Christophe Raynaud

Marie-Laure Locatelli

J.-F F Mogniotte

R Arvinte

G Grosset

M.-L Locatelli

Study of mobile ionic charges by thermally stimulated currents in 4H-SiC MOS capacitors with thick SiO2 layers

Keywords: silicon carbide, thick oxide, oxide traps, mobile ions 1. Introduction

come     Study of mobile ionic charges by thermally stimulated currents in 4H-SiC MOS capacitors with thick SiO2 layers

Silicon Carbide as a wide band gap semiconductor has been extensively studied in the past 25 years.

Thus a large number of discrete devices have been demonstrated. The technology is now mature enough to provide commercial devices in the area of high voltage electronics. 600V, 1200 V and even 3300 V Schottky diodes are commercially available with different nominal current ranges, up to 50A for 1200 V diodes. Best laboratory results achieved breakdown voltage of 10.8 kV in 2003 [1].

Development of SiC MOS devices have been more complicated. Indeed, since the revival of SiC in 90's (with the development of the sublimation method for crystal growth, and the first commercialized 1" wafer), studies on the thermally grown oxide to form SiO2/SiC interface, the main useful one for MOS devices, have always shown a higher defects density than on silicon devices [2], especially on p-type layers. However some progress have been made, especially with the nitridation process and the interface state density and the oxide traps density have been noticeably reduced [see for example 1c and reference therein]. It is important to note that deposited oxides are generally of poorer quality than thermally grown layers, in terms of interface states and bulk impurities (fixed and mobile charges) [2,3]. Recently deposited oxides on 4H-SiC show a low interface state density in the range 9×10 11 cm -2 eV -1 [3] and low densities of fixed and mobile charges, but these oxides are thin (< 50nm).

Research progress finally have resulted in the commercialization of power MOSFETs (Metal Oxide

Semiconductor Field Effect Transistor) that are available up to 1700 V. However, some problems remain to be solved. For instance, power MOSFETs are currently studied because they suffer from reliability issues such as a shift of the threshold voltage during their functioning [4], or premature breakdown of the devices due to short-circuit [5]. Some of these problems which are crucial to develop reliable power devices for industrial applications are suspected to be correlated to the presence of defects in the gate oxide. For other power devices e.g. JBS (Junction Barrier Schottky) or bipolar diodes, able to sustain voltages as high as possible in regards to their theoretical capabilities, periphery of the component has to be carefully designed. By periphery, we mean the termination of junction, e.g. JTE (Junction Termination Extension), or guard rings, but also the passivation layer above the semiconductor surface. This passivation layer is generally formed by a primary layer directly deposited or grown on SiC epilayer, i.e. SiO2 (before the metallization process). A secondary layer encapsulates the first one. This secondary coating may be a polymer. Concerning the primary passivation layer, a large thickness can be needed and the quality of this oxide (i.e. the density of the charge therein) is crucial, because it directly influences the efficiency of the periphery protection [6].

We have performed TSIC (Thermally Stimulated Ionic Current) measurements and SIMS (Secondary Ion Mass Spectroscopy) analyses on MOS capacitors, having an oxide layer equivalent to the one required for passivation layer, i.e. in the order of 1 µm. It is far thicker for standard MOSFET transistor structures. SIMS allows to identify the chemical species present in the oxide, and their densities, but with some uncertainties in their distribution. TSIC analyses can give information on their densities and also on their trapping-detrapping properties at the interfaces (Metal/SiO2 and SiO2/SiC interfaces). Thus these two techniques are rather complementary. TSIC technique is not commonly used and no literature is available concerning TSIC on 4H-SiC. All available data concern silicon devices ; only one study obtained on 6H and 3C-SiC polytypes has been published [7] to our knowledge.

Experimental details

Description of the samples

In order to fabricate high voltage devices with periphery protections including a thick silica passivation layer, we have realized MOS capacitors as test structures, on the same wafer, and using the same fabrication process characteristics. A N-type epilayer (of 35 µm thickness, and 8.5x10 14 cm -3 doping level) grown on a 4H-SiC N + -type substrate, purchased from Showa-Denko, has been used.

The oxide layer was obtained initially by wet oxidation of the 4H-SiC epilayer surface in order to grow a thin SiO2 film of ~30 nm in thickness. Further, a thick SiO2 layer of ~1.5 µm was deposited by PECVD (Plasma Enhanced Chemical Vapor Deposition) process using TEOS (tetraethoxysilane). An ohmic contact at the substrate back side was obtained by metal deposition and an annealing was performed in order to realize the bottom electrode of the MOS structures. The front side electrode (gate) consists in an Ag/Ni/Ti stack with a total thickness of around 2 µm, deposited by successive sputtering steps. Circular front side electrodes of 1.5 mm in diameter were shaped by photolithography. Then, after a secondary passivation layer spin-coating and photolithography, a final annealing (polymerization cure) was applied to the wafer.

Description of the test bench

C-V (Capacitance-voltage) measurements have been made using a Keysight impedance analyzer E4990A, or a power device analyser B1505 when voltages higher than 40 V were required. As the MOS capacitors are on wafer chips, a probe station has been used to contact the gate and the substrate (back side). TSIC measurements have been performed using a Keithley SourceMeter K2636,and a cryostat in which the DUT (device under test) was glued with silver lacquer, to ensure a good thermal contact. The temperature range available is ~80 K up to ~600K. A vacuum of ~ 10 -6 mbar is obtained after a first heating to 600 K. The applied voltage is limited to 200 V by the apparatus.

Results and discussion

Preliminary measurements: CV at room temperature and at high temperature.

C-V measurements have been realized at room temperature at a frequency of 100 kHz (the highest frequency leading to still negligible series resistance effects), on a device with a diameter of 1.5 mm (Figure 1), in the two scan directions. From the maximum capacitance Cox in accumulation regime (V >0), the deduced oxide thickness is eox = 1.57 µm. As expected in 4H-SiC, no inversion layer is observed due to the very weak minority carrier generation rate. So the depletion regime is used to determine the doping level and the flatband voltage VFB by plotting (Cox/C)²-1 vs. VG (inset of Figure 1). From the slope, the doping level is estimated to be 8.0×10 14 cm -3 , which is in agreement with the epilayer data sheet.

No difference is observed between C(V) curves in the two scan directions when the gate voltage VG is limited to -100 V (arbitrary chosen for first precautious measurements). When biasing down to -200 V in depletion mode, a little shift of ~2 V is observed on VFB between the curves obtained in the two scan directions at 300K. VFB = -59,4 V using the sweep from accumulation to depletion mode, and -VFB = -61.6 V using the sweep from depletion to accumulation mode. The difference of ~2V in VFB values may be due to the presence of deep states (interface states which have a longer response time and that can discharge during the bias sweep, either because they are deeper in the bandgap, either because they are a little deeper in SiO2 than interface states) which need a longer time to discharge, so that a scan from -100 V to positive bias is not long enough to allow deep traps to emit their electron. On the contrary, scan from -200 V to positive bias is longer and emission of electrons from deep traps is more probable and VFB is a little reduced. Of course, such high values of VFB are no more observed in thermally grown oxide, usual values are in the [-5V, 5V] range [8]. However, =

(Cox/C)²-1 V G (V)
these large negative values of VFB indicate the presence of a large amount of positive charges in the oxide Qox. If these charges are fixed charge at the SiO2/SiC interface their density should be at least of 8×10 11 cm -2 to result in VFB ~ -59 V. But it is more probable that they are also mobile charges or charges trapped in the oxide bulk. Similar C-V measurements have been performed at 560 K. A clear difference is observed between the two sweep directions, resulting in a higher VFB shift ( ~37 V, resulting in VFB ~-100 V when sweeping from +200V to -200 V). Note that the hysteresis loop between the two C-V curves shows a clockwise rotation at 300 K but a anti-clockwise rotation at 560 K. The anti-clockwise rotation should be relevant of the presence of mobile charge in the oxide. Indeed, if positive mobile charges are present in the oxide, when VG >0, they are at the SiO2/SiC interface and so they induced a negative flatband shift. When they are at the gate/SiO2 interface, they do not induce any flatband shift, and the C-V from depletion to accumulation is shifted toward the right. It results in an anticlockwise rotation! A shift of ~37 V should be relevant to a mobile charge density of ~5×10 11 cm -2 . To compare, mobile charges in SiO2 grown on silicon are inexistent, or so low that undetectable and without consequence. Two of the most recent publications about alkali ions in SiO2 grown on Si date from 1984 [9] and 1988 [10], and in these studies, alkali ions have been intentionally introduced in the oxide. This is the reason why there is a lack of knowledge about ions mobility in SiO2.

Previous to TSIC measurements, isothermal I-V measurements have been performed at 300 K between -200 V and 200 V and no significant current ( > 1 pA) have been measured. It is consistent with the high thickness of the oxide layer (e.g. no Fowler-Nordheim mechanism is possible with such a thickness).

TSIC measurements

In order to understand the origin of this large negative flatband voltage shift, TSIC measurements have been performed. This technique has been developed by Hickmott [11] to study the effect of sodium ions in SiO2 grown on silicon, and also by Choquet et al. [10] to study Na and K ion in SiO2. This technique consists in measuring with a picoammeter the current I flowing across the oxide, i.e. between the gate and the substrate of a MOS structures as a function of temperature T.

The structure is preliminary heated at high temperature (in our case at 600 K for the worst tests) under a constant gate bias (the trapping voltage) VG, so that the mobile ion, if there is, are driven by the electric field in the oxide towards the interface. If the mobile ions are positive, which is generally assumed for alkali ions such as Na + or K + , they are driven and trapped at the SiO2/SiC (ox/SiC) interface when VG >0, and at the metal/SiO2 (G/ox) interface when VG <0. After that the structure is cooled down to 100 K. Then, the gate bias is changed to -VG (the detrapping voltage) and the temperature is increased with a fixed rate a. In our case, a has been fixed to either 0.1 or 0.2 Ks -1 so that the heating sweep does not exceed ~90 min. When the mobile ions are detrapped from the interface, the picoammeter measures a current peak at a temperature named Tmax. Knowing the heating rate, the area A under the I-T curve is related to the density of the detrapped mobile charges by the relation A = qa Nm. Details of the technique are described in Ref. [10]. Tmax is related to the energy E of the detrapping by (Eq. 1)

where n is a factor ranging from 10 10 to 10 13 s -1 eV -1/2 and k the Boltzmann constant. Solving (Eq.1) using a mean value of n = 5×10 11 s -1 eV -1/2 allows to calculate E for each value of Tmax. The oxide electric field xox is calculated by (Eq.2)

So for each value of VG, Tmax can be related to the corresponding xox. The plot of E vs. xox 1/2 and its slope c is useful to compare our results to other published results. Since VFB varies drastically with voltage at high temperature, a variation of the electric field during the heating sweep should be considered. Figure 2 shows different TSIC spectra as a function of a positive detrapping voltage (positive -VG applied during heating). The inset is a zoom on the current between 200 and 350 K. Mainly 4 peaks are observed during the sweeping from low to high temperature. The maxima of these 4 peaks seem to be dependent on the applied voltage. Two of them appear at ~250 K (peak A) and ~300 K (peak B) and are of relatively small magnitude (i.e. 1-10 pA). The two others appear at higher temperatures (between 450K and 530 K for peak C, and at 550 K for peak D) and are of larger amplitude. These peaks are relevant for detrapping at the G/ox interface.

2 max max E E exp kT kT ae ö u = - ç ÷ ç ÷ a è ø G F B ox ox V V e - x =
To be accurate, the current must be recorded also during temperature scan without reversing the voltage supply at low temperature. The current so measured is called the baseline. It should consist in leakage current which should occur without the contribution of the mobile charges. Since each scan is time consuming, the baselines have been measured for trapping voltages of 120 V, 200 V and -120 V (Fig. 3). Therefore the real TSIC measurement is obtained by subtracting the baseline from the scan obtained at the same trapping voltage value. A surprising feature of these baselines is the presence of a peak at ~540 K. We believe that it is due to an incomplete trapping at high temperature before cooling down . Two reasons can be considered : i) maybe the time of trapping is not long enough; ii) maybe the temperature of 600 K is not high enough. The inset in Figure 3 shows a small peak that appears on the baseline for positive VG. We have no explanation for this peak, unless it is due to the ionization of carriers in the N-type epilayer, which could lead to the formation of an inversion layer and modify the electric field in the oxide. However, the magnitudes of these baselines are very low and therefore they do not modify the analyses of the TSIC spectra. Peak C and D show higher current, and they do not appear at low VG value due to the temperature limitation of the cryostat. Peak C begins to appear at -VG = 80 V. Charge densities corresponding to peak C and D are found in the range 10 12 -10 13 cm -2 , as reported in Table 1. Figure 4 shows the spectra of the negative detrapping gate bias after trapping at high temperature.

The detrapping curves shown in Figure 4 are similar with the trapping ones presented in Figure 2 for voltages below -80V and above. The densities of mobile charges have been calculated for -VG =-120 V and are in the same range as the positive detrapping voltages. For comparing our results with the published ones [10 and references therein], it is convenient to calculate the detrapping energy of mobile charges vs. the oxide electric field, and, if possible, its variation with xox. Detrapping energies have been calculated as detailed previously by solving (Eq.1)

for each peak at every detrapping voltage. xox have been calculated using (Eq.2) with VFB = -59 V, which is a reasonable assumption considering Fig. 1 for measurements subsequent to a positive trapping voltage. After positive trapping voltage, xox have been calculated using VFB = -100 V. For these high values, the main mobile charges have been indeed drifted to the ox/SiC interface and we can assume that the flatband voltage is at its minimum. The spectrum obtained with VG = -80 V has not been used to determine the activation energy because it is not possible to evaluate correctly the electric field in the oxide, which probably is very low. x 1/2 ox (MV 1/2 cm -1/2 ) c = -0.43

For peak D, the activation energy is about 1.5 eV (for VG = ±120, ±160 and ±200V) which is not dependent on gate voltage, i.e. on the electric field. It means that the traps that capture mobile charges are similar at the two interfaces (gate/oxide and oxide/semiconductor). For peak C, the detrapping energy is lower, i.e. in the range ~1.2eV and varies with the applied voltage, therefore with xox. Figure 5 shows the detrapping energies for the different observed peaks. The c value obtained for peak C is -0.43 eV MV -1/2 cm 1/2 at the ox/SiC interface and -0.72 eV MV -1/2 cm 1/2 at the G/ox interface.

Peak B corresponds to a trap with an energy in the range 0.76 -0.8 eV at the G/ox interface and 0.64-0.71 eV at the ox/SiC interface. The variation at the G/ox interface is not clear. Although, at the ox/SiC interface it seems to decrease with c = -0.24 eV MV -1/2 cm 1/2 . Peak A is observed only at the G/ox interface with an energy similar to that of peak B at ox/SiC interface.

Previous studies [10] on thermal oxide grown on 6H-and 3C-SiC have shown that traps for mobile charges are similar in 6H-SiC, 3C-SiC and Si structures, therefore these interfacial traps are not fundamentally dependent on the interface. One of these trap energies was in the range 1.3-1.5 eV with c ~-0.6 eV MV -1/2 cm 1/2 , which is similar to the peak C at ox/SiC interface

The other peaks present no similar characteristics with the traps in 3C or 6H-SiC structures [10]. The difference may be due to the different nature of the oxide : i.e. thermally grown in Ref. [10] vs.

deposited in our study. However, the presence of a thin interfacial thermally grown oxide can explain some of the similarities between trap C (ox/SiC face) and the one observed in [10].

Yet, the detrapping energy value depends on the traps nature, and it is probable that the value of c depends also on the nature of trapped mobile charges. Therefore, it is worth to identify the physical nature of the mobile charges that are presumably positive alkali ions. With this aim, SIMS measurements have been performed.

SIMS analyses

The presence of Na and K alkali ions was investigated by SIMS measurements, performed by EAG Laboratories, with a Physical Electronics Model 6600 equipment, using oxygen bombardment with positive-ion detection. The concentration detection limits are 10 13 and 2×10 13 atoms/cm 3 for K and Na species respectively. The SIMS depth concentration profiles of both Na and K within the SiO2 passivation layer under study are shown in Figure 6. These profiles indicate clearly the presence of both species at the surface of the oxide layer, with concentrations above 10 17 atoms/cm 3 close to the surface. A tailing of the Na profile through the silica layer, and a peak at the ox/SiC interface (~10 16 atoms/cm 3 ) can be observed. However, the peak at the ox/SiC interface may be considered as an artifact of the SIMS measurement. Although, during the analysis an optimization was performed in order to minimize the Na migration impact. Compared to Na, a lower K concentration was observed into the depth of the oxide layer. The K concentration seems almost stable at ~3×10 14 atoms/cm 3 up to the ox/SiC interface whereas a peak of ~2×10 15 atoms/cm 3 can be seen. The K species are not as mobile as Na species during the measurement, which can reflect a more reliable concentration profile. In this case, an evaluation of the K ion dose present in the whole oxide layer was determined.

Thus, a dose of 5.26×10 12 atoms/cm 2 was derived from the SIMS profile (principally related to the surface region contribution). This dose value is similar with the charge densities values obtained for peaks C and D, as described above. As a result, the surface contamination by Na and K ions as determined by SIMS analysis seems to be the source of positive mobile ions involved in the results obtained by C-V and TSIC measurements.

Conclusion

Thermally stimulated ionic current measurements have been performed up to 200 V and 600 K in order to study the oxide interfaces traps in MOS structures fabricated with a thick oxide deposited by PECVD on n-type 4H-SiC epilayers. These thick oxides are not intended to realize gate oxide but to make passivation layers. SIMS measurements show the presence of K + and Na + species in a large concentration. TSIC spectra shows a detrapping of mobile ions in at least four kinds of traps, with energies at ~1.5 eV, 1.2-1.4 eV, and others between 0.6 and 0.8 eV. Note that one of these peak seems to be only present at the oxide/gate interface. These traps are different (energy values and their dependences with the electric field) from traps detected in thermally grown oxide on 3C and 6H-SiC epilayers, except for a trap observed at the ox/SiC interface (i.e. trap C). This difference can be due to the different oxide natures (deposited vs. thermally grown). On the other hand, the similarity at the ox/SiC interface can be due to the presence of a thin intermediate wet oxide.

Figure 1 :

 1 Figure 1 : C-V characteristics measured at 100 kHz on the 1.5 mm Ø capacitors at 300 K and 560 K.

Figure 2 :

 2 Figure 2 : TSIC spectra recorded under several positive detrapping gate bias -VG varying from +10 V

Figure 3 :

 3 Figure 3 : TSIC baseline recorded under several gate bias VG (-120V, +120 V and 200 V) after a

Figure 4 :

 4 Figure 4 : TSIC spectra recorded under several detrapping gate bias -VG varying from -80 V to -200

Figure 5 :

 5 Figure 5 : Synthesis of the activation energies extracted from TSIC spectra for detrapping voltage -

Figure 6 :

 6 Figure 6 : SIMS depth concentration profiles of Na and K within the SiO2 passivation layer. O and C

Acknowledgment

This study was conducted in the framework of 'FilSiC' research program. The authors would like to thank the French Ministry of Economy and Finance for the funding of this research project. They also gratefully acknowledge Lionel DUPUY, Amélie TERRAT and Alexandra SERVEL for the test stuctures fabrication.

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.

Extracted parameters from these spectra are summarized in Table 1. Peak A gives an energy E ~ 0.67 eV and peak B an energy E ~0.78 eV. For -VG = 20 V, peak A does not appear, instead at 200 V peak A is higher than peak B. Up to ~100 V, the density of the charges which correspond with peak A and B is ~4×10 11 cm -2 . This density increases upto 8×10 11 cm -2 for -VG = 200 V. It is possible that for lower VG value, the mobile ions have not been all trapped during the time the structure is heated at high temperature.


Nolan Eley
email: nolan.eley@sorbonne-universite.fr

Sarabeth Mullins

Peter Stitt

Brian Fg Katz

Virtual Notre-Dame: Preliminary results of real-time auralization with choir members (Invited Paper)

Keywords: Heritage acoustics, room acoustics, performance evaluation, real-time auralization

Heritage acoustics studies are typically constructed around a listener's perspective, using prerecorded source material. The room acoustic response is generated via numerical simulation, such as via geometrical acoustics with proper model calibration. As part of a series of studies investigating the acoustics of heritage spaces and the impact of the acoustics on musical performance, a real-time auralization architecture has been developed and adapted to the requirements of recreating the acoustic conditions within the Cathedral of Notre-Dame de Paris for a choir ensemble. This paper reviews the processing architecture of the system, expanding on previous Experimental Virtual Archaeological Acoustics (EVAA) developments, and an objective level calibration performed for reproduction via headphones and a 32-channel loudspeaker array. A preliminary subjective evaluation was carried out with members of a medieval choir, having experience in Notre-Dame, concerning qualities of the rendering and the calibration procedure. Following the evaluation by this user group, revisions have been made to the rendering architecture with the goal of providing a realtime performance simulator for a four-member ensemble using an anechoic room and individual binaural renderings over open headphones. Further evaluations will concern relevance of additional modifications of the rendering system to improve realism, perception of architectural and acoustic changes related to the historic records of the space, and the impacts of these on musical performances of different genres and periods through objective and subjective analysis of the performances using the real-time auralization rendering system.

I. INTRODUCTION

Room acoustics play an important role in the composition, performance, and perception of music, and the link between acoustics and performance practice has been observed and documented for centuries [START_REF] Schiltz | Church and chamber: the influence of acoustics on musical composition and performance[END_REF]. Historically, the interaction between acoustics and performance practice has been loosely documented and based on experience, however, the number of serious scientific investigations into the matter has been increasing in recent decades. Seeking to find which acoustic qualities affect musicians and to what extent, these studies have used either real acoustic environments [START_REF] Chiang | Subjective Assessment of Stage Acoustics for Solo and Chamber Music Performances[END_REF]- [START_REF] Luizard | Singing in different rooms: Common or individual adaptation patterns to the acoustic conditions?[END_REF], virtual acoustic environments (VAE) [START_REF] Kato | Musicians' Adjustment of Performance to Room Acoustics, Part III: Understanding the Variations in Musical Expressions[END_REF]- [START_REF] Garí | Analysis of trumpet performance adjustments due to room acoustics[END_REF], or both [START_REF] Marshall | Acoustical conditions preferred for ensemble[END_REF]- [START_REF] Luizard | Singing in physical and virtual environments: how performers adapt to room acoustical conditions[END_REF]. While the findings have been mixed, some general trends can be extracted. For example, reverberation time tends to have one of the most measurable effects on a musician's performance [START_REF] Kato | Musicians' Adjustment of Performance to Room Acoustics, Part III: Understanding the Variations in Musical Expressions[END_REF]- [START_REF] Fischinger | Influence of virtual room acoustics on choir singing[END_REF], [START_REF] Kawai | Experiment on adjustment of piano performance to room acoustics: Analysis of performance coded into MIDI data[END_REF], [START_REF] Kalkandjiev | The Influence of Room Acoustics on Solo Music Performances[END_REF]. More recently, however, spatially defined acoustic parameters have emerged as showing great importance compared to more traditional omnidirectional parameters [START_REF] Panton | Chamber musicians' acoustic impressions of auditorium stages: Relation to spatial distribution of early reflections and other parameters[END_REF]. Timbre and loudness tend to be among the most affected performance parameters [START_REF] Luizard | Singing in different rooms: Common or individual adaptation patterns to the acoustic conditions?[END_REF], [START_REF] Kato | Effect of Room Acoustics on Musicians' Performance. Part II: Audio Analysis of the Variations in Performed Sound Signals[END_REF], [START_REF] Garí | Analysis of trumpet performance adjustments due to room acoustics[END_REF], [START_REF] Kalkandjiev | The Influence of Room Acoustics on Solo Music Performances[END_REF]. Performance parameters related to timing have also been described in many studies as having some correlation with reverberation time, [START_REF] Kato | Musicians' Adjustment of Performance to Room Acoustics, Part III: Understanding the Variations in Musical Expressions[END_REF]- [START_REF] Fischinger | Influence of virtual room acoustics on choir singing[END_REF], [START_REF] Kawai | Experiment on adjustment of piano performance to room acoustics: Analysis of performance coded into MIDI data[END_REF] although the exact correlation varies. One other important conclusion found in several studies is that consistent patterns of adaptation sometimes emerge in individual musicians even if there is an absence of common adaptation patterns among a group of musicians [START_REF] Kato | Effect of Room Acoustics on Musicians' Performance. Part II: Audio Analysis of the Variations in Performed Sound Signals[END_REF], [START_REF] Garí | Analysis of trumpet performance adjustments due to room acoustics[END_REF], [START_REF] Luizard | Singing in physical and virtual environments: how performers adapt to room acoustical conditions[END_REF].

The utilization of VAEs in these types of experiments allow the subject to experience a wide variety of acoustics (including acoustic spaces which no longer exist) in a very short amount of time, making comparisons between different acoustic environments easier and more reliable by avoiding long time intervals between experiences [START_REF] Gade | Investigations of musicians' room acoustic conditions in concert halls. part i : Methods and laboratory experiments[END_REF]. However, reliable results from such a study depend on an accurate reproduction of the acoustic properties of the space in question [START_REF] Gade | Acoustics for Symphony Orchestras; status after three decades of experimental research[END_REF]. With recent advances in auralization demonstrating a high degree of plausibility [START_REF] Lindau | Assessing the Plausibility of Virtual Acoustic Environments[END_REF], [START_REF] Postma | Subjective evaluation of dynamic voice directivity for auralizations[END_REF], studies using VAEs have become more commonplace.

Expanding on previous Experimental Virtual Archaeological Acoustics (EVAA) developments, this preliminary study sought to assess the perceptual and ecological validity of this auralization architecture, specifically for studying the interaction between musicians and acoustics with an eye towards future studies in acoustic heritage. Four professional singers performed in a VAE and provided feedback through questionnaires. The implementation and calibration of the VAE are described in detail. The results of the questionnaire are discussed followed by recommendations for improving the system for use in future studies.

II. CONTEXT

The EVAA system is a technical framework used for ongoing research and documentation of acoustic heritage and historically-informed performance utilizing virtual reality and real-time auralization [START_REF] Katz | EVAA: A platform for experimental virtual archeological-acoustics to study the influence of performance space[END_REF] and is developed in the context of a suite of ongoing archaeoacoustic research projects occuring at Sorbonne Université [START_REF] Postma | Virtual Reality Performance Auralization in a Calibrated Model of Notre-Dame Cathedral[END_REF]- [START_REF] Postma | The influence of visual distance on the room-acoustic experience of auralizations[END_REF]. Studies utilizing similar VAEs to auralize heritage sites have taken a variety of approaches including both a posteriori and real-time convolution of dry audio signals with relevant room impulse responses (RIRs) [START_REF] Postma | Virtual Reality Performance Auralization in a Calibrated Model of Notre-Dame Cathedral[END_REF], [START_REF] Autio | Historically Based Room Acoustic Analysis and Auralization of a Church in the 1470s[END_REF]- [START_REF] Postma | An archeoacoustic study of the history of the Palais du Trocadero (1878-1937)[END_REF].

This study is a part of the "Past Has Ears" (PHE) project, an ongoing research consortium investigating and developing a methodology for archaeoacoustic inquiry and using heritage sites in France, Italy, and the United Kingdom as case studies [START_REF] Katz | The Past Has Ears (PHE) : XR Explorations of acoustic spaces as Cultural Heritage[END_REF]. The study lays the groundwork for further research into the acoustic heritage of Notre-Dame de Paris, the heritage site at the center of the French component of PHE. The EVAA system delivered a virtual reconstruction of the acoustics of the cathedral based on a geometrical acoustics model which was calibrated to measurements taken prior to the 2019 fire as outlined in [START_REF] Postma | Virtual Reality Performance Auralization in a Calibrated Model of Notre-Dame Cathedral[END_REF].

The acoustics of Notre-Dame are the subject of ongoing historical, sociological, and musicological interest as a matter of intangible heritage [START_REF] Katz | The Past Has Ears (PHE) : XR Explorations of acoustic spaces as Cultural Heritage[END_REF], with particular attention paid to the role it may have played in the development of musical practice during the Middle Ages. The School of Notre-Dame was a musical movement active in the 12th and 13th centuries in Paris and is notable for its innovations in the nascent practice of polyphony [START_REF] Hoppin | Medieval music: Richard H. Hoppin. The Norton introduction to music history[END_REF], [START_REF] Wright | Music and Ceremony at Notre-Dame de Paris[END_REF]. Since the School developed in close temporal proximity to the initial opening of the cathedral, musicologists are interested in researching what influence Notre-Dame may have had on the local composers during the time period. A perceptually validated VAE system is therefore relevant to furthering the interdisciplinary research of the acoustic heritage and impact of Notre-Dame.

III. OVERVIEW

A. Study Participants

The four singers who participated in the study are members of "Rue des Chantres", a medieval ensemble with a particular focus on the musical compositions of the Notre-Dame school of polyphony in the 12th and 13th centuries. As specialists in historically informed performance, they have performed alone or as an ensemble inside Notre-Dame de Paris, the Basilique Cathédrale Notre-Dame d'Amiens, and other venues associated with medieval music. As the aim of this study was to perceptually validate the virtual replication Notre-Dame's acoustics prior to the fire of 2019, the musicians were chosen for their familiarity with the acoustics of the cathedral.

The musicians sang excerpts of both sacred and profane medieval repertoire which they had previously prepared. While no time limit or minimum duration was requested of the musicians, the musicians spent between 10 min to 15 min interacting with the VAE in each reproduction condition (multichannel loudspeaker and headphone reproduction). The interactions were musician-led, with each participant free to interact with the VAE as they chose. The participants sang both full excerpts and subsections while standing or moving within the VAE. They also interacted with the VAE by singing plo- sives, fricatives, and sibilants independent of a larger musical context.

B. Reproduction System Architecture

The EVAA system uses a VST3 plugin to perform convolution of a monophonic input with RIRs that allows for dynamic directivity [START_REF] Katz | EVAA: A platform for experimental virtual archeological-acoustics to study the influence of performance space[END_REF]. The previously documented version of the system used a uniform partitioned algorithm with multi-thread processing in two versions: high spatial update latency/minimum convolutions and minimum latency/convolution matrix. The current version used in this study has been updated to use non-uniform partitioned convolution [START_REF] Gardner | Efficient convolution without input-output delay[END_REF], since this allows for more efficient processing and the study used the matrix convolution algorithm in order to ensure good spatial reactivity for the subjects. The directivities were defined as 3rd-order spherical harmonics using a soprano voice directivity pattern [START_REF] Shabtai | Generation and analysis of an acoustic radiation pattern database for forty-one musical instruments[END_REF]. The virtual receiver was 3rdorder Ambisonics. This leads to a 16 × 16 filter matrix.

Due to the length of the impulse responses in Notre-Dame, only the first 1 s was generated via GA simulation, using the CATT-Acoustic (v9.1) and TUCT (v2.0e:1.02) software [START_REF] Dalenbäck | CATT-Acoustic v9.1 with TUCT v2[END_REF]. The remainder of the RIR was approximated by using the IEM FdnReverb [33], a feedback delay network (FDN) reverb that takes higher order ambisonics (HOA) [START_REF] Daniel | Représentation de champs acoustiques, application à la transmission et à la reproduction de scènes sonores complexes dans un contexte multimédia[END_REF] as input. The TUCT simulation used 2 000 000 rays while implementing algorithm 1. The source representing the singer location was modelled as a coincident pair with a receiver located roughly 2 cm away from the source, positioned in the choir of the cathedral. The general location of the singer positions are indicated in red on the floor plan (see Fig. 2), in between the choir stalls oriented along the long axis of the cathedral. The resulting RIRs are preprocessed in order to remove the direct sound as this is not needed for auralization. A more thorough discussion of the creation and calibration of this GA model can be found in [START_REF] Postma | Virtual Reality Performance Auralization in a Calibrated Model of Notre-Dame Cathedral[END_REF], [START_REF] Postma | Creation and calibration method of acoustical models for historic virtual reality auralizations[END_REF]. The output of the EVAA plugin was input to the FDN reverb and both outputs were passed to the Ambisonic decoder.

The implementation of the FDN for the remainder of the RIR assumes that the energy decay rate is a single exponential (i.e. linear). This is not truly the case, as was highlighted in [START_REF] Katz | An Acoustic Survey of the Cathédrale Notre-Dame de Paris before and after the Fire of 2019[END_REF]. However, for this preliminary study, it is not expected that the secondary decay will contribute significantly to the musicians performance. A supplemental model will include a second FDN to provide this later decay.

The IEM BinauralDecoder [33] was used for binaural rendering when reproducing the EVAA system output via headphone. This plugin uses the MagLS decoding method [START_REF] Schörkhuber | Binaural Rendering of Ambisonic Signals via Magnitude Least Squares[END_REF] with the widely used KU 100 dummy head HRTF. Decoding over loudspeakers was done using Spat decoder set to AllRAD [START_REF] Zotter | All-round ambisonic panning and decoding[END_REF] and max r E weighing. A 32-channel loudspeaker array was used for loudspeaker reproduction.

The signal from the singers was captured using a wireless lavalier microphone (DPA 4060) mounted on a headset, with the microphone resting several centimeters from the singer's mouth. The singers also wore a head-mounted motion capture device in order to capture the orientation and movement of the singers for properly rendering the directional characteristics of their voice. An OptiTrack motion capture system and its associated Motive (v1.8.0) software were used for this purpose and the tracking information was routed to Max 8 for further processing.

The experiment took place in the semi-anechoic (T 30 = 0.14 s at 1 kHz) motion capture/virtual reality room at Sorbonne Université. The test sessions were run with the plug-in hosted in Max 8, sample rate of 44.1 kHz with buffer sizes of 256 samples (5.8 ms).

C. Experimental Overview

The singers sang their prepared repertoire with the simulated acoustics being conveyed through multichannel loudspeaker reproduction and through headphone reproduction using binaural rendering. In addition to these two performances, subjects were also asked to explore the dynamic directivity of the system by turning while vocalizing in order to assess the impact of its inclusion. As a novel component of the EVAA system architecture, the dynamic directivity module was included to evaluate its perceptual significance to a singer within the VAE. Since the source and receivers are located inside the choir, the early-returning lateral energy from the choir stalls was more likely to be noticeable by the singers as they interacted with the VAE.

The subjects were also each explicitly asked about the overall level of the system response at the beginning of each experience through loudspeakers and headphones in order to assess the effectiveness of the level calibration procedure (see Section IV). The gain was then adjusted to their desired level before beginning. Throughout the experience the subjects were free to comment, and these comments were recorded contemporaneously. The singers also filled out questionnaires related to the perceptual validity of the experience.

In addition to participating as singers, the musicians also participated as listeners, in order to comment on the perceptual validity of a nearby listener position. With an aim at a future expansion of the EVAA system to study ensemble performances, the listener position was modeled after a position that would be typical in an ensemble setup, roughly 1 m away from the singer. This auralization was created in the same way as the singer position, however the direct sound was not removed. The listeners were set up in another room so they could not hear the singer directly. They were free to comment throughout the process, and they also filled out questionnaires related to the perceptual validity of their experience.

The questionnaires used in this study were aimed at evaluating the validity of the system for music performance. Subjects were asked to rate their experience singing in the simulated acoustics in comparison to their previous experience singing in Notre-Dame in terms of reverberation time, reverberation color, envelopment, and impression of room size. For example, if they rated the reverberation time as longer, they would be indicating that the virtual acoustic environment had a longer reverberation time than Notre-Dame. The questions were administered to the participants as a 5-point Likert scale with the middle point indicating a "similar" rating and the remaining points labeled appropriately for each specific category. The questionnaires were administered to the participants in French.

Additionally, the subjects were asked to rate the virtual acoustics in terms of naturalness and overall acoustic impres-sion on a 1 to 5 scale. The same questionnaire was administered to the subjects when they participated as listeners. The questionnaires were based on previous work assessing musicians' evaluations of acoustics [START_REF] Gade | Investigations of musicians' room acoustic conditions in concert halls. part i : Methods and laboratory experiments[END_REF], [START_REF] Miranda Jofre | Evaluation of stage acoustics preference for a singer using oral-binaural room impulse responses[END_REF]- [START_REF] Ko | Augmented stage support in ensemble performance using virtual acoustics technology[END_REF]. In addition to the formal questionnaires, the singers' comments were recorded throughout the duration of the experiment.

IV. LEVEL CALIBRATION PROCEDURE

In preparation for the experiment, a level calibration was performed in order to determine the proper level of the simulated acoustics, relative to the direct sound of the singers. As both headphones and a multichannel speaker array were used to convey the simulated acoustics, a level calibration was performed for each of these methods. Several procedures have been developed for this purpose including [START_REF] Garí | Analysis of trumpet performance adjustments due to room acoustics[END_REF], [START_REF] Kalkandjiev | The Influence of Room Acoustics on Solo Music Performances[END_REF], [START_REF] Laird | Energy-based calibration of Virtual Performance Systems[END_REF], [START_REF] Brereton | Singing in Space(s): Singing performance in real and virtual acoustic environments-Singers' evaluation, performance analysis and listeners' perception[END_REF]. None of these reported methods have achieved perfect subjective validation so this calibration step should be used to achieve a relatively reliable starting level rather than relied upon to determine an absolute fixed level. Minor adjustments can be expected based on musicians' subjective experience, although, these should hopefully not deviate too far as this would indicate a serious inaccuracy in the calibration procedure.

For this study we utilized a slightly modified version of the procedure outlined in [START_REF] Laird | Energy-based calibration of Virtual Performance Systems[END_REF] which compares the late support parameters, ST late [START_REF]1(E) Acoustics -Measurement of room acoustic parameters -Part 1: Performance spaces," standard[END_REF] (see eq. 1), between the VAE and the real acoustic space to produce a gain coefficient by which the simulated acoustics could be amplified or attenuated. However, instead of using ST late measured in the real acoustic space, this parameter was obtained from the GA model as this was the source of the auralization. A set of beam RIRs was produced from the GA model corresponding with the same source and receiver configuration. A loudspeaker (Genelec 8030A) produced an exponential sine sweep which was captured by an omnidirectional microphone (DPA 4060), a few centimeters away, the same microphone and position used for the singers in the VAE. This signal was then run through the EVAA system using the set of RIRs described above using a soprano singer (fortissimo) directivity pattern. This was then captured by an omnidirectional microphone (DPA 4006) positioned 1 m away. This sweep was deconvolved in order to create an impulse response, which was then used to calculate the ST late parameter. This measure was compared to the same parameter calculated from the RIR produced by the GA model and a difference, in dB, was calculated between the two. This gain was then applied to the output of the system during the experiments.

The level calibration procedure for the headphones was essentially the same as the above, only in place of the omnidirectional microphone (DPA 4006), a binaural dummy head was used (Neumann KU 80, DPA 4060 microphones placed at the entrance of each artificial ear canal) and a pair of open headphones (Sennheiser HD 600) placed over the ears.

V. RESULTS AND DISCUSSION

The results of both the listener and singer questionnaires (see Fig. 4 and Fig. 5) indicate that the VAE was evaluated as being relatively similar to Notre-Dame. However, this is according to sample of 4 participants who based their comparisons on previous experiences which occurred at least two years prior to this study. Therefore, any conclusions drawn from this experiment should take this into consideration.

In terms of naturalness, the virtual acoustics were rated, on average, as "somewhat natural" while the mean overall acoustic impression of the singers was a four out of five, however, the listener position was judged slightly below three.

In addition to these questionnaires, we also asked each singer to comment on the dynamic directivity of the system. Three out of four singers said that they could not notice a difference in the sound if they rotated while vocalizing. However, one singer commented that he noticed a slight difference when rotating that added to a sense of envelopment. The singers were also asked to compare their experience singing through loudspeaker playback with their experience with playback through a pair of open headphones. Two of the singers preferred the simulated acoustics being conveyed through loudspeakers while one singer preferred the playback through headphones. Another singer had mixed feelings, saying that the acoustic feedback from the headphones sounded more realistic but that it was more difficult to hear himself.

In order to gauge the effectiveness of the level calibration procedure, each of the singers was asked about the overall level of the room response. Through loudspeakers, only one singer requested that the gain of the system be increased by approximately 2 dB. Through headphones, three out of four singers requested a similar gain change. This suggests that the level calibration procedure is generally well-designed, especially for loudspeaker reproduction. However, the disparity between the desired level through loudspeakers compared to headphones is something that merits further investigation.

Despite rating the reverberation color of the VAE as similar to Notre-Dame, in oral feedback following the recording session the participants noted an unnatural and metallic coloration in the higher frequencies of the VAE. This feedback prompted a closer examination of signal flow through the EVAA system and revealed a fault in the calibration of the IEM FdnReverb mentioned in Section III-B. The FDN portion of the system was recalibrated to bring the total synthetic IR of the EVAA system into closer alignment with the measured RIRs of Notre-Dame. Further examination is required to determine if the FDN settings were responsible for the entirety of the coloration issue.

VI. CONCLUSIONS AND FUTURE WORK

When presented in isolation, the calibrated EVAA environment was perceived as generally plausible and similar to Notre-Dame. While this assessment is dependent on the recollection of study participants, it is an encouraging sign that development on the VAE is progressing towards realism. The inclusion of dynamic directivity in the system architecture did not appear to tangibly improve the participant's experience of the system. As most participants assumed a singing posture quickly and did not deviate from the posture and position unless prompted, it is thought that future iterations of this system intended for use by choral ensemble may not require motion tracking to maintain immersion. Additionally, the gain initialization and calibration procedure seems to work well for setting preliminary levels prior to participant interaction with the VAE, especially with the multichannel loudspeaker reproduction system.

The study also revealed room for improvement, most especially regarding the high-frequency coloration noted by participants. Participant feedback prompted a re-examination of the FDN used to simulate the late reverberation of the cathedral, and a flaw in the initial calibration was discovered as a result. Further inquiry is needed to determine if the FDN was the sole source of coloration in the reproduction. The musicians also reported a general preference for the multichannel loudspeaker system rather than binaural headphones. Since the eventual goal of the current project is to examine the musicians' performance as an ensemble, future work will focus on addressing musician concerns about comfort, low perceptions of envelopment, and difficulties hearing oneself while singing. Development on the EVAA system architecture will continue in preparation for a larger installation supporting experiments with the full ensemble.

Funding

  has been provided by the European Union's Joint Programming Initiative on Cultural Heritage project PHE (The Past Has Ears, phe.pasthasears.eu), the project PHEND (The Past Has Ears at Notre-Dame, No. ANR-20-CE38-0014, phend.pasthasears.eu), the Chantier Scientifique CNRS/MC Notre-Dame, the CNRS Mission for Transversal and Interdisciplinary Initiatives (MITI), and the Paris Seine Graduate School Humanities, Creation, Heritage, Investissement d'Avenir (No. ANR-17-EURE-0021 -Foundation for Cultural Heritage Science).

Fig. 1 .

 1 Fig. 1. View of 3D geometrical acoustics model of Notre-Dame.

Fig. 2 .

 2 Fig. 2. Floor plan of Notre-Dame with the location of the source/receiver singer-cluster indicated (•).

Fig. 3 .

 3 Fig. 3. Schematic diagram of EVAA system architecture.

Fig. 4 .

 4 Fig. 4. Individual responses (•) and means ( | ) to singer questionnaire.

Fig. 5 .

 5 Fig. 5. Individual responses (•) and means ( | ) to listener questionnaire.

ACKNOWLEDGMENT

The authors would like to thank the musicologists and musicians who participated in the study. The authors would also like to acknowledge Franck Zagala and David Poirier-Quinot for helpful advice and support.


Brian F G Katz
email: brian.katz@sorbonne-universite.fr

David Poirier-Quinot
email: david.poirier-quinot@sorbonne-universite.fr

Jean-Marc Lyzwa
email: jmlyzwa@cnsmdp.fr

La Vierge 2020 : Reconstructing a Virtual Concert Performance Through Historic Auralisation of Notre-Dame Cathedral (Invited Paper)

Keywords: Heritage acoustics, room acoustics, auralization

In honour of the International Year of Sound and for the 1 year memorial of the Notre-Dame cathedral fire, a team of researchers and sound engineers created a virtual reconstruction of a concert in the cathedral, using close-mic recordings made on 24-April-2013 of a performance of La Vierge by Jules Massenet. This reconstruction was carried out during the period of strict COVID-19 confinement. With 83 musicians, 6 singers and a 160person choir spatially distributed throughout the cathedral, the original performance offered a spatial composition highlighting the complex acoustics and interactions between source and listener positions. Individual tracks were convolved with spatial room impulse responses, created from a calibrated geometrical acoustic simulation. Several listening positions were binaurally rendered, along with an artistic mix created by a sound engineer, offering a unique 3d-audio experience approaching the reality of the moment in the past. Distributed on-line, the website presenting this virtual reconstruction has been visited from users around the world. A listener opinion survey was included on the website. This paper presents an overview of the source material, the production workflow, and the challenges of realising such a production during confinement. Finally, an overview of visitor statistics and survey results is presented, providing insight into the reception of the virtual recreation and the interest towards future productions.

I. INTRODUCTION

In honour of the International Year of Sound (sound2020.org) and for the 1 year memorial of the Cathedral of Notre-Dame de Paris fire, a team of researchers and sound engineers created a virtual reconstruction of a performance recording made in the cathedral, recreating the monumental acoustics and the event. As part of the celebrations of the 850 year anniversary of the construction of Notre-Dame de Paris, on 24-April-2013 the cathedral welcomed a performance of La Vierge. To share this performance with those who could not attend, and to honor the cathedral, damaged by the fire on 15-Apr-2019, acoustic researchers and sound engineers have taken these recordings and an acoustical 3D model of Notre-Dame, created from measurements, to recreate the concert from several listening positions; the place of the conductor, in the first row, middle audience, and rear of the cathedral, offering a unique 3d-audio experience approaching the reality of the moment in the past. This approach offers the possibility of realistic listening at a given position, which is of course not necessarily the ideal seat, nor is it comparable to the type of artistic mix we have grown accustomed to with broadcast or CD material which provides often a heightened readability and understanding of the work when compared to what is physically possible.

II. CONTEXT A. The Acoustics

Notre-Dame de Paris is among the most well-known worship spaces in the world. Its large volume, in combination with a relatively bare stone construction and marble floor, lead to rather long reverberation times. On the evening of 13-April-2015, almost 4 years to the day before the 2019 fire, a series of acoustic measurements were carried out following a small concert rehearsal. Subsequent to the fire, access was granted to the reconstruction site and acoustic measurements were carried on 30-June-2020. The full details of the measurement method, results, and comparisons with measurements made a few decades earlier can be found in [START_REF] Katz | An acoustic survey of the Cathédrale Notre-Dame de Paris before and after the fire of 2019[END_REF], [START_REF] Postma | Acoustics of Notre-Dame Cathedral de Paris[END_REF].

Results from these measurements can be represented by the calculated reverberation time. Figure 1 summarizes these results for three different measurements carried out in the Cathedral of Notre-Dame de Paris over more than 3 decades.

Further analysis of the energy decay curves showed a nonlinear decay typically associated with coupled volume acoustic conditions. This would seem reasonable considering the large and complex volume of the cathedral. These decay curves were analysed and the characteristic bending point in the decay were identified. Results of this analysis are shown in Fig. 2, highlighting the differences in early versus late reverberation times.

While the acoustics appears significantly altered since the fire, this is not a subject of the current study.

B. In Music History

For more than 850 years, Notre-Dame de Paris has been the sounding box for the city's sacred music. 1 Built to amplify the sacred word, spoken or sung, the Cathedral of Notre-Dame de Paris is a multiform sound space with the clamor of pilgrims circulating in the ambulatory and the chanting of services protected by the choir's enclosure. The Schola singing the mass and the humming of the low masses in the radiant chapels used to coexist until today, when before the fire, the call to silence for noisy visitors, the sound of the masses, the bursts of sound from the great organs and the concerts alternated.

Soon after its construction at the end of the 12th century, this Gothic cathedral became the emblematic place of musical creation in Europe, which historians call the " École de Notre-Dame". Documents testify to the intense musical activity during this period and it is possible to think that the spectacular development of this Parisian polyphony coincided with the organization of the liturgy in the new acoustic space inaugu-rated in 1182 2 Luckily we have the Treatise of Anonymous IV 3 , in which a witness to events describes musical practices circa 1275 in the choir of this cathedral where the sound of the organum and semi-improvised conductus could rise, towards the apse, before being noted in various manuscripts which testify to the richness of the Magnus liber organi (see [START_REF] Rosner | Magnus liber organi de Notre-Dame de Paris. Les quadrupla et les tripla parisiens[END_REF], followed by other volumes). During the period known as Ars Antiqua, i.e. the transition period between the École de Notre-Dame and Ars Nova (1320), the cathedral gradually lost its musical supremacy, although the chapter maintained the number of altar boys recruited for the Mastery and financed the construction of the choir organs (circa 1334) and the organ loft (circa 1415).

Over the centuries, the Gregorian chant remains the most important practice. The melodies escape from the closed choir mixed with the sounds of the organ, the bells, and sometimes with complex polyphonic works of the Franco-Flemish counterpoint (15th century). The history of music will remember the names of great masters and composers such as Antoine Brumel ( †1513), André Campra ( †1744), Jean-Franc ¸ois Lalouette ( †1728), or Jean-Franc ¸ois Lesueur ( †1837), who, after the revolutionary period, composed the famous March of the Coronation, for Napoleon's entry into the cathedral in 1804, and various pieces for the Coronation Mass. That was a great opportunity to hear the organ built by Franc ¸ois-Henri Clicquot (1783), restored afterwards by Aristide Cavaillé-Coll, while Viollet-le-Duc proceeded with the monumental restoration of the building.

The vaults heard everything during the 20th century, starting with the concerts of the greatest French organists and the most diverse musical groups, not forgetting the repertory of the liturgy. "In 1990, Cardinal Lustiger asked Jean-Michel Dieuaide to rethink the cathedral's permanent musical system. This reflection was carried out in collaboration with Guillaume Deslandres, who founded the association Musique Sacrée à Notre-Dame de Paris (MSNDP), of which he became the first director. (...). As in its early days, music continues to play a considerable part in the influence of Cathedral of Notre-Dame de Paris. The weekly organ auditions, the monthly organ recitals, the concerts given by the Maîtrise (choir of the cathedral), all testify to the high standards and excellence to which all those involved in music at Notre-Dame today apply" 4 .

C. Massenet's La Vierge

Passed on to posterity for the meditation of Thaïs or the letters aria from Werther, the figure of Massenet (1842-1912) remains attached to that of the French opera before Debussy's Pelléas et Mélisande. 5 Massenet also cultivated the oratorio, as evidenced by Marie-Madeleine (1873), Eve (1875), La Vierge (1880), and La Terre promise (1900). The distance between opera and oratorio, or between sacred and profane love, is tenuous at the end of the 19th century, which saw the birth of Franck's Les Beatitudes (1879) and Gounod's La Redemption (1882). It is therefore not surprising that Massenet's work was premiered at the Paris Opera on 22-May-1880, during the first of the Historical Concerts of the new director Vaucorbeil, alongside works by Lully, Rameau, Gluck, and Grétry.

Massenet, a pupil of Ambroise Thomas, who won the first Prix de Rome in 1863 and was appointed professor of composition at the Paris Conservatoire in 1878, had made a name for himself with the orientalist opera Le Roi de Lahore (1877) when he tackled his oratorio La Vierge, a figure he would find again in the lyrical miracle Le Jongleur de Notre-Dame (1902).

In 1875, Massenet met the young poet Charles Grandmougin who wrote for him the booklet of this sacred legend in four parts: The Annunciation, The Marriage at Cana, Good Friday, and The Assumption. He completed his score in August of 1879. The four parts retrace four key moments in the life of the Virgin Mary, contrasting situations exploited by Massenet. More dramatic, but also shorter, the two central parts draw on the operatic side with grandiose choirs (No. 5: Choeur du festin), funerals (No. 10: c'est Jésus qu'on mène au trépas), but also moments of choral recitation (Jésus s'est arrêté) reminding of Berlioz's (1803-1869) Roméo et Juliette ouverture. The two extremes would rather lean towards Berlioz's L'Enfance du Christ, with this initial, almost monodic Pastoral. The fourth scene remains the one that our 21st-century sensibilities understand best: the instrumental Andante religioso (No. 10: Last Sleep of the Virgin) is taken up again in the Choeur des Anges with a wonderful harp accompaniment, while the ecstasy of La Vierge, Infinite Dream, Divine Ecstasy, literally transports the listener to the afterlife before the Magnificat closes the ensemble.

D. Review of the 24-April-2013 performance

The following is an excerpt translation of the concert performance review by Patrice Imbaud:

Sacred legend in four scenes, for soloists, choirs, and orchestra. Libretto by Charles Grandmougin. Norah Amsellem, soprano. A brilliant work by its melodic line and the richness of its orchestration, grandiose and moving, served by a stage of more than 250 musicians, placed under the direction of Patrick Fournillier, undisputed specialist of the work of Jules Massenet, who leads from hand to hand, from end to end, with precision, commitment, and delicacy. Norah Amsellem, too often absent from the French stages, although she has had an international career for several years, knew how to give the Virgin all her humanity, through her remarkable singing, her perfect diction and her stage presence.

III. OVERVIEW

The creation of this virtual concert reconstruction followed several stages : recording, acoustic simulation & auralization, and mixing. Each of these elements is described briefly here. Links to more detailed presentations of each element are provided.

A. Concert recording

This recording of La Vierge by Jules Massenet was made on 24-April-2013 in the Cathedral of Notre-Dame de Paris. The work consists of an orchestra of 83 musicians, 6 solo singers and 2 choirs comprising 160 choristers. For this production, the performance was arranged with the objective of spatially staging the performers (see Fig. 3), highlighting the unique acoustics of Notre-Dame and the interactions between the different positions of the groups and the listener: Tribune on the courtyard side of the transept (distance 33.4 m from the conductor). A total of 56 microphones (main system, spot-mics, and ambiance) were deployed in order to capture this work. Full details of the recording setup are available in the recording documentation report [START_REF] Lyzwa | Compte-rendu de captation : La Vierge -Jules Massenet (1842-1912)[END_REF].

B. Acoustic simulation & Auralization

A computer room acoustic model of the Cathedral of Notre-Dame de Paris was created, as shown in Fig. 4. The geometry of the cathedral was determined from a 3D laser scan point cloud and architectural plans and sections. To each element of the model was assigned a specific acoustic material, related to the acoustic absorption and scattering properties, representative of the physical surface's roughness.

This geometrical acoustics model was then fine-tuned through calibration (see Fig. 5) to match the acoustic parameters obtained from measurements [START_REF] Postma | Creation and calibration method of virtual acoustic models for historic auralizations[END_REF]. Beyond acoustic parameter comparisons, this calibration was further evaluated through perceptual comparisons [START_REF] Postma | Perceptive and objective evaluation of calibrated room acoustic simulation auralizations[END_REF].

Within the calibrated model, each soloist and/or section of the orchestra which were recorded on a separate track were defined as an acoustic source, with a corresponding representative directivity pattern. Virtual listener locations were then defined, and the acoustic propagation characteristics of the cathedral, for each source-receiver combination, were simulated using a geometrical acoustics prediction software (CATT-Acoustic v9/TUCT26 ), a process taking several weeks considering the large number of sources and complex room geometry. This calculation produced Room Impulse Responses (RIRs) for each source-receiver combination.

With these RIR results, which are represented as signal filters, each recorded track is processed via convolution with the corresponding filter, thereby placing the recorded performance in the virtual room simulation, a process called auralization as the audio parallel to visualization. Details of this procedure can be found in [START_REF] Postma | Virtual reality performance auralization in a calibrated model of Notre-Dame Cathedral[END_REF].

In virtual acoustic renderings, the goal is to recreate the spatial acoustic cues that we as humans use to "decode" the soundscape around us. Such cues are represented by what is termed the Head-Related Transfer Function (HRTF), and like the RIR of the cathedral, it is specific to the geometry, here being an individual's head, torso, and especially ear shape.

As it was not possible to render this concert performance individually for every listener, it was necessary to choose the HRTF, or the "ears", of an individual which all spectators will listen through. As a subject of active study by the acoustic research team, numerous listening tests have been carried out previously comparing how we hear through other people's ears. For this project, we selected the most often best rated set of ears from our most recent study. This was HRTF number 1032 from the LISTEN HRTF database [START_REF]IRCAM LISTEN HRTF database[END_REF], [START_REF] Warusfel | LISTEN -Augmenting everyday environments through interactive soundscapes[END_REF]. This HRTF was therefore incorporated into the geometrical acoustics prediction software as a directional receiver, looking directly forward towards the choir section of the cathedral. For the cases when ambiance recordings were spatialized directly through binaural processing (see further discussion in Section III-D), the same HRTF was used to ensure spatial perception coherence.

C. Mixing / Post-processing 1) 2013 Artistic production mix: This production opted for a sound recording aimed at reproducing all the complexity of this spatial musical scene: we wanted to place the listeners at the conductor position, while immersing them in the unique acoustics of the cathedral. We chose this position as interesting because it allows the conductor to establish, control, adapt, qualify, and correct the instrumental balance of the work very precisely according to the acoustics.

The recording described in Section III-A was intended for a broadcast production by the CNSMDP which reproduced this staging as faithfully as possible. The work was produced in three formats:

• Binaural stereo • Transaural stereo (binaural 3D audio for 2 speakers) using

Transpan7 [START_REF] Baskind | Binaural and transaural spatialization techniques in multichannel 5.1 production[END_REF], [START_REF] Lyzwa | Utilisation de techniques binaurales et transaurales en prises de son et en post-productions multicanales 5.1[END_REF] • 5.1 (enhanced with transaural for 3D) The stereo productions (i.e. binaural & transaural stereo) are available on-line8 . The 5.1 format production included transaural elements to create a 3D mix using Transpan, introducing frontal L-R and rear Ls-Rs transaural streams.

This mix was realized in three stages:

1) The first stage was to achieve a realistic balance of all the different microphone tracks:

• The main capture system: an array of 5 omnidirectional microphones (DPA 4041) placed above the conductor captured a global vision of the sound scene.

• The auxiliary microphones : positioned near the sources, captured each section of the orchestra, soloists and choirs, which they reinforce relative to the main system recording.

2) The second stage consisted in integrating the natural acoustics of the recording space, recreating the different sound planes, such as placing the strings in front of the woodwinds, the woodwinds in front of the brass, etc.

Each auxiliary microphone was adjusted in time with respect to the main system as a function of the distance separating it from the latter. Adjustment of these delays makes it possible to emphasize and stabilize the different sound planes. Finally, the ambient array was, made up of 8 omnidirectional microphones (DPA 4006) located on either side of the central nave in pairs at 8 m intervals from the orchestra, captured the acoustics and sound color of the cathedral at different positions. These elements were mixed with the other tracks to give the listener the feeling of being inside the cathedral. 3) From this mixing base, we carried out fine-tuning in accordance with the indications of the musical score in order to support or move away a particular musician or singer according to the nuances described by the composer. The objective of the mix was to bring the realism of the recorded environment while keeping the maximum musical readability, to allow the work to be heard inside the space of the recording with the strictest respect for compositional intention. 2) 2020 Virtual reconstruction mix: The approach to the virtual reconstruction mix was different from the artistic mix described in Section III-C1. For this production, the mix was based on processing the auxiliary microphones with the synthesised natural acoustics of the cathedral. We therefore started with the 2013 recording using only the auxiliary microphones, the microphones closest to the sources. Due to their proximity, these microphones predominantly pick up the direct sound of the instruments, voices or choirs they point to.

Mixing: The relative levels and the balance of all the microphones are adapted in order to obtain a harmonious balance between the different instrumental and vocal ratios while respecting the balance desired by the conductor and described by the score. These subtle adjustments are the result of a good knowledge and expertise of the acoustics of the place, having to this date recorded more than twenty musical works in Notre-Dame de Paris.

The recorded tracks at the conductor's position are used to provide a reference for adjusting levels at the reconstructed conductor position. Using those established levels, the sound is then rendered at the other listening positions, without modifying any levels, in order to maintain the same "natural" balance across the orchestra.

Each track was processed with the BRIR filter corresponding to its position in relation to the virtual listener. These BRIRs were managed with the Pyramix workstation (Merging) and processed using Aalto University's SPARTA MutiConv9 [START_REF] Mccormack | SPARTA & COMPASS: Real-time implementations of linear and parametric spatial audio reproduction and processing methods[END_REF], a plug-in which implements the convolution of each microphone signal with the respective BRIR filter.

This mixing is an unusual approach for this type of musical production where, in general, it is advisable to privilege the precision and clarity of restitution of the various instruments and singers, providing high readability. In contrast, here we want to give the listener an extremely realistic listening sensation of a complex soundstage at a specific location in the recording venue. The balance sought here was to give the listener the immersive sensations they would have had in the cathedral on the day of the concert, for each of the 4 positions chosen. The balance perceived by the listener will be modified according to their listening position in the cathedral, with an expectation of loss of precision as they move further away from the orchestra.

D. Limitations

There were many recognized limitations to this production. Some of the most pertinent limitations are discussed below.

As the sound recording was carried out in the very reverberant acoustics of the cathedral and not in an anechoic chamber, each microphone track was still somewhat impacted by the acoustics of the cathedral. Each microphone also picked up to some degree neighboring instruments, in addition to the intended instrument it was directed at. The main drawback of this approach was that it resulted in a certain spatial blurring, an imprecision in the localization of certain instruments or instrument groups as they were reproduced over several virtual sources. Due to the imperfect recording conditions of the concert of microphones capturing other instruments at times, fine adjustments were attempted to remove (mute) them when possible. However, when both instruments were playing, this cross-talk between recorded tracks could not be removed. The result was that at times some instruments may appear to come from multiple positions within the orchestra, somewhat blurring the spatial precision of the virtual concert. One can also feel at times phase effects which can be unpleasant on certain passages. In addition, there were a few moments when musicians may have slightly bumped their microphones, or rustled their scores, which of course was included in the production.

While a significant effort was made to get close proximity recording of all performers, this was not always possible. For example, the procession of the children's choir in the 4th scene, when the chorus slowly approaches from behind the conductor, was only captured with the ambiance microphones. As these ambiance recordings include the room acoustics, they were not processed through the virtual acoustic model, as this would have produced a double reverberation effect. To integrate these elements into the virtual concert, they have been binaurally rendered directly using Sorbonne University's Anaglyph 10 [17], a binaural rendering plug-in, placing them at the performers' approximate positions relative to the listener during the performance.

A hard-coded binaural rendering approach was employed, fixing both the head/ear morphology of the virtual listener, as well as its orientation. The use of a non-individual HRTF can lead to degradations of various perceptions, including but not limited to spatial aspects [START_REF] Simon | Perceptual attributes for the comparison of Head-Related Transfer Functions[END_REF]. An alternative would be to rely on dynamic decoding over headphones, which was previously used for the audio-visual virtual fly-through of the cathedral [START_REF] Postma | Virtual reality performance auralization in a calibrated model of Notre-Dame Cathedral[END_REF], with only a short extract of the concert. While the spatial precision of such real-time Virtual Reality (VR) presentation is currently limited to 1st or 2nd-order Ambisonics, depending on the streaming platform, the ability to move one's head (best when using a device with gyroscope tracking) really benefits immersion in the virtual cathedral.

As a final point, we would like to indicate that part of the simulation work, and the entirety of the mixing and creation of the website, were carried out during the 1st COVID-19 confinement period in France: working on mostly portable and home machines, with all their limitations, being much less efficient and more limited compared to the 2013 production carried out in a professional studio.

IV. PUBLIC DIFFUSION

The production was made available on the 2020 anniversary commemoration of the fire on the website of the Acoustics Working Group of the Chantier Scientifique CNRS/MC Notre-Dame [START_REF]Hearing the Past : A virtual immersive audio concert for the ears[END_REF], including a brief description of various elements and the inclusion of a questionnaire. The 1st set of audio examples provided a 5 min 31 s montage of the production for each of the various listening positions, including an equivalent extract from the 2013 artistic mix in order to provide an element of comparison built on a more conventional approach to musical productions (see Fig. 6). This was followed by four full movements highlighting certain notable elements. Finally, the 3rd audio set contained the full 1 h 23 min concert, for the same ensemble of listening configurations.

The project was included in the International Year of Sound 2020-2021 program, in the section "Special Projects" due to it lacking a specific performance date. The publication/release was also announced on various social media platforms.

A. Website access statistics

As of 15-May-2021, since the opening of the site, it has been visited ≈ 4800 times (Google Analytics), with ≈ 3900 visits occurring in the month of April 2020. However, taking into account bots and search engines, we note from the analytics that just 453 of these visits explored the page for more than 3 min, and 194 for more than 1 h. It should also be noted that the production was available for download from the site in the event of poor internet connection, while also accessible directly via the Mediatheque of the Conservatoire National Examining the geographical distribution of visits, 61 % were in France, the host country for the project. The remaining geographical origins spanned Europe, Australasia, and the Americas, with a few connections from the African continent (see Fig. 7).

To provide some indication of technologies employed by visits, as this can help developers, producers, and distributors anticipate likely platforms, we note that 66 % of visits were from "desktop" computers, 29 % from "mobile", and 5 % classified as "tablet". Of mobile and tablet devices, 49 % were via Apple's iOS. Concerning browser technology, 38 % of visits used the Chrome browser, 28 % Safari, 17 % Firefox, with other browsers individually never representing more than 6 %.

B. Questionnaire

The questionnaire (Google Forms) allowed us to gather more information, and more importantly, impressions of the spatial audio reconstruction from the public. Of those individuals who visited and listened to the production, 31 completed the on-line survey, representing ≈7 % of the population who visited the site for more than 3 min.

1) Background/situational data: As with the website statistics, several connection related information were gathered which may prove generally useful. To begin with, internet connections occurred 5 times more frequently over WiFi than either wired connections or GSM. Regarding listening devices, 67 % listened over wired headphones, 17 % over wireless headphones, and the remaining evenly split between activenoise control headphones and loudspeakers, despite indications to listen over headphones. On a scale from 1 to 5, with 5 being best, participants rated their headphones with an average quality (± standard deviation) of 3.5±0.9 and the background noise level of their listening environment 3.9 ± 0.8.

The range of ages of respondent covered a large spectra, from under 18 to over 75 years old. As shown in Fig. 8, the largest represented age group was 25 to 34, followed by 45 to 54 years old.

Regarding personal experience, respondents were asked about their frequentation habits regarding listening to concert performances in churches/cathedrals, specifically in Notre-Dame, and also concerning visitation of Notre-Dame de Paris. As reported in Fig. 9, the clear majority of them had little to no experience of listening to a concert within Notre-Dame, though all had visited the cathedral at least once. The audience experience supports the intention of the production in bringing a new public into the cathedral though virtual acoustic reconstructions.

Due to the nature of the production, it was also of interest to know what previous experience respondents had with room acoustic simulations and binaural rendering. Results show a range of familiarity, from none to professional experience, with the latter category representing ≈20 %. This could be due to the association of the event with the International Year of Sound, as well as its dissemination on various special interest social media channels.

2) Production specific data: With the intention of the production to bring the public into the virtual performance reconstruction, while also highlighting the variations in its acoustics as a function of position, several questions were posed regarding appreciation of the production. Specifically, "How noticeable were the effects of changing position?" and "Did the virtual reconstruction sound like listening to a live concert?". Results, summarized in Fig. 10, show that for the vast majority of people the change in position was very audible and that the reconstruction was quite realistic, though there is room for improvement.

A final question concerned the potential negative impact of a lack of visuals, as this production was audio only, the equivalent of a live immersive radio event. Responses for this question show that respondents found the absence of image generally, though not extremely, problematic. We note that our previous virtual reconstruction effort for this same project included visual 360°VR rendering in conjunction with the spatialized audio [START_REF] Postma | Virtual reality performance auralization in a calibrated model of Notre-Dame Cathedral[END_REF]. However, it should be noted that this previous audio-visual production contained only a 2 min extract of the same concert. It was not imagined that the general public would be interested, equipped, or accepting of spending potentially over an hour listening to this production in VR, at least without significant effort in creating new and more lively visuals.

In order to have a sense of global appreciation, a ranking regarding quality was included. As shown in Fig. 11, the rear position tended to have lower ratings while, among the physically realistic positions, the conductor's position was generally viewed as having higher quality. The artistic mix, which presented a sound scene that was not limited to the physical reality of sound propagation, was generally highly ranked. However, in addition to the general trends, it is important to note that each of the five conditions was ranked highest by at least one respondent, and vice versa, each was also ranked lowest by at least one respondent. This lack of general agreement reinforces the notion that, as individuals, each listener has different expectations and preferences with regards to their preferred soundscape configuration. This variability has been similarly observed for example in seating preferences in concert halls, where preferences are determined by multiple factors and individual interests, such as clear distinction of separate instruments versus a general blending across musicians and with the hall's acoustics. This is equally seen in opera and cinema where the addition of visual elements is equally important, with preferences over immersion versus observability of the attending public can be found.

The final questions concerned future productions. Respondents were overwhelmingly positive about interest in future virtual productions within Notre-Dame de Paris, with 87 % in favor. With an eye to future potential, we also asked about interest in live-streaming of events from the virtual cathedral, which was also viewed positively, with 84 % in favor.

V. CONCLUSION

We have presented an overview of the various elements involved in the creation of a virtual concert reconstruction within a numerical model of the Cathedral of Notre-Dame de Paris, using a multitrack recording of a large orchestral production of La Vierge, oratorio by Jules Massenet. The historical importance of Notre-Dame de Paris as well as the context of the composer and the composition were provided. The technical conditions of the 2013 concert recording and original resulting spatial audio production were described. This was contrasted with the various technical details employed to carry out the virtual concert reconstruction using the same original recording material. The production's intent was to highlight the spatial nature of both the concert and the different perceptions of the performance with the cathedral's voluminous expanse.

Following the overview of historical, technical, and audio engineering aspects, data collected over 13 months of availability on-line, including a listener questionnaire provided along side the audio production material were presented. In general, the production was experienced by listeners of practically all ages and from around the world. While all respondents to the questionnaire had visited Notre-Dame before, they had listened at most to a single concert in the cathedral previously. As such, the audience coincided with the projects intention, exposing a relatively naive public to a 3D acoustic concert performance in the cathedral, currently under restoration after the fire in April 2019.

With the general positive opinion of the production, and the encouragement by respondent's to continue such work, even extending to live concert performance in the virtual cathedral, this work is hopefully only the first of several which will be produced in the context of the Past Has Ears at Notre-Dame (PHEND) project, currently underway.

Funding

  has been provided by the European Union's Joint Programming Initiative on Cultural Heritage project PHE (The Past Has Ears, phe.pasthasears.eu), the French project PHEND (The Past Has Ears at Notre-Dame, Grant No. ANR-20-CE38-0014, phend.pasthasears.eu), and the Chantier Scientifique CNRS/MC Notre-Dame.

Fig. 1 .Fig. 2 .

 12 Fig. 1. Mean reverberation time (T 20) over omnidirectional microphones with standard error bars. 2020 results show those for both sweep stimuli and impulse source gun-shots. (From [1].)

Fig. 3 .

 3 Fig. 3. 3D view within the cathedral highlighting the positions of the various musicians during the performance and the ( ) conductor's position. Note the position of the trumpets and Soprano in the gallery, on the far side of the transept, and the children's choir principal position at the far end of the nave and for their procession down the audience aisle.

Fig. 4 .

 4 Fig. 4. View of the 3D Geometrical Acoustics model of Notre-Dame.

Fig. 5 .

 5 Fig. 5. Comparison of room acoustic parameters EDT and C50 calculated on the measured (±JND, Just-Noticeable Difference for each acoustic parameter) and simulated RIRs highlighting the quality of the calibration.

Fig. 6 .

 6 Fig. 6. Website player frame with audio extracts, visual representation of the listening positions for use with the questionnaire, and indication to listen over headphones.

Fig. 7 .

 7 Fig. 7. Geographic distribution of website connections to the production homepage.

Fig. 8 .

 8 Fig. 8. Age range of respondents to the questionnaire.

Fig. 9 .

 9 Fig. 9. Respondents concert and Notre-Dame de Paris visit (top) frequenting rates and (bottom) familiarity with room acoustics simulations and binaural rendering.

Fig. 10 .

 10 Fig. 10. Respondents perception and sensitivity to reproduction conditions.

Fig. 11 .

 11 Fig. 11. Histogram of quality judgement ratings for the different positions, including the artistic mix.

  received a rather cold reception, due to the heavy program, which presented works by many other composers together. . . Sacred legend in four scenes, from a text by Charles Grandmougin, evoking the main episodes of the Virgin Mary's life through the biblical accounts of the Annunciation, the Wedding at Cana, Good Friday and the Assumption, this oratorio, with its terribly human accents in the expression of feelings, emancipates itself from the liturgy to become a true concert piece. One finds in it one of the great feminine figures of Massenet's imagination, with its joys, its rages, its weaknesses, its pain; which did not fail to provide subject to criticism, some reproaching Massenet for having composed too theatrical a music on such a mystical subject. . . A music, indeed, full of dramatic effects, colors, rhythm, different climates sometimes very surprising, like the very profane Galilean dance.

Maitrise de Notre-Dame de Paris, French Army Choir, Sotto Voce Children's Choir. Orchestre du Conservatoire de Paris, dir. Patrick Fournillier. Presented as part of the celebration of the 850th anniversary of Cathedral of Notre-Dame de Paris, La Vierge, by Jules Massenet (1842-1912) is a rare work, first performed in 1880 at the Paris Opera, under the direction of the composer. At its premiere, initially scheduled at Notre-Dame, this work was highly regarded and

This section is an excerpt from the text provided on the project website[START_REF]Hearing the Past : A virtual immersive audio concert for the ears[END_REF] by Frédéric Billiet, Professeur en musique médiévale, Sorbonne Université.

The main documents evoking this practice in organo, vel triplo vel quadruplo were published in 1932[START_REF] Handschin | Zur geschichte von notre dame[END_REF]. See more recently[START_REF] Wright | Music and Ceremony at Notre Dame of Paris[END_REF].

Anonymous IV points to two instances of Perotin's organum quadruplum -Sederunt principes and Viderunt omnes -in which the rhythmic swirling and interweaving of vocal parts make for a strikingly original sound-world.

See www.notredamedeparis.fr:"L'école épiscopale".

This section is an excerpt from the text provided on the project website[START_REF]Hearing the Past : A virtual immersive audio concert for the ears[END_REF] by Lucie Kayas, Doctor in musicology, Professor of musical culture at the CNSMDP.

• The orchestra and the main choir were positioned in front of the transept crossing. The orchestra was raised on a 2-level stage. The choir was located on a 5-storey tier in the rear extension of the orchestra. • The children's choir intervened in the cathedral choir at the start of the piece, behind the transept crossing. It was located 35.7 m from the conductor. The children's choir was not visible to the public at this location. A small ensemble, consisting of two harps, a positive organ and

www.catt.se

alexisbaskind.net/transpan

www.conservatoiredeparis.fr/fr/medias/recherche

research.spa.aalto.fi/projects/sparta vsts

anaglyph.dalembert.upmc.fr

ACKNOWLEDGMENT

The team would like to thank the Rector-Priest of the Cathedral of Notre-Dame de Paris, Monsignor Patrick Chauvet, for granting us access to the Cathedral for both recordings and measurements. This production would not have been possible without the performers and musicians: Maîtrise Notre-Dame de Paris, Lionel Sow, chef de choeur; Choeur de l'Armée franc ¸aise, Aurore Tillac, chef de choeur; Choeur d'Enfants Sotto Voce, Scott Alan Prouty, chef de choeur; Orchestre du Conservatoire de Paris, Quentin Hindley, chef assistant; Laura Holm, soprano; Charlotte Desaux, soprano; Caroline Michaud, mezzo; Benjamin Woh, ténor; Samuel Hasselhorn, baryton; Norah Amsellem, soprano; Patrick Fournillier, direction. We also thank Prof. Frédéric Billiet for contributing the section on the musicological context of the Cathedral of Notre-Dame de Paris.