Davide Dardari
email: davide.dardari@unibo.it

Nicoló Decarli
email: nicolo.decarli@unibo.it

Anna Guerra
email: anna.guerra3@unibo.it

Francesco Guidi
email: francesco.guidi@cea.fr

The Future of Ultra-Wideband Localization in RFID (Invited Paper)

Keywords: RFID, Localization, UWB, UHF

In the new scenarios foreseen by the Internet of Things (IoT), industrial and consumer systems will be required to detect and localize tagged items with high accuracy using cheap, energy autonomous, and disposable tags. To meet these challenging requirements, the adoption of passive ultra-wideband (UWB) radio-frequency identification (RFID) appears a promising solution to overcome the limitations of current Gen.2 RFID standard. In this paper we provide a survey on recent developments in the field of UWB-RFID by discussing the main advantages and open issues in providing high positioning accuracy with energy autonomous devices. Successively, we envision the possible cutting-edge technologies for next generation UWB-RFID as a key enabler for the IoT.

I. THE NEED FOR LOCATION-AWARE SERVICES

In recent years, especially after the introduction of the second generation (Gen.2) UHF standard, the radio-frequency identification (RFID) technology is rapidly replacing bar codes in items tagging thanks to the capability to work even in the absence of direct visibility and to store/retrieve information on/from tags. The cheapest RFID tags with the largest commercial potential are passive, in which the energy necessary for tag-reader communication is harvested from the reader's interrogation signal or the surrounding environment and the information is transmitted through backscattered signals [START_REF] Finkenzeller | RFID Handbook: Fundamentals and Applications in Contactless Smart Cards and Identification[END_REF]. Passive RFID technology has become more and more pervasive due to the availability of extremely low-cost tags (a few cents), thus making the range of actual and potentially new applications practically unlimited. As a consequence, the requirements for future RFID systems are becoming more and more demanding. Specifically, next generation RFID is expected to provide not only reliable identification but also sensing and high-definition tag localization functionalities [START_REF] Zhang | Real-time locating systems using active RFID for Internet of Things[END_REF], [START_REF] Costanzo | Tag, you're it: Ranging and finding via RFID technology[END_REF]. On the one hand energetically autonomous and not invasive sensors (tags) could be used for biomedical (e.g., smart plasters), to monitor drugs for efficient hospital management or in the food chain to prevent the risk of food counterfeit and adulteration. On the other hand, the capability to localize in real-time with high-accuracy (few centimeters) tags pervading the environment would enable unexplored context-aware applications and huge market perspectives such as in the field of item searching in logistic or shopping mall scenarios. It is possible to envision that in a near future every object will be tagged to take part of augmented reality-based applications [START_REF] Van Krevelen | A survey of augmented reality technologies, applications and limitations[END_REF], which allow virtual imagery to exactly overlay physical objects in real-time as illustrated in Fig. 1.

Art exhibition

Ground Floor 3 mt WiFi Tags Fig. 1. Envisioned future scenario: a user, with its own personal device, can localize and interact with tagged objects placed in the surrounding environment.

These perspectives highlight the main limitations of actual HF-and UHF-RFID technologies which were initially conceived just to replace bar codes and hence only for identification at higher distances. Limitations regarded the absence of sensors on tags and localization capability, the limited operating range (below 10 m with UHF tags, below 1 -2 m using HF tags), the need of dedicated and energy-hungry hardware to read the tag, and lack of integration with mobile communication standards. Nowadays, such limitations have been in part overcome through the introduction of significant improvements in current standards such as embedded sensors on tags to monitor physical parameters, the integration of HF readers in last generation smartphones (NFC standard1 ), and the availability of some solutions offering rough localization capabilities. Unfortunately these improvements did not come for free. In fact, they have been achieved at the expense of a further reduced operating range (e.g., NFC smartphones can read tags up to 10 -20 cm) or more complex readers (e.g., using large antenna arrays to localize tags based on angle-ofarrival (AOA) measurements) (see Sec. II).

It is clear that the above-mentioned requirements cannot be completely fulfilled by the current first and second generation RFID. Nowadays, high-performance wireless sensing and localization are offered by distinct technologies such as wireless sensor networks (WSNs) (e.g., ZigBee standard [START_REF] Verdone | Wireless sensor and actuator networks: technologies, analysis and design[END_REF]), and real-time locating systems (RTLS) using battery-powered tags [START_REF] Dardari | Indoor tracking: Theory, methods, and technologies[END_REF]. There is the need to merge the advantages of currently separated RTLS and WSN worlds through a technological shift.

For what high-accuracy localization using energy autonomous tags is concerned, some important progresses have been made towards this direction. An example is given by the merging of the ultra-wideband (UWB) technology, so far used in active RTLS systems, with the backscatter modulation principle, which is the basis of UHF-RFID.

For this reason we first briefly revise current ultra-high frequency (UHF) localization techniques in Sec. II, and successively, in Sec. III, we pose the attention to UWB-based solutions. In particular, we first explain the UWB backscattering principle, successively we illustrate the recent results achieved in related research projects by discussing the main advantages and open issues when providing high positioning accuracy with energy autonomous devices with respect to positioning systems using Gen2. RFID. In the last section, a look is given at possible cutting-edge solutions which are expected in the future to make next-generation RFID a key enabler for the "Internet of Things" (IoT) paradigm.

II. LOCALIZATION USING GEN.2 RFID

Before introducing emerging technologies using UWB signals, we briefly describe some recently proposed solutions that offer positioning capabilities using standard Gen.2 RFID. In fact, even though Gen.2 RFID was not conceived for positioning, rough localization information can be obtained by exploiting one or a mix of the following signal features: received signal strength indicator (RSSI) (Fig. 2-a), carrier phase (Fig. 2-b), and AOA (Fig. 2-c). Other approaches can be found in [START_REF] Arnitz | Characterization and modeling of UHF RFID channels for ranging and localization[END_REF], [START_REF] Arnitz | Multifrequency continuouswave radar approach to ranging in passive UHF RFID[END_REF].

RSSI-based solutions rely on the fact that, in ideal conditions, the larger the distance of the tag the smaller is the received power, and multiple RSSI measurements can be combined to derive distance and hence position information. Despite its simplicity, this method is very sensitive to random signal fluctuations caused by multipath that could make the correlation between RSSI and distance weak, and hence the position estimation poorly accurate and extremely unreliable, even if some improvements of the localization accuracy have been obtained [START_REF] Zhang | Item-level indoor localization with passive UHF RFID based on tag interaction analysis[END_REF]. Fig. 2-(b) shows the case in which the reader collects several carrier phase measurements from different locations (e.g., using an handheld terminal or moving reader) as response of an intended fixed tag. Accurate tag positioning in the order of a few centimeters can be obtained in principle by properly combining them as in virtual antenna arrays. Unfortunately, due to the 2π periodicity of phase measurements, corresponding to the signal wavelength (about 30 cm at UHF), phase-based localization suffers from phase-ambiguity that makes them suitable only in well-controlled environments with simple geometries and limited multipath [START_REF] Nikitin | Phase based spatial identification of UHF RFID tags[END_REF]- [START_REF] Digiampaolo | Mobile robot localization using the phase of passive UHF RFID signals[END_REF].

Precise signal AOA measurements can be obtained using antenna arrays with sufficiently narrow beams. By combining at least 2 AOA measurements or one AOA and one range measurement it is possible to determine the position of the tag. To achieve high angle resolutions, AOA schemes require large antenna arrays to realize narrow beams with consequent large dimension of readers and high cost of the infrastructure [13].

Due to all these limitations, a technology change is required in order to provide enhanced localization performance while preserving system complexity and low cost. In the following we focus on the impact of the joint adoption of UWB and RFID technologies to overcome such limitations.

III. ULTRA-WIDEBAND RFID

A. The UWB Technology

According to the FCC definition published on Feb./2002, each signal characterized by a bandwidth larger than 500 MHz is considered UWB regardless the way it is generated. The classical and simplest method to obtain UWB signals is by means of impulse radio UWB (IR-UWB). In an IR-UWB system, a sequence of short pulses (typically with duration around 1 ns) per information bit is transmitted in order to collect more energy and allow multi-user access [START_REF] Win | History and applications of UWB[END_REF]. The short duration of pulses guarantees a fine resolution in the measurement of signal time-of-arrival (TOA) and multipath discrimination so that time-based localization algorithms can benefit to obtain accurate localization estimates. The UWB technology is currently utilized as baseline in high-performance short-range RTLS using battery-powered active tags according to the IEEE 802.15.4a and IEEE 802.15.4f standards as well as proprietary schemes [START_REF] Sahinoglu | Ultra-Wideband Positioning Systems: Theoretical Limits, Ranging Algorithms, and Protocols[END_REF].

c (k) n c (k) n = 1 c (k) n = -1 m (k) (t) Y d (m) n d (m) n d (m) n , c (k) n

B. UWB-RFID Solutions

Given the aforementioned potentialities, the joint use of RFID and UWB technologies represents a very appealing solution to fuse the advantages of RTLS and RFID. Several works and research projects have analyzed this option, especially considering active UWB-RFID schemes [START_REF] Dardari | Pseudo-random active UWB reflectors for accurate ranging[END_REF]- [START_REF] Lazaro | Active UWB reflector for RFID and wireless sensor networks[END_REF]. For example, in [START_REF] Dardari | Pseudo-random active UWB reflectors for accurate ranging[END_REF], [START_REF] Lazaro | Active UWB reflector for RFID and wireless sensor networks[END_REF] active reflectors at tag side are exploited to reinforce the backscattered signal. The strict constraints on energy consumption make solutions based on chipless and passive tags more appealing as proposed in [START_REF] Dardari | Ultrawide bandwidth RFID: The next generation?[END_REF]- [START_REF]Detection of multiple tags based on impulsive backscattered signals[END_REF]. Due to the longer operating range and better management of multiple tags, tag-reader communication performed through the modulation of the signal backscattered by the tag seems better suited for localization purposes than chipless implementations as it will be detailed in the next section.

C. The UWB Backscatter Principle

Backscatter modulation is based on the idea that the antenna reflection properties of the tag are changed according to information data. This principle is currently adopted in traditional passive UHF-RFID tags based on continuous wave (CW) signals to carry information from the tag to the reader. The same idea has been proposed in [START_REF] Dardari | Ultrawide bandwidth RFID: The next generation?[END_REF] using UWB interrogation signals instead of CW signals to exploits the benefits offered by UWB in terms of TOA measurement resolution.

With reference to Fig. 3, when a train of UWB pulses is transmitted and encounters the antenna of the tag, it is partially reflected back depending on antenna configuration. The backscattered response consists of structural and antenna scattering modes which are plotted separately in figure for convenience. The structural mode occurs owing to the antenna's given shape and material and is independent from how the antenna is loaded. On the other side, antenna scattering mode is a function of the antenna load; when connected to a short circuit, the backscattered pulse is characterized by a sign inversion with respect to the open load condition. This property can be exploited to establish a communication link between the tag and the reader. Moreover, by estimating the round-trip time (RTT) of the reflected signal, by measuring its TOA with respect to the interrogation signal timing, it is possible to infer the distance between the reader and the tag with high accuracy. Unfortunately, signals scattered by the surrounding environment (clutter) as well as the antenna structural mode dominate the signal received by the reader, thus making the detection of the antenna mode component (which carries data) a main issue in passive UWB-RFID.

As a consequence a proper design of the signaling scheme, as that suggested in [START_REF] Dardari | Ultrawide bandwidth RFID: The next generation?[END_REF] becomes necessary. In particular, consider the following interrogation signal, composed of a coded sequence of UWB pulses p(t), which is emitted by the mth reader

s(t) = Nc-1 n=0 d (m) n p(t -n T c ) (1)
where T c is the chip time or pulse repetition period, d

, n = 0, 1, . . . , N c -1, is the (spreading) bipolar code identifying the mth reader with length N c .

Once activated, the backscatter modulator in the tag starts switching the antenna load between open and close circuit at each chip time T c according to the modulating signal2

m (k) (t) = Nc-1 n=0 c (k) n Π(t -n T c ) (2)
which depends on the tag's bipolar code c

(k) n
, for n = 0, 1, . . . , N c -1, being Π(t) = 1 for t ∈ [0, T c ] and 0 elsewhere. As a consequence, the polarity of the reflected signal (antenna mode) changes at each chip time according to the kth tag's code c

(k) n s (k) b (t) = s(t) ⊗ h (k) (t) • m (k) (t) = Nc-1 n=0 d (m) n c (k) n p(t -n T c ) ⊗ h (k) (t -n T c ) , (3)
where h (k) (t) is the channel impulse response related to the mth reader-kth tag link. This operation ensures the creation of a unique backscattered signal signature allowing the discrimination of the kth tag among other tags (generating the multi-tag interference) and clutter.

The signal backscattered by the tag propagates back to the reader's antenna through the tag-reader link giving

r(t) = s (k) b (t) ⊗ h (k) (t) + Nc-1 n=0 d (m) n w C (t -n T c ) + n(t) = Nc-1 n=0 d (m) n c (k) n w(t -n T c ) + Nc-1 n=0 d (m) n w C (t -n T c ) + n(t) (4)
where n(t) is the additive white Gaussian noise (AWGN), w(t) = h (k) (t)⊗h (k) (t)⊗p(t) is the reader-tag-reader channel response to p(t) (antenna mode only), and w C (t) is the clutter obtained from each emitted pulse. Note that the useful received signal is the result of double convolution of the interrogation signal with the one-way channel impulse response h (k) (t).

As can be noticed in ( 4), the first component appears as a UWB sequence spread using the combined code d . This difference allows the discrimination between the useful signal coming from the kth tag and the clutter as well as from the interference generated by other tags. In fact, if at the receiver the signal is de-spread using the combined code d (see Fig. 3) and the tag code c (k) n is designed so that it is balanced, then clutter can be (in principle) removed. In particular, the de-spreading operation consists in correlating the incoming signal with a local template s 0 (t) obtained using the combined code d

(m) n c (k) n s 0 (t) = Nc-1 n=0 d (m) n c (k) n w(t -n T c ) . (5)
In the presence of the kth tag, at the output of the correlator it is

Y = (Nc-1)Tc 0 r(t) • s 0 (t) dt =N c E w + E w Nc-1 n=0 d (m) n c (k) n d (m) n + Z (6) =N c E w + Z since d (m) n d (m) n = 1 ∀n, Nc-1 n=0 c (k) n
= 0 by design, and where E w is the energy of w(t) and Z is the thermal noise at the output of the de-spreader. From ( 6) one can note that the presence of the kth tag can be detected if the intensity N c E w of the useful signal at the output of the despreader is sufficiently high with respect to the thermal noise Z. The receiver complexity derived from the need to estimate the channel impulse response, and hence w(t) in ( 5), can be significantly alleviated by resorting to partial-coherent receivers [START_REF]A low complexity scheme for passive UWB-RFID: Proof of concept[END_REF].

This basic protocol can be easily extended by including data payload in the backscattered signal [START_REF] Dardari | Ultrawide bandwidth RFID: The next generation?[END_REF]. Notice that when TAG Tx/ Rx Fig. 4. Luggage sorting on a conveyor belt using UWB-RFID tags [START_REF]A low complexity scheme for passive UWB-RFID: Proof of concept[END_REF].

the same tag is interrogated by at least 3 readers located in known positions, 3 reader-tag distances are directly computed from the signal RTT and the tag can be localized (in 2D) without ambiguity by means of multi-lateration techniques [START_REF] Dardari | Indoor tracking: Theory, methods, and technologies[END_REF]. In addition, a proper code design allows simultaneously interrogations of multiple tags [START_REF]Detection of multiple tags based on impulsive backscattered signals[END_REF].

D. Implementations of UWB-RFID Systems

The basic passive communication and ranging scheme illustrated in the previous section is based on some simplifying assumptions that are not met in real scenarios. In practical implementations the following issues have to be addressed:

• Poor link budget -Because of the two-way link, the path-loss can be severe and only partially compensated by increasing the transmission power because of the conservative regulation limitations in the UWB band.

As can be seen in ( 6), the signal-to-noise ratio (SNR) can be improved by increasing the number N c of pulses associated to each interrogation signal at the expense of a longer identification time.

• Lack of synchronism -Tags are passive devices so no front-end is present in the receiver to extract synchronization information from the UWB interrogation signal. The main consequence is that tags are completely asynchronous. This problem can be solved by a proper design of the code and the introduction of side synchronization signals (e.g., in the UHF band) to provide a coarse synchronization, as it will be explained later [START_REF] Decarli | Passive UWB RFID for tag localization: Architectures and design[END_REF]. To facilitate code acquisition at the receiver in [START_REF]Detection of multiple tags based on impulsive backscattered signals[END_REF] (quasi)-orthogonal codes allowing multiple tags operating simultaneously with limited reciprocal interference have been investigated.

• Energy supply -Although UWB backscattering is characterized by an extremely-low power consumption since only the UWB switch is required to perform signal modulation, the circuitry at tag side (control logic in addition to the UWB switch) must be properly powered so that some limited source of energy (e.g., harvested from the environemnt) is needed . Due to the impossibility to extract significant energy from the UWB link because of regulatory issues, a solution is to exploit the UHF band to transfer the required energy to the tag as done in Gen.2 RFID [START_REF] Decarli | Passive UWB RFID for tag localization: Architectures and design[END_REF].

The above mentioned issues have been tackled in some research projects leading to prototypes providing proof-ofconcept of the UWB-RFID technology. An example of implemented UWB-RFID system realized in the context of the EU project SELECT for precise luggage sorting in airports is depicted in Fig. 4 [START_REF]A low complexity scheme for passive UWB-RFID: Proof of concept[END_REF]. To simplify the processing at the reader side, a partially coherent receiver has been designed as a trade-off between complexity and ranging performance. In this set up it was demonstrated that it is possible to detect and sort tags up to distances of 4.5 m, with errors within 30 cm.

In the context of the Italian project GRETA, the energy efficiency and eco-compatibility aspects of UWB-RFID systems have been investigated [START_REF] Decarli | Passive UWB RFID for tag localization: Architectures and design[END_REF], [START_REF] Decarli | The GRETA architecture for energy efficient radio identification and localization[END_REF]. Coexisting UHF and UWB interrogation signals have been considered to transfer the energy to the tag and provide some synchronization reference. In Fig. 5 an example of tag architecture is reported in which the energy necessary to wake-up the tag, power the control logic and the UWB backscatter modulator is harvested from the UHF link. The synchronization signal can be obtained by modulating the amplitude of the UHF carrier with a periodic square wave having period T c which timing is extracted by the clock extractor and used to drive the code generator. A simpler alternative method is to synchronize the code generator at the falling edge of the UHF wake-up signal [START_REF] Decarli | Passive UWB RFID for tag localization: Architectures and design[END_REF]. In this case a local clock generator must be included in the tag and clock drifts have to be properly managed at the reader.

Due to the coexistence of UHF and UWB bands and to reduce tag's dimensions, a dedicated dual band antenna has been designed as shown in Fig. 6 [START_REF] Fantuzzi | A novel integrated UWB-UHF one-port antenna for localization and energy harvesting[END_REF]. Such an antenna provides Fig. 6. Single port UHF/UWB antenna prototype on paper substrate [START_REF] Fantuzzi | A novel integrated UWB-UHF one-port antenna for localization and energy harvesting[END_REF].

quite good performance also in the UWB band considering it is built on paper-based substrate. Experimental characterizations have demonstrated the possibility to reach up to 6 m with 15 cm ranging accuracy in backscattering configuration. In [START_REF] Decarli | Passive UWB RFID for tag localization: Architectures and design[END_REF] other possible architectures are discussed including those in which the UWB link is exploited only for ranging and localization purposes while the UHF part is composed of a standard Gen.2 UHF tag for backward compatibility.

IV. OPEN ISSUES AND FUTURE PERSPECTIVES

A. Main Challenges

Despite the recent progresses obtained, the UWB-RFID technology is not mature yet for a widespread adoption in context-aware applications. In fact, several aspects, most of them in common with UHF RFID, have to be addressed to make it an appealing and users-accepted solution.

The most critical one is related to energy efficiency; in fact when a tag is interrogated only about 1% of the energy emitted by the reader is captured by the tag while the remaining 99% is wasted in the environment. In addition to the scarce power transfer efficiency, the localization of tags still requires ad-hoc devices and costly infrastructures that makes the integration of RFID readers in future smartphones not feasible yet. Associated to the low power transfer efficiency, the limited reading range (less than 10 meters) obliges the deployment of a network of dense readers to localize and track items in large areas (e.g., in stores). Backward compatibility with Gen.2 RFID is another important aspect that might significantly condition the market acceptance of new technologies. Last but not least, the potential introduction of billions of tags should be sustainable from an eco-compatibility point of view, in other words, tags should be disposable, which is in contrast with the need of high performance electronic circuits to manage UWB signals. THz band [START_REF] Pursula | Millimeter-wave identification -a new short-range radio system for low-power high data-rate applications[END_REF]. The incoming fifth generation (5G) smartphones will integrate mmW interfaces to boost the communication data rate beyond 1 Gbps. A part from the extremely large bandwidth available at such frequencies, which is beneficial for accurate positioning, mmW technologies offer also other interesting opportunities.

B. Future Perspectives

First of all, the small signal wavelength (5 mm at 60 GHz) allows to pack hundreds of antenna elements in a small area, even integrable into smartphones. An example of existing 400-elements array is given by [START_REF] Kaouach | Wideband low-loss linear and circular polarization transmit-arrays in V-band[END_REF]. Such large number of antenna elements permits to realize a near-pencil beam that is electronically steerable. Thanks to the extremely narrow beam formed, the possibility to focus the power flux towards the tag and transfer the energy at several meters will become possible with much higher efficiency than that achievable with today's technology. Therefore there will be the possibility to energize, detect and localize tagged items using smartphones at several meters of distance enabling augmented reality applications. Moreover, no dedicated infrastructure would be required as large antenna arrays already used for communication could be employed as reference nodes with the advantage of permitting both accurate TOA and AOA estimates (see Fig. 7).

In another possible scenario, access points deployed for indoor communications and equipped with mmW antenna arrays localize the tags present in the surrounding environment, and focus the beam to allow the tag to accumulate the energy (e.g., during the night) so that they will be operative whenever a mobile device interrogates them, as shown in Fig. 7 [START_REF] Guerra | Energy sprinklers for passive UWB RFID[END_REF]. Efficient and smart energy transfer together with high-performance energy accumulation (e.g., using supercaps [START_REF] Sudevalayam | Energy harvesting sensor nodes: Survey and implications[END_REF]) could open the possibility to exploit efficient active communications (active tags) in place of backscatter commu-nication.

As previously stated, tags have to be eco-compatible. In such a context, the use of paper for the implementation of microwave components and systems is receiving an increasing attention, as it is a cheap, renewable and biocompatible material [START_REF] Decarli | The GRETA architecture for energy efficient radio identification and localization[END_REF]. Among the available technologies for the manufacturing and integration of microwave components, the substrate integrated waveguide (SIW) technique is a potential candidate to deal with high frequency and UWB signals [START_REF] Moscato | Compact substrate integrated waveguide (SIW) components on paper substrate[END_REF].

In conclusion, in this survey we have highlighted that a technological shift is essential to satisfy the demanding requirements of future IoT systems offering high-accuracy localization. UWB-RFID is a promising candidate in this direction because it conjugates the high-resolution discrimination of UWB signals with the backscatter principle of passive RFID. However, for its widespread adoption as nextgeneration RFID, a significant research effort and synergy between different but tightly intertwined fields such as lowpower electronics, antenna design, communication theory and signal processing, is required.

Fig. 2 .

 2 Fig. 2. Main localization techniques using UHF-RFID.

Fig. 3 .

 3 Fig. 3. UWB-RFID backscattering scheme.

  term representing the clutter appears as a UWB sequence spread using the reader's code d (m) n

Fig. 5 .

 5 Fig. 5. UWB-RFID tag with energy harvesting in the UHF band.

Fig. 7 .

 7 Fig. 7. Energy transfer mechanism to energize passive tags using mmW/THz massive antenna arrays.

http://www.nearfieldcommunication.org/tecgnology.html

For the sake of explanation, we suppose the clock of the tag is synchronous with that of the reader.

ACKNOWLEDGMENT

This work has been funded in part by the European H2020 project XCycle (Grant 635975), the GRETA project (Grant 2010WHYSPR) funded by the Italian Minister of Research, and the H2020-EU.1.3.2 IF-EF Marie Curie action MAPS (Grant 659067).

The authors wish to thank R. D'Errico and all the partners of the GRETA and SELECT projects for the fruitful cooperation.


Jean-Paul Thibaud

Commented City Walks

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

The theme of public space crystallizes certain theoretical, methodological and epistemological considerations in descriptive approaches. As a conceptual category, the "public" links back to the phenomenal nature of reality and the facility of appearing to be through the eyes of others and of perceiving together [START_REF] Arendt | Condition de l'homme moderne[END_REF]. What becomes public comprises what may be perceived by everybody via his/her senses, what may be instituted by a "common appearing" [START_REF] Quéré | Qu'est-ce qu'un observable ? In L'espace du public. Les compétences du citadin[END_REF]. As such, social life may not be reduced to a sum of singular experiences; it mobilises a community of perceptions and develops on the basis of behaviour that may be observed by everyone. This inter-subjective dimension of public space is based on the fact that others take account of my presence, manifest their actions and enjoy the same access as me. Moreover, perception only makes sense in the context of the places and circumstances in which it unfolds. The use we make of our eyes and ears depends on the micro-ecological framework of the encounters and events with which we are confronted. The possibilities for access to others involve both physical conditions of visibility and audibility and perceptive structuring of sensorial data. Put another way, attention distribution, exposure to others and organisation of perspectives take place based on what it is possible for us to see and hear.

Most sociological approaches that deploy a descriptive approach reserve a central role for context-based social behaviour. Intricate accounts of ordinary gestures, postures, looks and words are usually an essential stage of field research. In particular, audio or video recordings make it possible to conserve a material trace of the activity in situ, to consult the documents as often as one wishes, to analyse several documents together and harness the possibilities offered by the technical medium. Viewing a piece of video without the sound, in slow motion or in freeze-frame mode, or using close-ups helps in analysing phenomena that could only be seen with great difficulty by the naked eye. As regards sound, systematic, repeated listening to recorded conversations makes it possible to analyse the manner in which verbal interactions are structured, either in terms of the prosody of the articulated language (vocality) or how series of words come together (sequentiality). One of the key contributions of these observations/descriptions is to highlight the contextual nature of social behaviour. These approaches constantly illustrate how behaviour only makes sense when placed in the context in which it appears, i.e., the spatial-temporal circumstances, actions in progress, and the expressions and behaviour of others.

Paradoxically, very little research has attempted to gauge the effectiveness of the sensorial environment in terms of how practical actions take place. Only very rarely do we get a detailed illustration of how the sensorial environment interferes with and helps to structure the reciprocal perception of urbanites. Indeed, the focus on behavioural adjustments and cooperation and social coordination processes favours an analysis of actors' interactional skills. This pragmatic approach questions how urbanites seek to make their behaviour clear, observable and intelligible, however it frequently neglects the sensorial context of this reciprocal accessibility (brightness and noise in the place in question). Everything happens as if there was maximum permeability of the sensorial milieu and the perceptive constraints and potentialities of the place were negligible. In other words, the hypothesis of the endogenous organisation of practical action tends to mask the physical, material and environmental conditions of social interaction. Erving Goffman undoubtedly has a special place in this type of approach. His constant focus on the micro-ecological frameworks of interactions led him to develop a conceptual apparatus that specifies the impact of place on actors' perceptive behaviour (notions of social situation, region, framework, etc.). His monumental work sketches out the possibility of differentiating between places in terms of conditions of access to others. However, the characteristics of the place are usually analysed in terms of physical obstaclesall manner of walls and partitionsthat function as barriers to perception, giving rise to a series of oppositions (stage/backstage, opening/closing, manifestation/dissimulation) that provide little indication of the complexity of the environs. Only rarely are we provided with a detailed illustration of how the built environment generates phenomena of reduced, contrasted or bloated observability. Furthermore, instead of focusing on the switch from backstage to centre stage, we wish to understand how the sensorial milieu helps to "showcase" life acted out in public, and how the sensorial qualities of a place are bound up with its public character. So how can we analyse the perceptions of passersby in public? What is observable data in terms of ambiance?

The main goal of the commented city walks approach is to gain access to the sensory experience of passers-by. The key is to acquire accounts of perception in motion. For this, three activities are simultaneously required: walking, perceiving and describing. As we will show below, this method is part of a wider interdisciplinary approach which draws on the engineering sciences (acoustics, lighting technology), design science (architecture, urbanism) and the social sciences (micro-sociology). In situ descriptions are nonetheless central to this approach: it is from such descriptions that hypotheses about ambience phenomena are formulated; further, they are the preferred means for interpreting various corpora. Commented walks are based on three central hypotheses.

Perception in context

Contemporary epistemology constantly claims that it is impossible for researchers to achieve a global perspective of their research topics. Regardless of the nature of an observation, we need to contextualise the conditions under which it was made possible. We first need to analyse this contextualisation in specific terms. The notion of context is twofold: both in terms of the methodological clause (the interaction between observable data and observation conditions) and of the research topic (ambience as a sensory context). What consequences can we draw from this dual contextual situation? For one, sensory ecology (Gibson 1986) reminds us that a subject's perceptive orientations cannot be dissociated from the "affordances" of an environment. As such, perception is not so much based on an environment as it is subject to an environment; it thus needs to be connected to the properties and characteristics of the site studied. Once we admit that environmental context has an impact on perception, the latter can no longer be studied in an abstract manner but rather needs to be examined as part of a perception-environment pairing. Moreover, examining the sensory activity of city-dwellers requires that we get as close as possible to their point of view. They use their senses based on and according to a pragmatic context: taking one's place in a queue, avoiding collisions on a journey, crossing an intersection on foot, waiting for an acquaintance in the street or just trying to find one's way are all practices which affect how we perceive things in public [START_REF] Thibaud | Regards en action. Ethnométhodologie des espaces publics[END_REF]. From this perspective, the way things are perceived cannot be dissociated from the course of action in which passers-by are engaged. In general, sociologists offer a second perspective in which they assume the role of a disinterested observer, external to the situation. And yet, given that city-dwellers themselves are observers of public life, can we not instead draw on their ability to observe and describe things? We wish to move away from academic and distanced observation and towards ordinary and engaged description. As such, the description of what is perceptible would no longer be dictated by the researcher but directly by the passer-by. Using the reflexive resources of passers-by provides an opportunity to contextualise their descriptions. Taken as such, the empirical study of urban ambiences requires us to examine the sensory environment of places, the perceptive behaviour of passers-by and the social activities in which they are involved. Ultimately, the aim is to point up the situationallyrooted nature of perception in order to develop an in situ approach.

The inevitable "movement" of perception

Whether we refer to phenomenology, sensory ecology or neurophysiology, it now seems illusory to want to dissociate perception and movement. All perception involves "movement", regardless of how small, to make the act of perception possible. As such, phenomenology is constantly pointing up the founding unity between "feeling" and "moving" (Straus 1935), the chiasmus forged by perception and movement [START_REF] Merleau-Ponty | The Visible and the Invisible[END_REF]. Rather than starting with the duality between object and subject, action and perception, we can look at their co-affiliation and concede that there appears to be something akin to an "appearance driver". From this perspective, movement cannot be reduced to simply changing place or moving from one spot to another; it calls up and reveals sensory qualities: "this amounts to saying that phenomenalisation originates in the world in which the subject is involved through his or her movements, or that it is the moving subject who, by going towards the world, makes it appear" [START_REF] Barbaras | Motricité et phénoménalité chez le dernier Merleau-Ponty[END_REF]. Setting the body in motion involves both a practical occupation of the world and an awareness of it. The principle of moving perception is not only based on an ontology of the flesh or a praxis of the perceiving body; it is also useful for understanding the sensory construction of public space. "Movement" is also a contingent possibility for the public sphere. Indeed, it is based on the plurality of perspectives and thus requires a variety of positions and points of view. Paradoxically, public space only begins to exist as a shared world, with unity and shared identity, once it has been seen from a variety of angles. In other words, for public space to exist, I must be able to put myself in another person's place and to change perspective. Finally, urban ambiences raise the issue of movement on two levels: in terms of motivity, as a condition for the existence of perception, and in terms of mobility, as a condition for the existence of public space. As such, rather than take a static approach, we wish to place movement at the very heart of our field research. To understand the city-dweller's sensory experience, we shall use a protocol for moving about in the city. Our research is based not only on situated perception but also on perception in motion.

The interlacing of words and perception

Western philosophy has a long tradition of opposing what is sensory and what is intelligible: perception on the one hand and concept on the other. Although the two indeed seem very different, we can nonetheless suppose the existence of closealbeit complex and variableties between the two registers. We believe that it is possible to assess perception based on what is verbally reported. We cannot of course perceive everything that our senses encounter, but only that which we can put into some sort of form. This is the distinction between sensing and perceiving. On a basic level, our sensory experience is based on a collection of unstable and undefined stimulations of which we have no memory; this is what Leibniz calls "the wave of small, indiscernible and vague perceptions". Apperception begins only when we let a gestalt emerge and we release figures from the sensory world. An individual's relationship with the environment is not purely reactive or reflexive; it involves a degree of configuration by the perceiving subject. Thus, spoken language is not only an instrument which allows a lived experience to be recounted after the fact, represented and shared with others; it actually participates in the experience fully and immediately. Hermeneutics as such insists on the "essential linguistic condition of all human experience" [START_REF] Gadamer | Vérité et méthode[END_REF] 1 . Taking a completely different approach, conversation analysis points up how concepts inform perception and how "classes and categories allow us to see" [START_REF] Sacks | Lectures on Conversation[END_REF]. In other words, all experience is conceptually built and is deciphered only through and in the language that it solicits. Events and phenomena perceived in situ are individualised on the basis of conceptual schema which give them shape and meaning. The descriptions we collected during our fieldwork give us access not only to the experience of passers-by but also reveal the configuration operations bound up with the ambience of a place. Our initial assumption is that a sensory environment can work as "speech trigger" and that local ambiences can be grounds for verbalisation. Just as words only take on meaning alongside a non-verbal backdrop, what is perceived inevitably involves a backdrop of inattentiveness. Describing an event presupposes that it was perceptible to us and that it became poignant enough to talk to us, to make us talk and to talk through us.

The entire approach is represented in the diagram below. We will then go on to describe the different phases individually.

Descriptions of perception in movement

Following a preliminary site observation phase (i.e. becoming familiar with the pedestrian network, the site's architecture, informal observation of social behaviour), we begin collecting reports of perception in movement. Passers-by (regular and nonregular site users) are asked to participate in the in situ experience.

Conditions of the experience

The research protocol is based on three types of instructions which structure the experience:

-Instructions related to the description: describe, as precisely as possible, the ambience of the place as it is perceived and felt during the walk. All sensory modalities can be used: visual, acoustic, tactile, heat, olfactory, kinaesthetic, etc. In order to know where an individual is during their description, they are asked to regularly provide spatial references, useful during the analysis phase.

-Instructions related to the route: the investigation site is established ahead of time, but the actual route taken can be left to a certain extent up to the interviewee. If they wish, the passer-by can stop momentarily, go back to a place or change pace. -Instructions related to the conditions of the experience: given the focus and attention required for such an experience, the walk lasts roughly twenty minutes, but may be extended if the person so wishes. All comments are fully recorded using a portable recording device. The route is travelled with the researcher who records the description but intervenes as little as possible and just tries to be a friendly listener, possibly restarting the conversation if the walker-observer is clearly having trouble.

During a second stage, the trajectory is reconstructed on paper with the participant. A map is useful when the places visited are particularly complex and it is difficult to reconstitute the route. If this is not the case, it is preferable to conduct the reconstitution without a graphic aid in order to encourage the use of memory (What are places called? What do we use to recall them? etc.). Then a short interview is conducted in which the following issues are tackled: possibility of distinguishing between the places visited according to their ambiences, dividing the walk into sequences, the most striking events during the walk, familiarity with the neighbourhood and how often it is visited, assessment of the experience, personal information (age, profession, site use frequency, etc.).

The experience is repeated roughly twenty times with different people who are contacted either through personal networks or directly at the fieldwork site (in this case, passers-by are engaged in a specific course of action: they are going to work, to take public transportation, for a walk, shopping, etc.). In order to ensure that the population and activity contexts are as diverse as possible, it is important not to rely solely on personal contacts.

If there is spatial coherence in the corpus (the same site for all of the commented walks), the search for diversity in the descriptions is based on three variables:

-A variety of routes. If left up to the walker-observer, the route chosen will not necessarily be the same for each person. Such spatial variations are interesting for more than one reason. Aside from the fact that they tend to cover a good part of the research field, they provide precious clues about how urban space is appropriated. They also allow for a comparison of descriptions according to the walker's orientation (directionality of the chosen route) and how a place was accessed (environs of squares and intersections). -A variety of circumstances. The sensory context of a place changes over the course of a day or week (as well as with the seasons). Descriptions of the same walk differ based on whether it was taken during the day or at night, whether it was a sunny day, taken alone or with other members of the public, during organised events or during quiet periods. Therefore, it is best to diversify the temporal conditions of the experience as much as possible. These two aspectsdirectionality of the route and temporality at the siteare the two most important aspects of the description context to consider during the analysis phase.

-A variety of points of view. This will depend on the people participating in the experience. Aside from the traditional variables such as age and sex, there are three other parameters at play when selecting walker-observers: sociocultural status which produces specific verbalisation processes, how well a site is known which will engage memories to variable degrees, and a person's visitor status (ordinary passer-by, tourist, local shopkeeper, homeless person, etc.) which will often lead to implicit representations. It is important to compare and contrast points of view, to tease out the similarities that go beyond the differences and reinterpret similar content from unique ways of describing them. Such cross-analysis is a means of reconstructing the inter-subjective dimension of the experience and of showing how a site can generate shared perceptions.

Ways of describing, means of perceiving

The descriptions recorded through this research technique are transcribed and aim to remain as faithful as possible to the fluctuations in recorded speech (e.g. pauses, probing, hesitation, onomatopoeia and stammering). The great diversity of comments means that the descriptions must be read repeatedly in order to progressively extract the perceptive "moorings" rooted therein. In this respect, analysing the comments is not so much about classifying the objects perceived during the walk (what is perceived?) as about examining how people relate what they perceive (how do we perceive?). Although they are not devoid of interest, we tried to move beyond two immediate observations. For one: the filter of representations in descriptions. We observed that some comments implicitly or explicitly contain personal judgements which guide people's comments. The way things are described is influenced by our feelings about (positive or negative), and by our use of a place (a tourist discovering a new place, the banality of everyday routine, a commercial strategy, etc.). Secondly: use of different types of discourse. Descriptions take on a different form depending on whether we are observing, qualifying, detailing, clarifying or listing. And yet beyond such purely descriptive aspects, there are also attempts to explain (hypotheses, reasoning) and assess (aesthetic or functional).

In addition to a sensory interpretation, several descriptors were selected during the analysis. Through different means, each provides access to situated perceptions and sensory contexts regarding the places visited. a. Spatial-sensory associations. Very often, passers-by call on their perceptive memory and describe the ambience of a site they are visiting by association. Some urban spaces thus work as veritable sites of sensory reference. Associating an ambience perceived in situ with places like a "train station", "airport", "swimming pool", "Parisian passageway" or "greenhouse" provides insight into the acoustic nature, lighting and temperature in a place. Such indications become explicit when the observer explains or justifies the relevance of the metaphors used.

b. Perceptive transitions. Another means to describe the ambience of a place is to note the obvious changes which occur. For example, we can note that in some places "it is calmer", "voices sound more distant", "there is less light", "it is brighter", "it is too hot", etc. By highlighting differences in intensity and changes in quality, the observer describes an ambience in their spatial-temporal setting according to the route taken and specific circumstances. Whether related to an event and/or a location, such perceptive transitions allow us to identify the sensory connections that exist between places.

c. The verbal field of appearance. Describing what we perceive not only provides an inventory of facts and obvious details that can be heard and seen. The use of verbs such as "seems to", "appears to" and "looks like" expresses a degree of perceptive uncertainty and ambiguity. As John [START_REF] Austin | Sense and Sensibilia[END_REF] has shown, such verbs are not always interchangeable; they expose the dilemma between a matter of fact and a question of language. By listing the circumstances in which such verbs are used, we can identify situations which are problematic from a perceptive point of view. The analysis must not be limited to understanding when others become visible or audible, but rather specifically how and under what conditions. A certain number of verbs provide invaluable information on contexts of observability and types of access to others: "to contrast", "detach oneself", "merge into", "be outlined against", "emerge", "be hidden", etc. Such terms are particularly interesting given how they describe the way the public is present by connecting the sensory details of space to the observer's configuring activity. More specifically, they allow us to name perceptive phenomena, understand how a form emerges from within the whole and identify different types of figure/background relationships.

d. Reflexive formulations. The last point used during the analysis involves the observer-walker's reflexive wording. This helps clarify the two main activities of the experience: on the one hand, the perceiving subject's perceptive orientation (e.g. "I'm looking up", "I'm turning around", "I'm listening"); and, on the other hand, the walker's momentum (e.g. "I'm hesitating", "I'm accelerating", "I'm stopping", "I'm drawn towards"). Such word choices indicate the possibilities available to passers-by to adapt to the circumstances at a given moment and potentially impact the practical context of the experience. As part of the public, observers have to respond to the situations and events they encounter along their way.

These different types of descriptive behaviour are not independent of each other; rather, they correspond with different ways of expressing an in situ experience. Given that they concern a single site, each rounds out the others and provides a possible breakdown of the practical-sensory field.

"Polyglot route"

Thus far, we have focused on the first phase of analysis which involves sorting and selecting a certain amount of descriptive data. In order to be fully operational, the data must undergo several additional operations. Firstly, they need to be replaced in the flow of the description according to what is said before and after. Most often, it is the connections between proposals that allow us to understand the meaning of each. Then we need to check the inter-subjective content of comments by doing crosschecks and comparisons. The recurrence and redundancy of similar comments from different observers is proof of a certain perception community. Finally, the descriptions are organised according to the places visited and the conditions involved. This provides us with a collection of information and hypotheses about the sensory-motor phenomena related to a visited site. The categories and terms employed to describe such phenomena are largely based on research conducted by the CRESSON2 research unit.

Reconstructing the walk

The challenge then is to synthesise all of the descriptions compiled during the first phase of fieldwork. To do so, we reconstruct the narratives into an "ideal" walk (in the sense of Max Weber's ideal type). For a given walk, we need to reconstruct the route which brings out the existing spatial-perceptive potentiality most effectively. Therefore, the difficulty is not reconstruction verisimilitude but its demonstrative strength. We do not actually do a complete re-writing, but rather a collage of fragments from the different descriptions. Changes in the person speaking are identified by a full stop. These "polyglot routes"a patchwork of diverse visitor commentsnonetheless follow the same trajectory logic. Two basic rules regulate this descriptive compression: fragments which best reveal the local sensory context should be selected, and the speaker's localisation and the route's directionality must be respected. The "reconstructed description" (in the column on the left) is accompanied by a "reader's guide" (in the column on the right) which indicates the sensory-motor phenomena identified. The "polyglot route" below can be read like a descriptive analysis or a result in progress. Mid-way between description and analysis, it gives voice to a place whilst shaping understanding.

The example below is drawn from part of a walk at the Grand Louvre in Paris. The walk begins in the courtyard around the pyramid and continues through to the Louvre's Grande Galerie. We are going to enter the Pyramid. So we're now entering, there, the sound has already completely changed since we've left all of the cars behind us, the sound is very muffled, it resonates a lot, like when you enter a railway station actually, or an airport, it's a bit like that kind of place. The noise is what's most striking here, it's filled with noise that gives you the impression that you're in a closed space. Here, there is a kind of muddled noise, the same noise you hear in swimming pools, you can hear everything and nothing at all because you can't actually understand anything or distinguish things. The sound of people is less precise than outside, there is a sort of background noise, a hum. You really need to strain your voice when you talk here, to make yourself audible; although the sound does not seem all that loud it is actually very present. So now, it's really like a greenhouse in here, there is a concentration of heat. Under the pyramid, we really feel much more stifled by the heat. Now my eyes automatically look up to the sky, because the sky looks really unique, through this glass roof, which outlines a piece of blue sky, although it was already there when we were outside, but I was not looking at it. So of course thereafter, when we move ahead, we're drawn towards a shaft. I think that this platform was designed for us to stop. Here, we are overlooking the large arena -it actually looks like a large arena -of the Louvre museum's entrance, under the pyramid; so there are people. The people are down there, it is a sort of small closed zone, a kind of… I don't know… micro-society. You usually get the impression of a living painting, an anthill, where you can see people waving to each other, meeting up, queuing for tickets. What I really like are the materials used which reflect, and are bright. The use of white marble.

Ambience at the project scale

The text above highlights different sensory phenomena expressed during the walk. Such an intermediary result still needs to be worked on. The polyglot routethe intermediary step in the analysisnow needs to undergo a secondary description which clarifies and contextualises the sensory-motor effects detected. We need to not only make the terms in the column on the right understandable, but also recontextualise the sensory-motor effects previously identified. Indeed, these are not necessarily stable or permanent; they can vary over time according to the number of visitors, the relative amount of natural light (daytime/night time ratios) and events at a given time. It is thus necessary to return to the field. The analysis of the comments obtained during the first phase of fieldwork provides a structured framework for observation of the site. The goal is to precisely identify the conditions under which the phenomena described by the passers-by emerge. The relationship between observation and description is thus reversed: the goal is no longer to describe what is perceived but to link the descriptions to what is observable on site. Aside from ethnographic observations which allow us to describe social behaviour and preserve a physical trace of the phenomena identified (recorded sound and images), we take architectural, acoustic, lighting and thermal and aeraulic measurements which subsequently allow us to analyse the physical nature of the perceived effects. Although we shall not develop this last point here, this is an integral part of the research method3 .

Contextualising phenomena

The final processing of the different corpora obtained in the second phase of fieldwork allows us to not only clarify the conditions and circumstances under which phenomena appear, but also to analyse the built infrastructure in terms of ambience. The challenge is then to understand how each phenomenon detected fits together and works in combination with the others to produce local sensory configurations. This analysis of a place's ambience is based on three additional approaches. The first is that of the built infrastructure. The goal is to describe the physical makeup of the place. More specifically, this first category aims to expose the interaction between the built environment and physical signals. The phenomena detected here are measurable. The second approach concerns the sensory features. The goal is to describe the perceptive component of the in situ experience. The phenomena detected here can be expressed. The third approach deals with the social features. The goal is to describe the public dimension of visitors' behaviour. The phenomena detected here are observable. Let us take the example of the Hall Napoléon in the Grand Louvre (the space located under the pyramid) to illustrate how data can be reinterpreted this way. Only the visual and acoustic aspects will be examined here.

Seeing and hearing in the Hall Napoleon

The basement as a bright background. Visually, the Hall Napoleon can best be described as a place of "overexposure", meaning that people are extremely susceptible to be seen by others. In other words, visitors situated in the basement are particularly visible to those looking down from the entrance level above. Everything is done to encourage those arriving to look at what is happening below. The layout almost exhorts visitors to observe.

-Visual attraction. From the outside, the monumental emergence of the pyramid works as a veritable channel of curiosity and attraction. Everything seems to exhort passers-by to enter the large glass space. Once inside the pyramid, the attraction effect persists and draws people's attention towards the basement: "we are drawn towards a shaft". Visitors become momentarily captivated by the view which dominates the foyer below. Not only do they need to take their time to identify how to access the basement, but they need to adapt their trajectory and go either to the right (escalator) or to the left (spiral staircase). The staircase and escalator are off-centre in relation to the entrance and break up the continuity of movement. However, the view is directly in line with the entrance and calls out to visitors, inviting them to pause.

-A commanding view. Upon entry, visitors are well above the foyer located in the basement (9 meters below); this elevation provides a commanding view of what is happening below: "we are overlooking the large arena". This dominant perspective allows the visitor to survey the entire scene in the foyer whilst remaining detached from the activity below. Further, the foyer's closed nature and the mezzanines along almost all sides accentuates the panoramic view and closely resembles an arena.

-Figure-backdrop contrast. The basement does not contain any furniture or visual obstacles and is thus a backdrop against which only the outlines of passers-by appear: "people clearly stand out from the floor". From the lookout up at the entrance, there is nothing to mask their presence. The brightness of the walls and the natural light flowing through the glass pyramid accentuate the luminosity in the foyer and make the people in the basement even more visible. Depending on the amount of sunshine, the luminance of the basement varies between 100 cd/m² and 35 000 cd/m². Seen from above, human forms are even more striking as they are reflected off the floor which is entirely light in colour.

The invitation to visit, the commanding perspective and the uniformly light background mean that visitors in the basement are overexposed. This visibility context is nonetheless subject to variation. For one, the number of visitors in the foyer affects visibility. When the area is relatively empty, each person becomes a visible entity that clearly stands out from the others; each is optimally exposed. At busy times, however, the forms which stand out correspond more to clusters of fluctuating groups and queues. Each individual becomes an indistinct element in a collective whole. Further, visibility also varies according to the lighting. When the sun is shining, natural light makes the stone appear brighter and accentuates the contrast between the bright surface in the basement and the visitors. The shaded areas in the basement (and, similarly, the exclusively artificial lighting at night) produce less contrast than the areas directly lit up by the sun. However, too much reflection from the sun on the floor can cause a glare and make it difficult for visitors up above to prolong their observation.

The indistinct hum in the background. Acoustically, the Hall Napoleon is filled with an indistinct hum into which visitors are immediately submerged. When arriving from outside, visitors immediately notice a distinct change in the acoustic ambience. The muddled ambient noise reinforces the sense of a collective presence yet conserves the impersonality of contact with others. Once again, several phenomena work together to produce this acoustic immersion.

-The reverberation effect. Composed of particularly reflective matter (stone and glass), there are several reasons why this large, closed space (35 000m

3

) is particularly prone to strong reverberation: "there is a strong resonation here". Sounds bounce off the surfaces in the surrounding environment and persist even after the source of a sound has stopped. The larger the crowd, the louder it sounds and the greater the indistinct hum which affects how noise is distinguished. Without visitors, the reverberation time in the foyer is close to 6 seconds (TR60).

-The humming effect. Several mechanical sources (escalators, ventilation) generate continuous noise of equal intensity which accompanies visitors throughout their stay: "a sort of permanent noise". The sound of such equipment is propagated throughout the space with a slight attenuation based on distance. This humming effect sets the tone of the site's ambience by creating a constant and uninterrupted acoustic backdrop. There is no pause in sound to let the ear relax and momentarily escape the acoustic continuum.

-The mask effect. The background noise under the pyramid is relatively loud and varies between 70 dB(A) and 72 dB(A) with visitors. This acoustic reality tends to reduce the intelligibility of speech, mask visitors' voices and require them to raise their voices: "you need to strain your voice when you talk here". Only a few higher pitched sounds (children's screams, ringtones, etc.) are decipherable.

-The mixing effect. The noise produced by the mechanical equipment (ventilation and escalators) blends with voices from the public to such an extent that visitors have trouble clearly dissociating the two types of noise. Speech also tends to merge into the site's acoustic backdrop: "voices become a kind of mechanical hum". Acoustically, the background noise spectrum is dominant between 250 Hz and 1 KHz, a spectrum close to that of speech.

The four effects identified all contribute in one way or another to the acoustic immersion of visitors. This immersion can be more or less pervasive depending on the density of people in a given space. We observed, for example, that the presence of visitors is an important factor in the loudness rating detected in the foyer. When empty, the average sound level produced essentially by equipment is 58 dB(A), whereas in busy periods it is around 71 dB(A). When there are fewer people in the early morning and late afternoon, this level dips a little below 70 dB(A). This analysis by sensory modality helps us understand visitors' perception scale and the socio-perceptive efficiency of the built environment. In the case of the Hall Napoleon, there is a divergence between visitors' acoustic and visual experiences: acoustically, entering the pyramid corresponds to an immediate change in the acoustic milieu involving strong acoustic immersion and a reduction in hearing range; visually, however, the transition occurs slightly later, when moving towards the basement (change in perspective) and involves an asymmetry in conditions of visibility between the spectators located up above and the actors located down in the foyer below. This type of analysis needs to be pursued in two ways: first by looking at other sensory modalities (thermal, olfactory, etc.) in the same way; and secondly, by trying to show how the different phenomena identified by sensory modality fit together and work to create an overall ambience.

Notes on methodology

The commented walks method can be applied relatively broadly: e.g. to neighbourhoods, shopping malls, museums, railway stations, underground networks, transport hubs, underground public spaces and large urban projects. However, the need to move through a circuit does impose a restriction on the scale of the site studied. As such, it would be difficult to apply this methodology to extremely small territories (for example inside a home) or on a very large scale (an entire city). This type of field research is therefore particularly well suited to studying urban ambiences in situ.

This approach to urban space touches on a fundamental challenge in social sciences methodology: determining what is observable. Commented walks obviously draw on the reflexive capacity of human actors and on their ability to understand, describe and interpret the situations in which they find themselves. This use of discursive conscience is surely full of lessons, but it needs to be combined with the practical conscience of the agents involved. In particular, we can ask ourselves to what extent an ambience can be verbalised and actually concerns the field of language. How is it possible to put into words what is first and foremost a matter of physical sensation and immediate experience? This vital and difficult question will not be resolved here. We shall limit ourselves to a few summary pointers that will help clarify the scope of the debate. To begin, language is not a simple representation of the world, a tool amongst others to interpret reality or a simple instrument which allows us to communicate our experience. More fundamentally, language is what mankind uses to "control things" and "confront the world" 4 . As such, we have posited that using speech to access ambiences allows us to understand how the latter are formed and shaped within and by language. In other words, the descriptions of perception obtained with the commented walks method primarily provided access to emerging sensory phenomena and to the configuration operations at work in perception. We can thus question the very possibility of verbalising immediatelyhere and nowwhat is felt or perceived in situ. Indeed, phenomenology has taught us that apperception and reflexive action are very difficult in the present. Becoming aware of one's own activity is more conceivable after the fact, once an experience is over. From a methodological perspective, this issue was addressed in two ways. First, moving, travelling through different and contrasting ambiences allowed people to take a prospective and retrospective approach. The existence of thresholds and transitions encouraged comparison and putting immediate experiences into perspective. The comments did not refer solely to the present experience but also to the -different or expectedtransitory phenomena which structure what has just occurred and which also allow us to anticipate what is to follow. In this case, the goal was not to extract oneself from an ambience completely, but rather to include one's temporality and variations from within. Second, each walk was followed by a semi-structured interview which allowed the participant to go over what had just been experienced. Short-term memory was called upon to reinterpret and round out the initial comments. Far from being resolved once and for all, these questions will have to be the focus of future research and observation to confirm the validity of such an approach.

To conclude, commented walks quite clearly lead us away from everyday life situations. Although factoring movement into the research approach allows us to get closer to the ordinary activities of city dwellers, we still need to distinguish between the different scenarios that exist. The descriptions of perception in motion deserve to be analysed according to the passer-by's course of action at the time of the experiment. We could as such point up how the sensory environment is shaped by the activity underway but even then we would still be a long way from a "natural situation" since the obligation to describe would continue to have an influence. That is why data gathered through "naturalistic" observation would allow us to put the final results into perspective. Conversely, the results obtained from commented walks raise the question of whether such naturalistic observation is actually possible. In analysing the sensory contexts of urban space, we demonstrated how they shape different types of visibility and audibility in public. If an ordinary passer-by can hear and see someone else more or less well depending on the sensory characteristics of a place, the same is true for ethnographers studying social behaviour. As such, the urban setting cannot simply be seen as a space in which behaviour occurs that merely needs to be recorded and analysed. What can we observe from in situ behaviour when passers-by appear overexposed, blurred in a poorly lit background or when they disappear from the frame altogether? How can conversations be recorded when background noise masks the voice or when the reverberation in a place limits the intelligibility of speech? The microecology of public relationships is not merely the focus of theoretical thinking; it also questions the investigative methods upon which it is based. The public space issue thus confronts the researcher with the paradox of what is observable: describing situated actions based on observations which are themselves influenced by the site. Addressing this paradox is undoubtedly one of the major challenges of an observation methodology for urban ambiences.

  The ordinary descriptions provided by passers-by are what constitute the basic corpus for our analysis. This experience involves taking a walk and describing what is perceived and felt during the excursion.

  Excerpt from a "polyglot route" at the Grand Louvre I am on a large and very sunny esplanade, you can feel the heat rising from the ground; I am drawn towards this pyramid like an open, central form towards which you are almost obliged to move, I mean, there is a very strong attractive force in the middle of this square. It's true that it's attractive, it really catches the eye. Here, you really feel like you're in the city, with traffic all around, the sound of people -albeit quite fainttalking, entering, waiting in the queue. The noises are quite faint, I can mostly hear children's voices, a bit like in a public park.

As Hans-Georg[START_REF] Gadamer | Vérité et méthode[END_REF] has argued: "Not only is the world only a world to the extent that it is expressed in a language, but language itself only really exists by the fact that the world finds its presence within it" (translated here).

"Sound effects" and "frames of visibility" are central concepts underpinning this type of analysis. For the former, see Augoyard & Torgue 2006; for the latter see Chelkoff& Thibaud 1993.

As an example, in terms of acoustics, we measured the sound levels (Leq), noise decay and reverberation time. With regard to lighting, we measured the luminance and illuminance. With regard to the thermal and aeraulic levels, we examined the air temperature, mean radiant temperature (wall temperature), air velocity and humidity.

See Gadamer, op. cit.


Giving Voice to Urban Atmospheres Jean-Paul Thibaud

In On Listening. Angus Carlyle and Cathy Lane (ed.) CRISAP, London : University of the Arts London, 2013 Largo da Carioca in downtown Rio de Janeiro on a hot November afternoon. A disconcerting experiment with one very simple instruction: cross this square blindfolded with the help of an attentive guide and, at the same time, describe one's immediate sensations and impressions. This means walking as if blind for about 20 minutes and having your impressions recorded. The experiment produces a series of sound recordings featuring the voice of the blind stroller along with the noises from Largo da Carioca in the background. Note the stifling heat and the uneven ground…impressions concerning the cries of hawkers and the noise of traffic…all sorts of remarks that attempt to put into words what is perceived and felt along the way.

However, what if we also listened to the voices themselves, their melody and rhythm, their hesitations, intonations, laughing, silence, exclamations and interjections…the intermittent vocal flows that describe the body in motion to the keen ear? What if we focused on the textures and sounds of the spoken word with its different inflections, dynamics and accents? What if we paid greater attention to the perceptible hum of background vocals on the recording? This could be presented in terms of the voice as expression and production of ambiances -or better still -the ambiant voice.

In addition to what is actually saidi.e., the semantic contentwe are attempting to listen to the musicality of the spoken word from the perspective of the senses. We are not interested in the timbre of the voice itself as the signature of a specific body, but in the variations and fluctuations of the voice in a situation, as shifts taking place in an ambiance. So this involves giving vent to to the sensitive body in one's dealings with the ambiant environment; the effort of, and undertaking of movement; the affective tonalities of experiences in progress and problems encountered; adjusting the voice to what is heard; paying attention to certain events; maintaining contact with the interlocutor, etc. In a nutshell, a myriad of phonic "gestures" that accompany and embody public life and render the dynamics of places, flows, publics and exchanges audible. So, without attempting to compile an exhaustive list, here are four types of particularly clear and striking phonic "gestures".

• Capturing gesture: a first listening reveals gestures that aim to capture the attention of passers-by. During the walk across the square, a series of interjections punctuate the descriptions that make it possible to start another comment. Speech does not flow in an unbroken manner but leaves room for silences of varying lengths depending on the nature and diversity of the surrounding stimuli. These speech-triggers -"ha!", "well…", "here…" highlight the moments when and how the attention of passers-by is fixed. All that is needed is street music or the hailing of a street seller somewhere in the soundscape for the passer-by to suddenly enter a darker zone where they encounter an obstacle to being able to speak again. There are also cases where speech is merely suspended: the passer-by has momentarily stopped speaking but continues to study an event or a phenomenon without putting it into words. Silence here indicates that the focus of attention is in progress, a lingering focus on what has been noticed that makes it more difficult to speak at the same time, e.g., when one hears other passers-by gradually approaching and then moving away. Because the ambiance in Largo da Carioca is a stimulating and dynamic one, it throws up frequent and diverse capturing gestures and gives rise to numerous micro events and shifts in attention. We can define it here as an amplified attention grabbing space.

• Articulation gesture: a second listening points up articulation gestures that reveal practical ways of physical engagement. This time the voice gives expression to a body in motion with all its tensions, fluidity and inertiamovements and series of movements. As an acoustic body, it extends and accompanies, renders and reinitialises the physical engagement involved in crossing the square. As such, although Largo itself does not leave much of a vocal trace along the waythe fluidity of the recording prevails herethis is not true at the periphery. The broken footpaths and uneven surfaces give rise to a rhythmical and melodious modulation of speech. Merely stepping up onto the path becomes audible and bears out the effort made by the passer-by. The voice takes part in the strollers' physical engagement and merely expresses what is being accomplished in its own way and this allows us to retrace the entire crossing of Largo step by step, word by word. The crossing is quite bumpy at its outer limits but easier on the actual square itself and not especially challenging physically. For present purposes, Largo may be defined as a relatively moderate space in terms of physical engagement.

• Accentuation gesture: a third listening focuses on accentuation gestures that reveal fluctuations in situational intensity. The Largo is particularly animated during rush hour periods and this gives rise to numerous occasions for intensification of experience: emerging dancing music, nearby stationary traffic that suddenly starts to move again, thickening crowds, surrounding voices becoming more insistent, rumblings of street theatre, etc. All of these situations contribute to intensifying experience during the crossing and maintaining a high level of tension and stimulation. The voice tends to accompany this collective dynamic by switching to a hastier rhythm, rising in tone and adopting a happier tone, and giving greater emphasis to what is being said. For example, the repetitive cries of street sellers sometimes turn into a mantra and the passer-by cannot help assimilating these and memorising their every inflection. In brief, the voice expresses not only the passer-by's physical engagement but the hold and tonality of the surrounding atmosphere. In a nutshell, Largo may be defined here as a space of high situational intensity.

• "Tuning" gesture: a fourth listening reveals "tuning" gestures that bring out diverse forms of surrounding dynamics. The most obvious example is of course the many times strollers vocally imitate music they have heard by playing a passage or reproducing an especially catchy rhythm. The music that has been heard can generate an impetus that is very close to vocal imitation. However, there are also other phenomena in which the fit and timbre of the voice operate via transduction by expressing and embodying the dynamic of a sensory phenomenon from the ambiant environment. For example, when the wind gets stronger, the passer-by remarks on this and pronounces "wiiiiiiiiind" in such a way as to make it last: he or she reproduces the type of weather that they experience through their voice. In other words, the mention of "the wind" is not merely a way of indicating its presence, it is also a means for those who experience it of expressing its existence from a temporal perspective (the elongated manner of pronouncing the word "wind" emulates the content of the gust of air itself). In other cases, the accelerated speech of the blind passer-by exacerbates the sudden acceleration of traffic or the plethora of nearby voices. Although Largo is criss-crossed by a multitude of ambiant flows, each with its own dynamic (musical tempo, pedestrian allure, passing currents of air, incessant rumour, etc.), it does not appear especially desynchronised. For our present purposes, we would define it as a rhythmical, highly resonant space.

So, what lessons should we draw from this attempt at listening to the ambiant voice of Largo da Carioca? First, the sheer plasticity and power of vocal expression in a given situation is perfect for pointing up the atmospheric nuances of places. This gives rise to various different impregnations that colour behaviour and sensitise experiences; clear focuses that frame vocal expression and accompany the passer-by throughout the experiment. But here again, does this not point us towards an ethnophony of ambiances? In this regard, the various aforementioned phonic gestures relating to capturing attention, physical engagement, situational intensity and surrounding dynamics constitute a few of the many registers that participate in situ in sensitive experience. A singular listening channel needs to be developed, one that is highly sensitive to the tonalities of ambiant situations and changes in the situated voice. On a concluding note, does the voice not act like an extraordinarily powerful resonator of the infinitesimal variations that comprise the urban soundscape?