PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

The Perceptual Effect of Reflective Surfaces on Acoustic Crosstalk
Cancelation Using an 8-Channel Linear Loudspeaker Array
Karim BAHRI; Jens AHRENS1
Audio Technology Group, Division of Applied Acoustics
Chalmers University of Technology, 412 96 Gothenburg, Sweden

ABSTRACTWe recently presented a user-tracked crosstalk cancelation system based on a linear array of loudspeakers.
The system uses Ambiophonics in the low frequency range and a superdirective nearfield beamformer inthe mid and higher frequency range. Efficient real-time execution is achieved by parameterization ofpre-computed beamformer weights. In this contribution, we investigate the perceptual effect of different
reflecting surfaces that are apparent in the reproduction room on binaural content that is presented throughour system. Our user study shows that a floor reflection does not have an audible effect. An increasing
amount of reverberation from the playback room makes perception more spacious and pleasant. Contrary toprevious studies, even a strong lateral reflection from a close wall did not impair localization significantly.
Keywords: Crosstalk cancelation, binaural audio, beamforming

1. INTRODUCTION
Crosstalk cancelation (CTC) has been pursued since the 1960s [1]. Most of the early implementations employed a pair of loudspeakers with a loudspeaker positioned ipsilateral to each of the earsof the listener, respectively. The crosstalk of each loudspeaker to the corresponding contralateral ear
was estimated by different means, and a cancelation signal was emitted by the other loudspeaker.
Numerous authors have contributed to the field. We refer the reader to the references in, for example,[2-4] for a non-exhaustive list. Starting in the 1990s, researchers have been evaluating the capabilities of loudspeaker arrays to perform acoustic CTC [5] with just as many contributors as totwo-channel CTC. We again refer to the references in, for example, [2-4].
CTC systems are usually designed assuming free-field conditions. The effect of reverberation onchannel separation was studied instrumentally in [6]. The effect of a low number of lateral earlyreflections on localization of sound sources in binaural content was studied in [7] using a
2-loudspeaker setup operating in an anechoic chamber. The reflections were produced using flat rigid
surfaces that were installed in the chamber. It was found that such isolated early reflections can cause
a localization bias towards the incidence direction of the reflection. Front/back confusions werehigher in the non-anechoic conditions with a bias towards localization in the front hemisphere where
the loudspeakers were located. The effect of the floor reflection was not studied.
It was found in [8], on the other hand, that the room response did not affect localization significantly when simulated with an image-source model.
Simple room simulations as the one in [8] do not account for relevant acoustic effects and cansound artificial when rendered spatially, and the manual handling of reflective surfaces as in [7]limits the range of acoustic conditions that can be tested. We therefore use measured impulse responses from the loudspeakers of a CTC system to the ears of a dummy head in different acoustic
environments in the present study to auralize the CTC system. This allows for switching between
environments by the click of a button and puts no limits on the range of acoustical conditions thatcan be covered.

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jens.ahrens@chalmers.se

2. SYSTEN DESIGN
The present study employs the system that we presented in [2-4]. It is a linear equi-spaced
8-channel loudspeaker array as depicted in Fig. 1. The geometry of the scenario under consideration
is depicted in Fig. 2. The core of the system is a convex superdirective nearfield beamformer that
directs a beam to one of the ears of the listener and produces a null at the other (contralateral) ear.
Assuming that the loudspeakers emit ideal spherical waves, then the transfer functions from one of(the two) input channels of the system to the ears of the listener that is assumed to be located centrally at 1 m distance to the array show a channel separation of at least 20 dB over the vast part of the
audible frequency range as depicted in Fig. 3.

Figure 1 – Example prototype using 8 Neu-

Figure 2 – System geometry; the enabledmann KH 80 DSP loudspeakers with a spac-

listening locations are along a line parallel toing of 154 mm

the array

Figure 3 – System transfer function to the two ears of the listener under ideal conditions (black lines) as
well as with simulated loudspeaker mismatch (gray lines) (data from [2])Fig. 3 reveals that the channel separation can drop significantly at frequencies below 1 kHz assoon as some amount of mismatch of the sensitivity of the loudspeakers and uncertainties in the
loudspeaker placement are included in the simulation.
This reduction of the channel separation at low frequencies is not surprising as we are looking at afrequency range in which the two control points, the ears, are separated by less than a wavelength.
We therefore chose to use the beamformer only above 800 Hz and recursive ambiophonic crosstalkelimination (RACE) [9], which is a simple two-channel CTC method that has shown to be effective
and degrade gracefully, below 800 Hz.

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Translations of the listener are enabled by pre-computing the beamformer weights for a set oflistener positions and subsequent parameterization as presented in [3,4].
Measurements on the prototype under anechoic conditions show that a channel separation ofaround 20 dB is apparent over most of the frequency range independent of the listener position [4](cf. also Fig. 6). We demonstrated in [10] that the design of the beamformer based on the measured
directivity of the loudspeakers increases the CTC only in a limited frequency range around 2 kHz.
The data from Fig. 6 do not take the loudspeaker directivity into account.

3. MEASUREMENT DATA AND INSTRUMENTAL EVALUATION
To assess the perceptual influence of reflecting surfaces and reverberation on crosstalk canceledbinaural content, we measured impulse responses from each of the loudspeakers of the array to theears of a KEMAR dummy head (DH) in different environments and for different head orientations insteps of 1°. Automatic rotation was performed using VariSphear [11]. This allows for performing
binaural head-tracked auralization of the virtual array as well as switching between different environments by the click of a button. The following environments were measured:
1.
2.
3.
4.

Anechoic chamber
Anechoic chamber with a floor reflection
Small dry laboratory room
Small dry laboratory room with a reflective side wall and the DH located in the center ofthe room
5. Small dry laboratory room with one reflective side and rear wall and the DH located in
the center of the room
6. Small dry laboratory room with a reflective rear wall and with the DH located 1 m awayfrom a highly reflective lateral wall (to the right of the DH)
Fig. 4 shows photographs of some of the setups. Fig. 5 depicts sample ear impulse responses. The
resulting channel separation for the environments anechoic (1), anechoic plus floor reflection (2),and dry room (3) is depicted in Fig. 6. We chose not to show data on channel separation for the other(more reverberant) conditions as it is not straightforward to differentiate direct sound and room reverberation, which may both trigger different perceptual mechanisms. Interpreting frequency-domainchannel separation can be misleading in these cases. It is evident from Fig. 6 that the floor reflection
does not have a significant effect on channel separation.

Figure 4 – Photographs of example measurement setups:
environment 2 (left); environment 6 (right)

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Figure 5 – Impulse responses from loudspeaker 4 to the left ear of the KEMAR manikin on a logarithmic
scale (from left to right): environment 1, environment 2, environment 5, environment 6

Figure 6 – Channel separation at the ears of the DH for three of the measured conditions; no separation of
direct sound and reverb was performed

4. PERCEPTUAL EVALUATION
4.1 Experiment Design
We auralized the CTC array in all 6 environments from Sec. 3 by means of head-tracked binauralsynthesis of the DH measurement data and using the SoundScape Renderer (SSR) [12,13]. The subjects were instructed not to move their head excessively as the CTC method does not account for this.
We nevertheless employed head tracking so that the auralization accounts for small head movements,
which can reduce distortion of the spatial perception [14].

7269The virtual loudspeaker array was playing binaural content, which consisted of anechoic malespeech spatialized in the directions straight ahead (0°), 30° to the left, and 90° to the left by means of
KEMAR HRTFs. The listener was positioned at 1 m distance from the array and symmetrically w.r.t.
the array. The virtual orientation of the listener was normal towards the array. Note that we foundthat making the listener look away from the array in normal direction can significantly reduce
front/back confusions [4]. We chose not to investigate front/back confusions in the present study as
the phenomenon reported in [4] has not been decoded yet.
The subjects were provided with a graphical interface with which they were able to switch seamlessly between the conditions while the speech signal was playing continuously. A total of 18 different conditions were present (6 rooms times 3 virtual source positions).
We found in a pilot study that the perceptual differences between the auralizations of the different
room conditions can span a broad range from hardly or not perceptible to clear multidimensional
differences. We therefore chose to run the study as an interview. The subjects were comparing the 6different conditions for each of the 3 virtual source positions separately and reported in free speech
to the experimenter what differences they were hearing. The subjects did not have further information on what it was that they were listening to.
4.2 Results and Discussion
9 grown-up subjects with self-reported normal hearing participated in the experiment. The average duration was 23 min.
We distilled the following from the subjects’ responses:•

The floor reflection has no audible influence (environment 1 vs. 2).

•

The lateral virtual source positions were perceived more spacious.

•

The effect of the dry room (environment 3) is minor. The perception is very similar to (environments 1 and 2).

•

An increasing amount of reverberation increases externalization. Internalization can occur in environments 1 and 2.

•

An increasing amount of reverberation makes spatial perception more plausible in general, forexample, in terms of localization accuracy, locatedness, and source width.

•

Front/back confusions occurred mostly in environments 1 and 2.

•

Environments 4 and 5 evoked the most plausible and pleasant perception.

•

Approx. half of the subjects perceived the virtual source as slightly elevated. Reverberation mitigated this effect.

•

The strong lateral reflection in environment 6 was perceived disturbing by most of the subjects.

•

No major effect of the strong lateral reflection in environment 6 on localization was reported.

Our results suggest that there is a range of room acoustic conditions that influence the presentation of binaural contend only to a minor extent (environments 1-5). This confirms the results from[8], which were obtained based on a simple room simulation. Strong and isolated lateral reflections
caused localization bias in the experiment presented in [7]. We cannot confirm this observation.
Environment 6 in our study comprised a similarly strong reflection but embedded in the room reverberation. This reflection did have a negative effect, however, not primarily on localization but
rather on a more general level. It seems that embedding a reflection in reverberation mitigates theperceptual impairment.

5. CONCLUSIONSWe auralized an 8-channel crosstalk cancelation array based on manikin data measured in different environments to assess the perceptual influence of the acoustic response of the environment on

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the perception of binaural content. We found that the floor reflection does not have a noticeableinfluence. An increasing amount of reverberation and early reflections support spaciousness andreduce front/back confusions. Even excessive reverberation yielded acceptable results.

ACKNOWLEDGEMENTS
We thank Georgios Zachos for building the arrays.

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