Individualized dynamic binaural auralization of classroom acoustics using avirtual artiﬁcial head
Mina FALLAHI1 ; Martin HANSEN1 ; Simon DOCLO2,4 ; Steven VAN DE PAR2,4 ; Dirk PUESCHEL3 ; Matthias BLAU1,4
1 Institut für Hörtechnik und Audiologie, Jade Hochschule, Oldenburg, Germany
2 Dept. Medical Physics and Acoustics, University of Oldenburg, Germany
3 Akustik Technologie, Göttingen, Germany
4 Cluster of Excellence Hearing4All, Germany

Abstract
Binaural Room Impulse Responses (BRIRs) are typically used for binaural auralizations, as they carry bothroom acoustic information as well as acoustical properties of the individual listener. In a dynamic reproduction
scenario, i.e. with head movements of the listener taken into account, BRIRs need to be measured for a large
number of listener head orientations, which can be very time-consuming. In this study, a planar microphonearray with 24 microphones, referred to as Virtual Artiﬁcial Head (VAH) was used to measure Room Impulse
Responses (RIRs) for a single listener position for different source positions in a lecture room. VAH ﬁlter
coefﬁcients for different head orientations were calculated and applied to the measured RIRs. The VAH ﬁlter
coefﬁcients were calculated by solving a constrained optimization problem, i.e. by minimizing a narrow-band
least-squares cost function subject to constraints on spectral magnitude error and the mean White Noise Gain.
The quality of the synthesized individual BRIRs was evaluated in a subjective listening test based on a dynamic
binaural scenario. The results showed that a typical reverberant room can be dynamically auralized with the
VAH for speech signals with perceptually convincing agreement to real loudspeaker presentation in the room.
Keywords: Binaural Room Impulse Response (BRIR), Virtual Artiﬁcial Head (VAH), dynamic binauralsynthesis

1

INTRODUCTION

The signals arriving at the two ears include important cues on sound reﬂection and diffraction caused by thelistener’s head, torso and external ear. These cues, which can be captured by Head-Related Transfer Functions(HRTFs), are crucial for a spatial perception of the surrounding acoustic world. The binaural technology commonly makes use of the so-called artiﬁcial heads, copies of the average human head and torso with microphones
in the ear canals, to capture the spatial properties of the sound in a recorded signal. HRTFs or their equivalencein the time domain referred to as Head Related Impulse Responses (HRIRs) are known to be individual andshould preferably be determined individually. Therefore, the non-individual anthropometric geometries of these
artiﬁcial heads can lead to perceptible deﬁciencies. In another application of the binaural technology, a given
room can be auralized binaurally by convolving an anechoic signal with Binaural Room Impulse Responses(BRIRs). BRIRs combine the information contained in HRIRs with the acoustical information of the room (e.g.
lateral reﬂections and reverberation). With binaural auralization, naturally sounding representations of different
rooms can be created, which establish an appropriate tool for the investigation and comparison of different
acoustical environments. Furthermore, including head movements during binaural signal playback with headphones improves the localization accuracy [1], helps to promote externalization [2] and plays an important rolein preserving the plausibility [3] and authenticity [4] of the virtual acoustic environment. In a dynamic binauralsynthesis, the listener’s head movements are tracked in real-time to choose the BRIR corresponding to the current head orientation to be convolved with the signal. This means, however, that in order to auralize different
reverberant environments dynamically, the BRIRs should be measured for different head orientations in eachenvironment, which is very time consuming, especially if the BRIRs are supposed to be measured individually.

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Figure 1. Virtual Artiﬁcial Head (VAH) used in this study: planar microphone array with 24 microphones [5]As an alternative, the Virtual Artiﬁcial Head (VAH) can be employed which has been shown to capture spatial
audio signals for headphone representations in a perceptually convincing manner [5]. The VAH consists of a
microphone array with N spatially distributed microphones with digital ﬁlters for each microphone, forming aﬁlter-and-sum beamformer. The same recording made with the VAH can be individualized post-hoc by mimicking the directivity pattern of the individual HRTFs. This is done by applying the individually calculated ﬁlter
coefﬁcients as complex-valued weighting factors to the signals of the N microphones. The major advantage ofthe VAH is that the ﬁlter coefﬁcients can be calculated for different head orientations. This allows for headtracking during signal playback without the need for changing the VAH’s orientation during the recording. The
VAH technology for a measured and simulated room was evaluated in listening tests and found to be perceptually convincing for dynamic binaural auralizations in [6], using simulated steering vectors. In the present
study, real measurements were accomplished with the VAH in the same room as in [6] to assess the extent
to which BRIRs synthesized with the VAH can be used for dynamic binaural room auralizations. The results
were perceptually evaluated in a listening test with comparisons to real loudspeaker presentations in a typicalclassroom.

2

FILTER COEFFICIENTS FOR THE VIRTUAL ARTIFICIAL HEAD (VAH)

2.1 Calculation of VAH ﬁlter coefﬁcients in a constrained optimization problem
The directivity pattern H( f , θ ) of VAH at frequency f and direction θ is deﬁned asH( f , θ ) = wH ( f )d( f , θ ),(1)with the N × 1 steering vector d, deﬁned as the free-ﬁeld acoustical transfer function between the source at any
given direction θ and the N microphones. The N × 1 complex-valued vector w( f ) of Filter Coefﬁcients (FCs)consists of the weighting factors for the N microphone signals. In order to synthesize the desired directivity
pattern D( f , θk ) of individual left and right HRTFs at a total of P discrete directions θk , k = 1, 2, ..., P, FCs canbe calculated by minimizing a narrow-band least-squares cost function JLS , deﬁned as
P

JLS (w( f )) = ∑ |H( f , θk ) − D( f , θk )|2 .

(2)

k=1

In order to achieve small synthesis spectral deviations at all P directions, the minimization of JLS was performed
subject to constraints imposed to the resulting Spectral Distortion (SD) at each of the directions θk , by setting
an upper and a lower limit, LU p and LLow , i.e. for all k
2

LLow ≤ SD( f , θk ) = 10 lg|wH ( f )d( f , θk )|dB ≤ LUp .
|D( f , θk )|2(3)

In addition, in order to guarantee the robustness of VAH against microphone self-noise or deviations in microphone characteristics and positions, a minimum desired value β was set for the resulting mean White Noise732

SD (dB) at Elevation 0°
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2kFrequency (Hz)Figure 2. Resulting Spectral Distortion (SD) for synthesizing left HRTFs at 0◦ and 15◦ elevations as well as
resulting left WNGm , using FCs calculated with: (a) P = 72 horizontal directions and β = 0 dB, (b) P = 3 ×
72 = 216 directions from elevations −15◦ , 0◦ and 15◦ and β = 0 dB.
Gain, W NGm , which is deﬁned as the ratio between mean output power of the microphone array over all P
directions and the output power of the spatially uncorrelated white noise [7], i.e.
W NGm = 10 lg(

1 P |wH ( f )d( f , θk )|2∑ wH ( f )w( f ) )dB ≥ β .
P k=1

(4)

For solving the constrained optimization problem of minimizing JLS subject to constraints deﬁned in Eqs. 3 and4, an iterative Interior-Point optimization algorithm was used. The solutions proposed in [7] were used as theinitial values.
2.2 Constraints and parameters for the calculation of the VAH ﬁlter coefﬁcients
According to Eqs. 2-4, the performance of the VAH with respect to meeting the constraints depends on a varietyof parameters such as constraint parameters LU p , LLow and β , the microphone array topology and the number
P of discrete directions included in the calculation of the FCs. Applying different constraint parameters to two
simulated microphone arrays of different topologies, it was shown in [8] that with properly chosen constraint
parameters and array topology, the VAH can perceptually outperform a traditional artiﬁcial head for sources in
the horizontal plane for musical content with respect to overall audio quality. However, since the microphonearrays were simulated and considered as perfectly robust and noise-free, the effect of different values of resulting
WNGm could not be assessed in [8]. In addition, the results were evaluated only for free-ﬁeld conditions. In
contrast, in the current study the planar microphone array with 24 microphones shown in Figure 1 was used
for measurements in a reverberant room. The two constraint parameters LLow and LU p were chosen as −1.5 dB
and 0.5 dB, respectively, leading to a maximum deviation of 2 dB in the resulting Interaural Level Differences
at all P directions, as was done earlier in [8].
The values chosen for the two other parameters, i.e. β and P, impacted the extent to which the resulting
SD would remain between the two desired limits of −1.5 dB and 0.5 dB. For example, Figure 2a shows theresulting SD and WNGm for synthesizing left HRTFs with FCs calculated with β = 0 dB and P = 72 directionsin the horizontal plane (5◦ azimuthal resolution). A minimum value of 0 dB for the resulting WNGm and −1.5
dB ≤ SD ≤ 0.5 dB at P=72 directions in the horizontal plane could be satisﬁed well up to 6 kHz. However,for non-horizontal directions such as at 15◦ elevation the resulting SD increased clearly. On the other hand,733

including P = 216 directions (3×72) from elevations −15◦ , 0◦ and 15◦ improved the resulting SD at elevation
15◦ , but deteriorated at the same time the resulting WNGm and the SD at horizontal directions, as shown in
Figure 2b. If more elevations were included in the calculation of the VAH FCs, the resulting SD and WNGm
would deteriorate even more for horizontal directions. Thus, the question arises, whether it is preferable totarget at high accuracy and robustness in the horizontal plane while ignoring non-horizontal directions or totarget moderate accuracy and robustness which are then more evenly distributed over all elevations. For thisstudy, three cases for P in combination with two cases for β , as listed in Table 1, were considered, resulting ina total of six sets of FCs.
In order to enable a dynamic binaural presentation, each of the six sets of FCs were calculated for 37×5 = 185
head orientations (azimuth angles −90◦ to +90◦ in 5◦ steps and elevations −15◦ to +15◦ in 7.5◦ steps). For
a given head orientation θh , h ∈ 1, 2, ..., P, FCs were calculated by taking the D( f , θk ), k = 1, 2, ..., P, and theshifted steering vectors d( f , θk ) with k = h, h + 1, ..., P, 1, 2, ...h − 1 into Eqs.(1) to (4).
Table 1. Overview of values chosen for the parameters P and β .
P = 1 × 72

3

(Elevation: 0◦ )labeled as El0

P = 3 × 72 = 216(Elevations: −15◦ , 0◦ , 15◦ ) labeled as El0±15

P = 3 × 72 = 216(Elevations: −30◦ , 0◦ , 30◦ ) labeled as El0±30

β = 0 dBlabeled as
β0
β = -10 dBlabeled as
β −10

METHODS

3.1 Measurement room
The room chosen for this study, which was the same as in [6], was a lecture room (7.12m ×11.94m × 2.98m)
with an average reverberation time of 0.58 s. A listener position was selected with ears at 1.30m height from theﬂoor, as shown in Figure 3a. The ﬁgure also shows the four source positions that were considered in the room:
Source 1 (Genelec type 8030c) was located ahead of the listener slightly higher than the ears, representing a
lecturer standing in front. Source 2 and Source 3 (Genelec type 8030b) were located left and behind the listenerat the right side, respectively, both at the same height as the ears. Source 4 (Event active studio monitor 20/20
bas V3) was mounted in front of the room on the right and at an elevation of about 20◦ .
3.2 BRIR acquisition
In the next step, the VAH was placed at the listener position and the Room Impulse Responses (RIRs) for all
four source positions were captured for each of the 24 microphones of the VAH (Figure 3 b). These RIRs werethen ﬁltered with the previously mentioned six sets of FCs, resulting in six versions of individually synthesized
BRIRs, referred to as VAH BRIRs. BRIRs were also measured with the KEMAR artiﬁcial head as well aswith a head-sized rigid sphere (radius = 8.5 cm) with two microphones positioned at ±100◦ on the equator,
referred to as KEMAR BRIRs and Sphere BRIRs, respectively (see Figure 3 c-d). In order to enable dynamic
binaural representations, the KEMAR- and Sphere BRIRs had to be measured 37 times for 37 orientations of theartiﬁcial head and the rigid sphere (azimuthal head orientations -90◦ to 90◦ , in 5◦ steps). Note that KEMAR’shead was rotated relative to the shoulders which kept a constant orientation. All RIRs were measured with asampling rate of fs = 44100 Hz, using the Multiple Exponential Sweep Method (MESM) [9] with sweeps of
20 s duration, ranging from 20 Hz to fs/2 with 4 s shift between subsequent excitations.

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Figure 3. (a) Listener position and the four source positions in the lecture room (Az: azimuth, El: elevation, R:
distance to listener. (b) VAH, (c) Kemar artiﬁcial head, and (d) rigid sphere at listener position in the room.
3.3 LISTENING TESTTo assess the perceptual quality of the six VAH BRIRs as well as the KEMAR- and Sphere BRIRs, a listening
test with dynamic binaural synthesis was performed. A total of 9 normal-hearing subjects (three female, sixmale) took part in the test. For all of them, individually measured HRTFs and Head Phone Transfer Functions (HPTFs) as well as six sets of individually calculated FCs were available. The headphone (Sennheiser
HD800) was equipped with a head tracker attached to the top bow as well as a push button to switch backand forth from headphone to loudspeaker presentation, as implemented in [6]. The listening test took place inthe same room as the measurement room. Subjects sat at the listener position during the test and were asked
to rate different headphone presentations, generated either with VAH BRIRs or KEMAR- and Sphere BRIRs,
in comparison to the presentation via loudspeakers in the room (the reference condition). The subjects were
not informed about the speciﬁc BRIR synthesis condition that was presented. The stimulus was a dry recorded
speech utterance of 15 s duration, spoken by a female speaker, which was convolved with the different BRIRs
as well as the inverse HPTFs prior to presentation via headphones. The evaluation was performed with respect
to ﬁve separate attributes: Reverberance, Source Width, Source Distance, Source Direction and Overall Quality. Subjects gave their ratings on a 9-point scale covering ﬁve German labels schlecht (bad), dürftig (poor),ordentlich (fair), gut (good) and ausgezeichnet (excellent) with four equidistant intermediate scale points. Eachsource direction appeared three times in the test in a randomized order and subjects were allowed and encouraged to switch freely between all of the different headphone signals and/or between loudspeaker and headphonepresentations.

4

RESULTS AND DISCUSSION

The perceptual ratings of all subjects for different perceptual attributes and source positions regarding different
BRIRs are shown in Figure 4. Almost for all perceptual attributes and source positions the VAH BRIRs with
El0/β0 and the KEMAR and Sphere BRIRs were rated similarly high, with median ratings between good and

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ex
go cellor fair od ent

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El0

El0 15

El0 30El0

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El0 30

0

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Rigid
Sphere

BRIRs

Figure 4. Results of perceptual evaluations of 9 subjects (averaged over three repetitions) with respect to ﬁve
attributes and four source positions, for 8 different BRIR versions (horizontal axis).

736

excellent. The two exceptions were the ratings given to Sphere BRIRs with respect to Reverberance, and theratings given to VAH BRIRs with El0/β0 with respect to Source Width, both for the signal corresponding to
Source 1. In comparison, generally lower ratings were given everywhere to VAH BRIRs with more directions
included in the calculation of FCs, i.e. El0±15 and El0±30, regardless of β . This is in accordance withthe poorer synthesis accuracy in the horizontal plane, as already discussed as an example in Figure 2, andindicates that accuracy and robustness for directions in the horizontal plane are more important than for nonhorizontal directions in a classroom scenario, eventhough not all sources were in the horizontal plane. VAH
BRIRs with El0/β−10 were in general rated lower than VAH BRIRs with El0/β0 . Since for the calculationof FCs for El0/β−10 the constraint imposed to the resulting WNGm was relaxed to allowable values down to−10 dB, the sensitivity of the VAH to deviations in microphone characteristics increased for VAH BRIRs with
β−10 . Regarding the time lapse of about four months between the measurement of the steering vectors for the
calculation of FCs and the recordings performed in the room, it seems likely that small unavoidable deviations
in microphone positions and/or sensitivities during this time period might have led to audible synthesis artifacts
in the VAH BRIRs with El0/β−10 . As a result, it is advisable to choose higher values for β .
Despite the different geometries of the Kemar artiﬁcial head and the rigid sphere compared to anthropometricgeometries of individual subjects, high ratings were given almost in every case to the KEMAR- and Sphere
BRIRs. We suspect that this is due to the presence of reﬂections in the reverberant room which improved theexternalization and helped mask the deﬁciencies related to non-individual BRIRs (see for example [4]). Anotherimportant aspect was that the listening test took place in the original room and hence, all other cues including
visual ones were perfectly available. This made it for example less likely that visual cues that are inconsistent
with auditory cues lead to in-head-localization or front-back confusions. Nevertheless, the ratings given to
VAH BRIRs with El0/β0 were similarly as good as ratings on KEMAR- and Sphere BRIRs. Applying theFriedman test with post-hoc multiple comparisons revealed in some cases signiﬁcantly better (p<0.05) ratingsgiven to KEMAR- and Sphere BRIRs compared to some of the other VAH BRIRs, but there were no signiﬁcantdifferences between ratings given to VAH BRIRs with El0/β0 and KEMAR- and Sphere BRIRs for any of theperceptual attributes or source positions. Regarding the impractical effort taken to measure the KEMAR- andSphere BRIRs for many different head orientations, and considering the comparable perceptual results given
to VAH BRIRs with El0/β0 , the VAH technology seems to offer the more promising alternative for dynamic
binaural auralizations of commonly used acoustical environments (lecture room) with realistic signals (speech).

5

SUMMARY AND CONCLUSION

In this study, individual binaural room impulse responses (BRIRs) in a lecture room were synthesized for 185
head orientations with the virtual artiﬁcial head (VAH). The synthesized BRIRs of different head orientations
were used to auralize the room dynamically. The quality of the synthesized BRIRs was evaluated regarding
different perceptual attributes in a listening test in comparison to the real loudspeaker playback in the room.
The results showed that it is possible to auralize a typical reverberant room dynamically with the VAH for
speech signals, with perceptually high agreement to the real loudspeaker signal. The accuracy and robustness
for directions in the horizontal plane is very important for sources in and near the horizontal plane. The
results showed also that the need for BRIR individualization might not be crucial in a reverberant room. It
should be noted, that this outcome relates only to the case of dynamic binaural representation. To which extent
the effect of head tracking had an inﬂuence on the evaluations should be addressed in a separate experiment.
Nevertheless, the major advantage of the VAH with respect to the possibility of applying head tracking could
be demonstrated in this study. Further investigations should concern the performance of the VAH in otheracoustical environments, for other source positions and using other signals such as music or broadband noise,alongside the enhancement of the VAH with respect to the microphone array topology.

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ACKNOWLEDGEMENTS
This work was funded by Bundesministerium für Bildung und Forschung under grant no. 03FH021IX5.

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