively to (1) the air, (2) the soft tissues, (3) the temporal bone, (4) the earplug, and (5) the skin inside the earcanal. It is worth mentioning that Fig. 2(b) shows two parts of the custom molded earplug separated by soft tissues [see number 4 in Fig. 2(b)].

FIG. 2

 2 FIG. 2. (Color online) (a) Right half part of the axial view of the partici-pant's head whose ears were occluded by the silicone custom earplugs. The red rectangle gives the extracted ROI localized around the earcanal and rep-resented in (b). The corresponding ROI for the open ear of the participant is given in (c). Numbers give details of the MR image and correspond respectively to (1) the air outside and inside the earcanal, (2) the soft tissues (fat, muscles, cartilage), (3) the temporal bone, (4) the silicone custom earplug, and (5) the skin inside the earcanal.

  FIG. 3. (Color online) Anatomical directions. The red and blue arrows indi-cate, respectively, the arbitrary definition of the positive and negative displacements.

FIG. 4

 4 FIG. 4. (Color online) (a) Coronal view of the right earcanal sectioned to delimit (1) the concha, (2) the first and (3) second bends, (4) the cartilage/bone junc-tion, and (5) the tip of the earcanal. Vertical lines are used to localize the different parts of the earcanal. Transparent red rectangles in (a) correspond to non-displayed areas in (b). (b) Polar representation used to plot the component values and the displacement magnitude of the earcanal walls. Numbers and circular lines correspond to numbers and vertical lines in (a).

  FIG. 5. (Color online) Displacement of the left earcanal occluded by the silicone custom molded earplug. (a) Polar plots and (b) maximum values for (a.1) and (b.1) the magnitude, (a.2) and (b.2) (solid-circle lines) medial/lateral (inward/outward), (a.3) and (b.2) (dashed-diamond lines) posterior/anterior (backward/ forward) and (a.4) and (b.2) (dotted/dashed-cross lines) inferior/superior (downward/upward) components of the displacement.

  (a.4)] component. Between the two bends, the posterior part of the earcanal is deformed [see Fig. 5(a.1)] mainly backward [see Fig. 5(a.3)]. The magnitude of the maximum displacement is approximately 2.2 mm [see Fig. 5(b.1)].

FIG. 6

 6 FIG. 6. (Color online) Displacement of the left earcanal occluded by the roll-down foam earplug. (a) Polar plots and (b) maximum values for (a.1) and (b.1) the magnitude, (a.2) and (b.2) (solid-circle lines) medial/lateral (inward/outward), (a.3) and (b.2) (dashed-diamond lines) posterior/anterior (backward/for-ward) and (a.4) and (b.2) (dotted/dashed-cross lines) inferior/superior (downward/upward) components of the displacement.

  .1)]. However, the tragus is deformed by the outer tip of the earplug stem [see Fig. 8(a.1)] and the maximum displace-ment magnitude for this region is 2.1 mm [see Fig. 8(b.1)]. Between the two bends, the second flange of the pre-molded earplug mainly deforms two regions: (i) the superior/ posterior part of the middle of the first bend [see Fig. 8(a.1)], which is deformed backward/upward [see Figs. 8(a.3) and 8(a.4)]. The amplitude of this maximum displacement is about 2 mm [see Fig. 8(b.1)]. (ii) The superior/anterior part of the middle of the first bend is deformed backward/downward [see Figs. 8

FIG. 9

 9 FIG. 9. (Color online) Displacement of the right earcanal occluded by the custom molded earplug. (a) Polar plots and (b) maximum values for (a.1) and (b.1) the magnitude, (a.2) and (b.2) (solidcircle lines) medial/lateral (inward/outward), (a.3) and (b.2) (dashed-diamond lines) posterior/anterior (backward/for-ward) and (a.4) and (b.2) (dotted/dashed-cross lines) inferior/superior (downward/upward) components of the displacement.

  FIG. 11. (Color online) Displacement of the right earcanal occluded by the push-to-fit earplug. (a) Polar plots and (b) maximum values for (a.1) and (b.1) the magnitude, (a.2) and (b.2) (solid-circle lines) medial/lateral (inward/outward), (a.3) and (b.2) (dashed-diamond lines) posterior/anterior (backward/forward) and (a.4) and (b.2) (dotted/dashed-cross lines) inferior/superior (downward/upward) components of the displacement.

  FIG. 12. (Color online) Displacement of the right earcanal occluded by the premolded earplug. (a) Polar plots and (b) maximum values for (a.1) and (b.1) the magnitude, (a.2) and (b.2) (solid-circle lines) medial/lateral (inward/outward), (a.3) and (b.2) (dashed-diamond lines) posterior/anterior (backward/forward) and (a.4) and (b.2) (dotted/dashed-cross lines) inferior/superior (downward/upward) components of the displacement.

  FIG. 13. Ellipsoidal cross section of the right earcanal between the two bends. The double dashed arrow indicates the minor axis of the ellipsoidal cross section in the superior/posteriorinferior/anterior direction.

FIG. 14

 14 FIG. 14. (Color online) Projection on the walls of the left earcanal (flipped in the medial/lateral direction) of the posterior/anterior component of the displace-ment for the (a) push-to-fit and (b) custom molded earplugs. Arrows indicate the direction for regions of maximum (both positive and negative) displace-ment. The system of coordinates recalls the anatomical directions with (S) superior, (I) inferior, (A) anterior, (P) posterior, (L) lateral, and (M) medial.

  (a) shows the subject's open earcanal and its earcanal occluded with [Fig. 15(b)] the custom molded, [Fig. 15(c)] the roll-down foam, [Fig. 15(d)] the push-to-fit and [Fig. 15(e)] the premolded earplugs. Compared to Fig. 2(b), Fig. 15(b) shows the custom molded earplug in a single section.

ACKNOWLEDGMENTS

The authors would like to thank the Laboratoire Auditif Laviolette from Trois-Rivie`res for the custom earplugs and the institut de recherche Robert Sauve en sante et securite du travail (IRSST) and MITACS for the funding of this research (Grant No. 2016-0020).

APPENDIX: MR IMAGES OF THE OPEN AND OCCLUDED EARCANAL