Evaluation of an ILD-based hearing device algorithm using Virtual Sound
Environments
Ruksana GIURDA(1,2) , Laurent S. R. SIMON (1,3) , Hannes W ÜT HRICH (4) , Norbert DILLIER(1,3)(1) ENT department, University Hospital, Zürich, Switzerland(2) Department of Electrical Engineering, ETH Zürich, Switzerland(3) University of Zürich, Switzerland(4) Sonova AG, Stäfa, Switzerland

Abstract
Hearing devices are mostly tested using controlled laboratory environments. These feature simplified auditory
representations of the real conditions under which the hearing devices will perform once released, such as one
speech signal played from one loudspeaker plus a noise signal playing from another loudspeaker. It can however
be insufficiently realistic for some of the processing included in the devices. In order to circumvent this limitation, Virtual Sound Environments (VSE) can be used to simulate complex sound fields using headphones orarrays of loudspeakers, and thus to offer more realistic conditions than most common hearing device evaluation
protocols. In this paper, a hearing device algorithm based on Interaural Level Differences (ILD) was evaluatedusing Binaural Room Impulse Responses (BRIR, used here as a reference), and artificial room effects reproduced via either Higher Order Ambisonics (HOA) or Vector-Based Amplitude Panning (VBAP). Results showthat the algorithm does not work when using 5th order HOA, and that the performance of the algorithm is lower
when using VBAP than when using BRIR.
Keywords: Hearing devices, 3D audio, Interaural Level Differences, Signal processing

1INTRODUCTION

Normal Hearing (NH) people show remarkable abilities in listening tasks, even in challenging conditions. Thoseabilities strongly rely on the use of the binaural cues such as Interaural Level Differences (ILD) and Interaural
Time Differences (ITD). By combining the information from the left and the right ears, normal hearing people
are able to focus on a single speaker in a mixture of competing/interfering talkers such as cocktail-party situations. They are also able to localize a target sound source in complex auditory environments. However, thecues used for such tasks are degraded in Hearing Impaired (HI) people. Depending on the type and severenessof the impairment, tasks such as sound localization, sound lateralization, discrimination of interaural level andtime differences, speaker discrimination are challenging and, in some severe cases, impossible. A brief review
of past studies conducted on the effect of hearing impairment on binaural interaction is given in [1].
Initial studies on spatial perception in HI people used limited conditions e.g., no room simulation, sources
coming from single loudspeakers. Recently, the development of Virtual Sound Environments (VSE) made it
possible to simulate complex environments and use them to understand and improve speech perception in suchenvironments [3]. In this study, VSEs are used to guarantee more realistic test conditions, particularly in termsof variety of sources, source positions, room effects, and dynamic of the auditory scenes. The increased realism
might be required for testing the efficiency of e.g., noise reduction or spatial processing algorithms.
The use of Virtual Sound Environments (VSE) for evaluating hearing devices in complex auditory 3D environments was already discussed in [4, 5, 6, 7, 8]. More particularly, the authors discussed the limitations of3D audio environments for evaluating hearing devices. In [9], the authors compared the reproduction of ILDs
with Distance-Based Amplitude Panning (DBAP), Higher Order Ambisonics (HOA), Multiple Direction Amplitude Panning (MDAP), and Vector-Based Amplitude Panning (VBAP). They showed that the ILDs were better
reproduced with VBAP by 3dB on average. The ILD errors could reach up to 17dB with DBAP. Given that2210

according to [10], sources located 90◦ to the side can lead to ILDs up to 25dB at high frequencies, this implies
that the errors of ILDs for VSE are not neglectible.
The impact of such errors on hearing device processing is not yet known. Yet it is important to consider these
3D audio reproduction artifacts when evaluating hearing aids: hearing devices offer tools that can help thewearer to focus on a target sound source in complex acoustic environments. Some of these tools are based on
algorithms which also rely on the estimation of binaural acoustic features. For that reason, a correct simulationof the binaural cues is fundamental when creating virtual acoustic environments for hearing device evaluation.
The current study therefore illustrates the effects of the ILD reconstruction errors of HOA and VBAP reproduction techniques on a hearing device algorithm for sound localization. This paper describes the experimental
setup, shows the results and discusses the limitations of the reproduction systems for the evaluation of hearing
devices.

2EXPERIMENTAL SETUP

In this study, a hearing aid localization algorithm based on binaural cues was tested using three different typesof sources: a real source, an HOA virtual source, or a VBAP virtual source.
The real source test signals were obtained trough the convolution of Binaural Room Impulse Responses (BRIR)
and anechoic speech signals. The BRIRs were recorded with two Behind The Ear (BTE) hearing aid satellites
placed on a KEMAR manikin. The database of the BRIRs comprises impulses recorded from 0◦ to 350◦ ,with a step size of 10◦ in the azimuth plane. The recordings were conducted in three different rooms with asource-receiver distance of 2m. The acoustical properties of the recording environments are shown in Table 1.
Table 1. Acoustic properties of the rooms.
DRR and RT60
at 2m [dB|s]

Room

Volume [m3 ]Cafeteria

488

5.4

0.61

Listening room

125

5.7

0.17

Classroom

262

4.1

0.59

Both of the HOA and VBAP recordings were made in a listening room (RT30 = 199ms) equipped with anarray of 32 8020 DPM Genelec loudspeakers:• 8 loudspeakers every 45◦ azimuth at an elevation of -30◦
• 12 loudspeakers every 30◦ azimuth at an elevation of 0◦• 8 loudspeakers every 45◦ azimuth at an elevation of +30◦
• 4 loudspeakers every 90◦ azimuth at an elevation of +60◦
Distance from the loudspeakers to the center of the array was 1.5 m, except for the 4 top loudspeakers, whichwere 98 cm away from the center.
In order to have a comparison as fair as possible between the methods, only the results obtained with the BRIRs
recorded in the Listening room, the one with the most similar RT60 , are considered here. In this way, the effectof the reverberation time is reduced and only the geometry of the rooms will affect the results.
Fifth order HOA and VBAP sound reproduction was done using Max1 , a visual programming tool for audio,
1 https://cycling74.com/2211

Figure 1. Schematic of the loudspeaker spherical array used to generated HOA and VBAP recordings.
and Spat2 , a spatial audio processing plug-in for Max. No room effect was added in Max nor Spat, meaning thereverberation recorded at the hearing aid satellites was that of the recording room. This means that the RT60
was not the same for all conditions. The smaller RT60 for the VBAP and HOA reproduction room implied thatthe performance of the tested algorithm should have been better with the simulated sound sources than with thevirtual sources.
The stimuli tested in this study included either one or two speech sources. The absolute localization performanceof the algorithm was tested using a single talker. The ability to identify and localize multiple sources was tested
using two competing talkers placed in specific positions.
The inputs of the tested algorithm are the recordings of the four microphones placed on the hearing devices.
The energy level difference between left and right devices is computed for each considered frequency band andonly the level differences in the high frequency bands is kept for further analysis. Based on the level difference
measured at each time frame and other relevant acoustic parameters, a decision model estimate in which oneof the six possible areas the detected sound source is located, as depicted on Fig. 2. The limits between areas
1 and 2 or 1 and 6 are set at ±30◦ . The localization performance of the algorithm is azimuth-dependent. In
this study, the comparison with different sound reproduction methods was carried out with sound sources placedalways in positions where the performance of the algorithm was expected to be good.

3RESULTS

In this section, the results for the localization algorithm obtained with the three different sound reproduction
methods, BRIRs, HOA and VBAP are presented.
Fig. 2 shows the acoustic scene simulated and tested in this study. Two speech sources are located on the left
and right side with respect to the hearing aid user. The right sound source was set to be slightly louder thanthe left one (≈ 5dB). The sources were placed at the azimuth positions of ±50◦ with the 0◦ considered at thefront of the hearing aid user. This acoustic scene was used to test the three sound reproduction methods hereconsidered. Only for VBAP and BRIRs, a further test with the speakers located at ±60◦ was conducted.
As mentioned previously, the results obtained with the binaural impulse responses are used as the reference.
Fig. 3 shows that sound sources are detected in area 1, 2 and 6 even though the sources are only located inarea 2 and 6. The phantom source detected in area 1 comes from the interaction of the two sources. Without
further processing, whenever two sources interfere in such a way that the measured level difference is close tozero, the algorithm will interpret these low ILDs as a source located at the front (for which the natural ILDs
would be close to zero). This aspect will not be further discussed as it is not relevant for this study and the2 https://forumnet.ircam.fr/product/spat-en/2212

area 1100
area 680Percentage

area 2

area 3area 5604020

Figure 2. Acoustic scene simulated with thethree sound reproduction methods. Sources
are placed at the azimuth positions of ±50◦with the 0◦ considered at the front of thehearing aid user.

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Figure 3. Localization results with BRIRs
when the sources are located at ±50◦ . This
output is used as reference in the comparison between the reproduction methods.

100

8080

Figure 4. Localization results for HOA when
the sources are located at ±50◦ .

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40
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a120

440

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100area
2Percentage

result with the phantom source detected in area 1 is still going to be considered as the reference. In addition
to the estimated location, the algorithm provides information about which source was louder over time: Fig 3shows that the source located in area 6 was louder than the source located in area 2. However, no information
about the loudness difference is provided.
Fig 4 and 5 show the results obtained with HOA and VBAP, respectively.

Figure 5. Localization results for VBAP
when the sources are located at ±50◦ .

The results shown on Fig 4 demonstrate that the algorithm failed in localizing the sound sources. With VBAP
the algorithm could detect more than one source but the interaction between the sources is quite different fromwhat is obtained with BRIRs. In fact, the percentage of detection in area 1 is more than 60 in case of VBAP
but only 19 in case of BRIRs.
To verify whether this difference in the source interaction was mainly due to the reproduction system, a second
test was run only for VBAP and BRIRs. Fig. 6 and 7 show the comparison between the localization results
obtained with BRIRs and VBAP when the sound sources are located at the true speaker positions of ±60◦ . In
this case, only one speaker per source is active and the difference between the results is smaller. In fact, thepercentage of detection in area 1 is 45 for VBAP and 24 for BRIRs. It is worth mentioning here that one may
expect to have a smaller detection in area 1 for the BRIRs since the sources are located further to the side.

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220Figure 6. Localization results obtained with
BRIRs when the sources are located at thetrue speaker positions of ±60◦ .

440ar4060

ea60

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Percentage

100

1Percentage

However, this expectation does not consider the actual geometry of rooms, the actual reflection pattern present
in the impulses and the difference , even if small, in the microphone directivity.

Figure 7. Localization results obtained with
VBAP when the sources are located at thetrue speaker positions of ±60◦ .

DISCUSSION

In [9], an objective comparison of various VSE was done in the same room and similar conditions to thosedescribed in this paper. In the study by Simon et al., ILDs produced by virtual sources located every 10◦ on
the horizontal plane were compared to the ILDs of real sources for the same source positions (either one of
the loudspeakers of the reproduction system, or an additional loudspeaker used only for the measurement ofreal source impulse responses). When a virtual source produced with VBAP is located at the same position as
one of the loudspeakers of the array, only that loudspeaker is active. Therefore at those positions, the ILDs
produced by VBAP virtual sources are the same as the ILDs of the real sources. This is not the case for HOA,
for which all loudspeakers are always active, although with different weights.
In the following, the expression ’estimation error’ will refer to the difference between the estimation of thealgorithm with HOA and VBAP and the estimation with real sources, i.e. BRIRs.
The results shown in Fig. 8 and 9 could explain why the ILD-based algorithm failed to work correctly in thecase of acoustic environments reproduced with HOA and VBAP. The plots show the difference in ILDs between
the real sources and the two other reproduction methods as a function of frequency and azimuth. The colorbar
range has been manually limited between 1 and 7 for an easier comparison. However, in the case of HOA,
the difference can reach 10 dB in the high frequency range. The algorithm considered here strongly relies on
the actual ILD value measured at any time frame. Whenever the ILD values are distorted, the algorithm will
misinterpret the acoustic scene. In particular, as already mentioned in section 2, the algorithm considers only
the ILD values in the high frequency range where the sound is more directive and the ILD is the dominantcue. The error in ILDs produced by HOA and VBAP differs in magnitude and nature. In the case of HOA, thesound field in the high frequency range is not faithfully reproduced due to spatial aliasing and to the limited
number of real sources producing the sound. Therefore, the directional cues are missing and the algorithm
cannot estimate the direction of arrival of the sound or, in case of multiple sources, which one is louder. In thecase of VBAP, there is a visible effect of the azimuth position of the source playing. The estimation error of
the algorithm was larger in the case of virtual sound sources, i.e. sources that are placed between two speakers.
In this case, more than one speaker at a time is active and the correlation between the sound fields generated
by each active speaker creates the illusion of a source placed where no physical speaker is present. The effectof the correlation is limited and results in a sound field which is not a faithful reproduction of the desired
acoustic scene. Comparatively, the estimation error was relatively small when the source was played at a true
speaker position. In this latter case, only one speaker is active and the differences between the estimations with2214

Figure 8. Difference of ILD in dB between
the real sources and the HOA5 reproduction system in the same listening room, atthe same positions. Measurements were done
every 10◦ .

Figure 9. Difference of ILD between the realsources and the VBAP reproduction systemin the same listening room, at the same positions. Measurements were done every 10◦ .

VBAP and BRIRs comes only from the difference in the reflections in the two tests.
Due to the multiple parameters involved in the localization estimation, it is difficult to evaluate the reproduction
systems with a single number. In fact, one should take into account the exact geometry of the room, the absorption coefficient of all the elements present in it, their relative location and the exact position of the sources withrespect to the receiver. In this study, the comparison was done between two rooms which had a very similar
reverberation time but the actual reflection patterns and geometry of the rooms were not considered. However,
all these parameters are not likely to produce an ILD difference up to 10 dB. Therefore, the differences in theestimations presented here are still related to the actual reproduction system used.

5CONCLUSIONSIn this study, the effects of three sound reproduction methods on a hearing-device localization algorithm that isbased on ILDs has been conducted. It was shown that the fifth order HOA was not able to faithfully reproduce
the sound field in the high frequency range leading to a wrong localization for the whole duration of the soundsignals. Better results were obtained with VBAP although it was shown that the mismatch between the reference(BRIRs) and VBAP is smaller when the sources are located at a true speaker position.

ACKNOWLEDGEMENTS
This work was funded by CTI grant Nr. 25276.2 PFLS-LS.

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