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

Improved binaural speech intelligibility by adding reverberation to
the target speaker
Julian Grosse, Steven van de Par
Acoustics Group, Cluster of Excellence “Hearing4All”, University of Oldenburg, Germany

ABSTRACTIn anechoic, multi-source listening situations, the auditory system is remarkably capable of understanding
speech. An improvement in speech intelligibility is observable when interfering and target speakers are
spatially separated compared to being co-located. The present study investigates the influence ofreverberation on speech intelligibility of a reverberated target placed in front of the listener in the presence ofsymmetrically placed anechoic interferers. A simulated binaural room impulse response (BRIR) is
systematically truncated at different durations to investigate the effect of reflections. Measurement
conditions consisted of co-located and separated speakers that were presented, using BRIRs for the target
speaker and HRTFs for the interferers. It was found that reflections can have an opposite effect on speech
intelligibility. Whereas for spatially separated speakers, added reverberant components impair speech
intelligibility, for co-located conditions speech intelligibility improves. This suggests that the auditory
system may utilize the spatial diffuseness of the target source in co-located conditions to perceptuallysegregate it from the interfering anechoic sources. In spatially separated conditions, the difference inperceived location of the sources may be the dominant cue for segregation such that diffuseness caused byadding reverberation does not further improve intelligibility.
Keywords: Binaural speech intelligibility, room-acoustics, reverberation

1. INTRODUCTION
It is well known that in multi-talker conditions, the speech intelligibility improves when the target
speaker is spatially separated from the interfering speakers [1]. Various contributing factors have been
mentioned such as better ear listening, binaural unmasking, and the contribution of binaural cues toauditory stream segregation [1,2].
Modelling approaches that can predict the improvement in speech intelligibility are often based on
Equalization and Cancellation processing [3, 4, 5] which was originally proposed in the context ofbasic psychoacoustic experiments of tone in noise masking [6]. The essential idea is that the left andright input signals that occurs at a certain moment in time within a specific frequency band is first
temporally aligned and equalized in level such that after subtracting of the modified signals theinterferer is mostly cancelled thus improving the target speech to interfering speech ratio.
The presence of reverberation can impair speech intelligibility. The precise reasons are not yet wellunderstood. In the context of an Equalization and Cancellation model, the reverberation will lower thecorrelation of the interfering signals at the two ears which will make cancellation less effective.
Alternatively, spatial cues will be modified due to the presence of reverberation which could maketheir contribution to auditory stream segregation less effective. The Equalization and Cancellation
model has been extended to include the effect of reverberation by partitioning the reverberated target
speech in components that contribute to speech intelligibility (direct sound and early reflections) andcomponents detrimental for speech intelligibility (late reverberation) [7,8]. These latter components
are considered as additional masking components [8].

5773In this contribution, we were interested to see how well a reverberated target speaker could beunderstood in the presence of two anechoic interfering speakers. Both co -located and spatiallyseparated configurations will be investigated as well as diotic presentations. More specifically, theBinaural Room Impulse Response (BRIR) will be truncated after various cut-off times. In this way, theeffect of early and late reflections on speech intelligibility can be investigated in order to determine theextent to which they are detrimental to speech intelligibility. As will be shown, the expectation that
more reverberation should impair speech intelligibility does not always hold and depends on thespatial configuration of the target and interfering speakers.

2. Method
The following section describes the experimental setup, i.e. the room-acoustical conditions, thespatial orientation of the interfering and target speakers and the manipulation of the BRIR toinvestigate the role of single reflections and reverberation on binaural speech intelligibility.
2.1

Room-acoustical conditions and spatial locations of the speech sourcesr = 1.5 m

60°

DS
e.g. τ=20 ms

Figure 1 – Room-acoustical conditions that were used in the subjective evaluation. Panel A (left) shows theroom dimensions, the orientation of the listener relative to the location of the interfering and target speakers,
respectively. Panel B (right) shows the first 30 ms of the BRIR at the target speaker location with an example
truncation time τ of only direct sound (DS) or e.g. 20 ms.

In order to investigate the role of reflections and reverberation on speech intelligibility in rooms,
the room acoustic simulation software RAZR [9] has been used to simulate binaural room impulse
responses (BRIR). It combines an image source model to simulate early reflections and a feedback
delay network for the late reverberant part.
Figure 1 shows the dimensions of the room and spatial orientation of the speakers that were used inthis study. Table 1 shows additional room properties. The position of the listener is somewhat
off-centered and the facing direction is slightly rotated to avoid a symmetrical setup. The target
speaker is located in front of the listener at a distance of 1.5 m. The interfering speakers are located at-60° and +60° to the left and right relative to the target speaker. The distance between listener andinterfering speakers is kept constant to 1.5 m to have an equal direct sound level independent of thetargets’ location.
The reverberation time (RT) is controlled via the RAZR simulation software by adjusting theabsorption coefficients that are equally distributed across the walls to obtain a RT of 0.8 s and 1.6 s.
This adjustment allows to preserve the temporal structure (in terms of the acoustical path lengths) of
the early reflections. Consequently, only the strength of the late reflections changes.

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Table 1 – Shown are the room properties, i.e. room dimensions, reverberation time, direct to reverberant ratio
and the distance between listener and speech source.
Dimensions (x,y,z)

Reverberation time

DRR

Distance

Room A

10 m x 6.5 m x 3 m

0.8 s-0.4 dB

1.5 m

Room B

10 m x 6.5 m x 3 m

1.6 s-2.9 dB

1.5 m

2.2 Stimuli & subjective procedure
The target speech stimuli consisted of ongoing speech tokens (spoken by a male) from the German
version of the OLSA-speech corpus [10]. The sentences consist of 5 words (name, verb, numeral,
adjective, subject) randomly chosen and without any semantical meaning with a duration of approx.
2.5 s. As interfering speech stimuli, two different female speakers were taken from two audiobooks
spoken in German language. The two independent interfering speech tokens were truncated to a totalinterval-duration of 3.5 s. The target speech was temporally centered within the interferers resulting in
an onset-delay between interferer and target speaker.
The spatially separated condition were created by convolving the tokens with the BRIR stemmingfrom one of the three locations. The target speech originated always from 0° and the two interferers
either from -60° or +60° allowing the auditory system to make use of binaural cues to segregate thetarget and interfering speakers or to use better-ear listening to improve speech intelligibility.
The co-located condition consisted of the target and the two interferers originated from 0° which
does not allow the auditory system to make advantage from binaural cues but still allows better-ear
listening
The diotic condition consisted of the left ear signal of the co-located condition but was presented toboth ears. The left ear was arbitrarily chosen. This monaural signal contained no binaural informationand does not allow the auditory system to make use of neither binaural cues nor better-ear listening.
In order to investigate the role of early and late reflections on binaural speech intelligibility, only
the target speakers’ BRIR was manipulated and the interfering speakers’ BRIR only consisted of thedirect sound (DS) to convey only undistorted directional information to the listener without any
reflections. The target speaker BRIR was truncated to:a) The direct sound (DS) to simulate anechoic/free-field condition,b) A duration of 10 ms, in which only ground and ceiling reflections occurred (i.e. monaural
information of the reflections)c) 20 ms which includes ground and ceiling (monaural) and left and right-wall (binaural)reflectionsd) 125 ms to cover all early reflections but without the late reverberatione) The full impulse response (full IR) that is technically defined by the reverberation time
illustrated in Table 1.
The symmetrically placed interferers were presented anechoic by convolving the tokens with thedirect sound of the BRIR to obtain a binaural signal with the interaural cues stemming from the proper
location of the interferers with respect to the spatial condition.
In order to investigate the effects of additional reflections on speech intelligibility and to avoid anadditional loudness cue caused by the reflections, the level was normalized independent of theduration of the truncated BRIR.
The signal to noise ratio (SNR) was controlled by taking the mean-value across both ears of theroot-mean-squared value of the binaural signals after spatialization.
In order to measure speech reception thresholds (SRTs), the interfering talkers were calibrated to a
constant level of 65 dB-SPL. To reach the 50% speech intelligibility point, the SNR between the target
and interfering talkers is adaptively adjusted depending on the answer of the subject. To obtain the
SRT50, a total set of 15 sentences per condition were used.

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Eight subjects in total participated in the subjective evaluation. Subjects were normal-hearing (puretone audiogram<20 dB). They participated in two sparate blocks at which one block consisted of 15
threshold measurements (3 spatial coditions x 5 truncations times) and each had a duration of
approximately 90 minutes, but subjects were instructed to have a rest whenever it was necessary.
Before a daily block, subjects had an initial training of six conditions which spanned a variety of thethree spatial conditions but were not part of the main experiment. All 15 conditions were randomized
across subjects whereas a room-acoustical condition was not divided across the two daily blocks
because literature shows that prior listening in rooms can improve speech intelligibility [11].
3.

Results
Figure 2 shows the results of the subjective evaluation for both room-acoustical scenarios.
Illustrated are speech reception thresholds (SRT) at 50 % speech intelligibility displayed by meanvalues and standard error across subjects for the spatially separated, co-located and diotic condition.
Panel (A) shows the results for the room with 0.8 s and (B) with 1.6 s.
When both, the target and the interfering talkers are presented in anechoic condition, i.e. only the
direct sound but with the binaural cues stemming from the particular location, the spatial separated
talker configuration shows a binaural benefit of about 5 to 6 dB compared to co-located and diotic
condition. This result is independent of the RT, because the level of the direct sound was kept constantacross room-acoustical condition. This behavior can also be seen for a truncation time of τ=10 ms. The
small differences can be attributed to the two additional (purely monaural) reflections stemming fromthe ground and ceiling and cause a comb-filter like effect in the spectra but does not add additional
binaural information to the target stimulus.
For the separated condition, the SRTs monotonically decrease from approx. -13 dB to -11 dB forroom A and to -9 dB for room B with increasing truncation time. Main differences between the rooms
are observable when the full BRIR is presented and results in SRTs of approx. 2 dB across spatial androom-acoustical conditions. This difference can be mainly attributed to the change of the strength ofthe late reflections.
The co-located condition shows with increasing truncation time an increase in SRTs of about 2 dB
for room A and approx. 4 dB for room B compared to the anechoic condition. This behavior seems to
be independent of the strength of the late reflections and converges to the separated condition after atruncation time of 125 ms.
(A)

(B)

Figure 2 – Speech reception thresholds (SRT50) measured in the presence of two anechoic interferers forroom 1 (A) with a reverberation time (RT) of 0.8s and room 2 (B) with RT=1.6s. The x-axis represents thetruncation time τ (ms) of the BRIR of the target speaker location and the y-axis represents the SRTs.
Illustrated are the mean across subjects and the standard error of the mean for the spatially separated (bluediamonds), the co-located (red triangles) and the diotic condition (green squares).

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The diotic condition reflects a constant behavior till a truncation time of 10 ms for room A,
increases to a truncation time of 20 ms and is saturated for the remaining part of the BRIR. Room Bshows a similar behavior of increasing SRTs after 20 ms even though the increase for room B is more
prominent and no saturation in SRTs are visible.
Comparing the separated and co-located with the diotic condition measured with the full BRIR
within and across both rooms, the binaural benefit in SRTs is approx. 5 dB for room A and 4 dB forroom B when a reverberant target speaker is masked with anechoic interfering speakers.

4. Discussion
In the speech intelligibility experiments we presented in this contribution using anechoic
interfering speakers and a target speaker that was convolved with a Binaural Room Impulse Response(BRIR) truncated at various truncation times, we found that for the spatially separated condition, an
increase in truncation time leads to reduces speech intelligibility (increased SRTs). This finding is inline with the common understanding that speech intelligibility is impaired by added reverberation.
Note that the target signal was always level normalized and thus was independent of the truncation
time. This implies that with increasing truncation time, the direct sound component and earlyreflections decrease in level. Already at 20ms truncation time, we see an impairment in speech
intelligibility indicating that for speech intelligibility it is better to concentrate all target signal energy
within the direct sound and at most first 10 ms early reflections.
Rather contrary to the findings with separated speakers, the co-located condition showed that withlonger BRIRs applied to the target speaker, speech intelligibility improves (i.e. SRTs decrease) up to
truncation times of 125 ms. Thus, this leads us to conclude that reverberant components up to 125 ms
contribute to speech intelligibility.
An explanation for this finding may be that for the co-located condition, the anechoic interfering
speakers and the target in case it is rendered only with a direct sound component create a spatial image
that provides no spatial cues that would allow to segregate the target speaker from the interfering
speakers. When, however, early reflections are added to the target speaker, the spatial diffuseness ofthe target increases, and now the spatial image of the target speaker extends beyond the position of theinterfering speakers. This would allow to spatially segregate the target from the interferers. A further
explanation for this improvement in SRTs might be due to the binaural information conveyed by the
first order reflections (sidewalls) that can be used to improve speech understanding and which are in
our set-up is co-located with the interfering speakers in the spatially separated configuration.
It is interesting that the BRIR components in the range from 20 ms until 125 ms can be both
detrimental, in case of separated speech sources, and useful, in case of co-located speech sources.
To support the contribution of spatial diffuseness cues, results can be compared to the diotic
conditions. In this case neither spatial localization cues, nor spatial diffuseness cues are available. In
this case, a pattern of SRTs is obtained that is similar to the separated condition, only for the diotic
condition SRTs are shifted up by about 4 to 6 dB.
Possibly the pattern of results for the co-located conditions could be predicted by an Equalization
Cancellation model for which the Equalization stage attempts to cancel the interfering speakers. When
spatial diffuseness increases, more target speech components will remain after cancellation. Such an
EC model would, however, in order to model the diotic conditions, require some stage that allocatessome of the late BRIR parts of the target speech to be detrimental components that mask the earlytarget speech components. Thus, this leads us to conjecture that the determination of what are
detrimental signal components will depend on spatial configuration in line with a similar conclusion
reached in [7].
Note also that typically Equalization Cancellation models do not include an explicit stage
modelling auditory stream segregation contributions to speech intelligibility altho ugh this has beenshown to be an important contributor to speech intelligibility [12].
In the experiments shown here, two different T60 time conditions of 0.8 and 1.6 s were used.
Despite the large difference in these T60 times, no strong difference in speech intelligibility was found.
This may be related to the relatively small source-receiver distance of 1.5 m which creates a relatively
strong direct sound component in the BRIRs and thus a relatively similar direct re verberant ratio (seeTable 1).
To summarize these results show that target signal components between 20 ms and 125 ms can bothbe detrimental and useful for speech intelligibility.

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5. Summary & Conclusion
We found that reverberation reduces speech intelligibility in case target and interferers are spatiallyseparated. In contrast to this, when adding reverberation only to a target speaker that is co-located with
anechoic interfering speakers, the reverberation leads to an improvement of speech intelligibility. It
was hypothesized that the spatial diffuseness of the target speaker helps to better segregate this speakerfrom the interferers.

ACKNOWLEDGEMENTS
The authors acknowledge the Deutsche Forschungsgemeinschaft for supporting this work that ispart of the grant titled “Contributions of Auditory Stream Formation to Speech Perception.”

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