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

Speech intelligibility in a realistic virtual sound environment
Naim Mansour1, Marton Marschall1, Adam Westermann2, Tobias May1, Torsten Dau1
1

Hearing Systems Group, Technical University of Denmark (DTU)
2

Widex A/S, Lynge, Denmark

ABSTRACT
Employing realistic, yet controlled sound scenarios for the evaluation of hearing aid (HA) algorithms in a
virtual sound environment (VSE) has the potential to improve HA users’ real-world listening experience andperformance. In the present study, so-called critical sound scenarios (CSS) were defined as acoustic scenesthat HA users experience as important, difficult and common, and selected through ecological momentary
assessment (EMA), inspired by results from recent studies. The scenarios were acquired in real scenes using
a spherical microphone array, and reproduced in an acoustically valid way inside the VSE. A speech
intelligibility task using the Danish Hearing In Noise Test (HINT) was implemented in one of the scenes tomeasure speech reception thresholds (SRT) for normal-hearing and hearing-impaired subjects. SRTs
obtained for the realistic VSE background noise were found to be higher than corresponding ones reported inheadphone-based studies, potentially as a result of the increased cognitive effort required to separate target
speech from the more realistic interfering speech background.
Keywords: Speech intelligibility, Experimental realism, Spatial audio

1. INTRODUCTION
The tremendous sophistication of auditory perception allows humans to make sense of andcommunicate in vastly complex acoustic scenes and situations. Consequently, impairments to thissystem can have a negative impact on social interactions of those affected (1). The development of
increasingly complex hearing devices over the past few decades has been aimed at partiallycompensating for these impairments. Complex algorithms inside hearing aids (HA) control the gain ofincoming sound in a time- and frequency-dependent way. In order to achieve maximum performance
when worn by users in their daily lives, it makes sense to evaluate and validate HAs and HA algorithms
in conditions that mimic those of the real world. Moreover, results of psychoacoustic listening tasks
involving normal-hearing (NH) and (aided) hearing-impaired ((A)HI) subjects might be affected by
the increased experimental realism these conditions can provide.
The design of such psychoacoustic experiments can be viewed in light of a trade-off between
control and realism. Experimental outcome measures benefit from being clear and consistent, with
their paradigms accurately described and replicable. In a listening task, this implies having control
over the precise acoustic conditions and variabilities in the stimuli. At the same time, a design that
does not incorporate the complexity of real-world conditions might not produce generalizable results.
Specifically, a listening task that uses headphones to capture speech-in-noise performance canspatialize stimuli using head-related-transfer functions, but this approach is coarse and its accuracy islimited by many factors, including headphone placement and the quality of said transfer functions. In
addition, accounting for head movements and fitting a HA is cumbersome in practice. Quadrophonicloudspeaker setups can be employed to spatialize the noise maskers, alleviating these problems, butthey generally still do not produce signals with realistic spatial properties. In contrast, conducting thetask in the real world would be perfectly realistic, yet real-world conditions tend to be poorly
controlled and highly variable. Therefore, an experimental setup that trades off control for realism inan optimal way has the potential to result in well-controlled, yet ecologically valid findings.
In addition to a controlled and realistic reproduction, the type of source signals (target as well as
masker) can affect experimental realism as well. Typically used speech-shaped noise maskers tend to
be quite dissimilar to an actual multi-talker acoustic scene. They do not contain low-frequency
modulation masking components, have shallower time-varying dynamics and lack intelligible
components which could lead to informational masking (2).

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Simulating spatialized room-acoustic maskers (such as those obtained through ODEON software(3)) has been shown to increase speech reception thresholds (4), but might still be limited in level ofrealism by the number and complexity of sources that can be simulated. In order to maximize realism,
a controlled, in-situ spatial recording of a realistic sound scenario is likely to be even more
representative, retaining auditory cues up to the limitations of the recording setup .
Finally, any recorded sound scenario would benefit in terms of its ecological validity from having
been selected based on actual HA user input. The use of ecological momentary assessment (EMA) data
captured in the real world has become increasingly prevalent in attempts to qualitatively andquantitatively determine which scenes HA users experience (5). Thus, choosing scenarios that arealigned with these experiences is a logical way to further improve experimental realism (6).
To evaluate the specific speech-in-noise paradigm of speech intelligibility (SI), the Hearing In
Noise Test (HINT) has been widely used, in various acoustic conditions. This test, along with otherapproaches, such as the matrix-based DANTALE II (7), results in NH SRTs that are well below zero, ataround -2.5dB in the Danish HINT (8), -3.9dB in a multi-lingual HINT pooling (9), and down to-8dB for the DANTALE II.
In contrast, research trying to categorize real-world signal-to-noise ratios (SNRs) has consistently
found a substantially higher range of SNR values in the majority of sound scenarios (10). Thus, whenrelating SI test scores to intelligibility in a realistic scenario, headphone-based tasks using artificial
stimuli might lead to an underestimation of the difficulties both NH and HI people experience.
The speech-in-noise experiment considered in the present study attempts to address some of thementioned limitations, by designing an ecologically valid SI task based on the Danish HINT. At thecore of the study lie two hypotheses. The first one states that a speech intelligibility paradigm that
employs realistic, spatialized stimuli will produce SRTs that are significantly higher compared to thoseobtained with an artificial approach. In the second hypothesis, those obtained SRTs are proposed to beconsistent with SNRs reported in the real world.
A set of critical sound scenarios (CSS) was selected based on a novel categorization of existing HAuser EMA data (Figure 1a). These scenarios were recorded in-situ with a spherical microphone array(Figure 1b). A 64-channel, fully spherical loudspeaker array inside an anechoic enclosure (Figure 1c),
capable of rendering arbitrary three-dimensional sound fields at its center through Higher-Order
Ambisonic reproduction techniques (HOA, 11), was used to reproduce these recordings as a so-called
Virtual Sound Environment (VSE). Finally, all components were combined into the design of a speech
intelligibility task for NH and HI subjects (Figure 1d-e) based on the Danish HINT.

a. Selectionb. Acquisitionc. Reproduction

d. SI for NH

e. SI for HI

Figure 1 – Design stages of the VSE SI task

2. METHOD
2.1 Sound scenario selection
To establish a high level of ecological validity in the speech intelligibility task, the acoustic
background stimuli were selected based on EMA data from 281 field reports by HA users, collected by
Smeds et al. (12). Their data were categorized into so-called critical sound scenarios (CSSs) in a novel
way, based on the orthogonal division between three reported EMA metrics: importance, occurrence
and difficulty (Fig. 2). Combinations of these three parameters seem to separate different real-lifesituations very accurately. Some examples in Figure 2 illustrate this.
Using CSSs in the “sweet spot” (the subset of scenarios that are important, difficult and common),as backgrounds for the SI task was intended and assumed to reflect conditions in which HA users
struggle. Important, difficult and less common CSSs are arguably also of interest for selection, since
they still reflect challenging conditions for the HA to perform in. Three sweet spot scenarios wereselected: a public lunch, a small festive event and an office meeting. The speec h intelligibility task inthis study was implemented only for the office meeting scenario.

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Figure 2 – Critical Sound Scenario framework
2.2 Sound scenario acquisition
The selected sound scenarios were captured in the real world with an em32 Eigenmike sphericalmicrophone array capable of 4th HOA recording (13). In addition, a Knowles Electronic Manikin for
Acoustic Research (KEMAR) equipped with HA shells was used to capture binaural ear and HA
signals. An overhanging omnidirectional microphone measured overall background levels. Figure 3
illustrates the scenario acquisition for the office meeting scenario.

a. Background scene recording

b. Impulse response recordingc. Conversation in noise recording

Figure 3 – Office meeting scenario acquisition stages
Spatial background recordings were obtained with the EigenMike and KEMAR in the listener
position, while 12 participants, seated and standing in the office meeting room, conversed as
illustrated in Figure 3a. Impulse responses were captured from the target position using a mounted
loudspeaker producing exponential sweeps, while all participants remained in place to avoid altering
the room reverberation (Figure 3b). This was done to later allow virtual embedding of the target speechmaterial into the room. In addition to these recordings, the scene was captured by the right ear of the
KEMAR positioned to the left of a participant sitting in the listener position, conversing with a partnerin the target position across the table (Figure 3c). This last stage allowed the capture of the backgroundscene during a target-listener conversation. At each stage, the signals were clock-synchronized tosingle-sample precision at a 48kHz sample rate.
2.3 Sound scenario reproduction
The spherical microphone recordings obtained during acquisition were encoded from the 32 raw
input channels to a 25-channel Ambisonic 4th-order HOA format. These HOA signals were thenrendered on a 64-channel spherical loudspeaker array (Figure 5b) using dual-band (basic, max-rE)
decoding, with a crossover frequency of 2400 Hz. HOA auralization was chosen for its physicallyfaithful rendering of sound fields in the sweet spot at the center of the array, ensuring their usability for
HAs as well as human ears.
To guarantee that the overall root-mean-squared (RMS) level would not fluctuate too widely
throughout the reproduction, an additional procedure was designed to select and combine portions of
the auralized recording, based on level estimates derived from the overhanging reference microphone.
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Figure 4 – Recorded background level distribution
The 10-minute-long reference microphone recording was segmented into frames of 5s (with 80%
overlap). Level values in dB were derived for each frame, and the level difference calculated between
consecutive frames. The upper and lower boundaries for allowed level differences were respectively
set to the 5th and 95th percentile of the difference distribution. This allowed the segment indices ofconsecutive frames within these boundaries to be retained, ordered by decreasing length (from 25s toabout 7s), and resulted in a 2.5 min. recording.
Next, the 64-channel synchronized spherical recording was decomposed into frames of the samelength and overlap, and the portions corresponding to the selected reference segment indices wereextracted. These frame segments were then individually calibrated to a fixed target level using a B&K
2669 free-field microphone, before being concatenated through windowed cross-fading with 1s
overlap. The frame pairs at the cross-fading boundaries were calibrated in the same way, to ensuresmooth transitions between segments. Finally, the resulting 2-min., 64-channel background recording
was calibrated binaurally inside the loudspeaker array, using a B&K Type 4128 Head and Torso
Simulator (HATS) with ear canals. As a result of this approach, the obtained background signal
retained its intelligible properties despite being level-equalized in a time-sensitive way.
The target sound pressure level for the sound scenario, 73.5dB, was chosen as the median value ofthe noise level distribution measured during the third phase of acquisition. This distribution was
calculated by selecting noise-only portions of the recording through the use of an energy-based
voice-activity detector (14), and shown in Figure 4 as a nested histogram (with increasing bin widths
of 0.2, 0.4, 0.8 and 1.6dB). The background noise level in the classical Danish HINT study (8) was set
to values between 65 and 75dB SPL.
2.4 Speech intelligibility assessment
The testing paradigm to evaluate speech intelligibility was kept exactly the same as that of theclassical Danish HINT. In its classical implementation, the well-established HINT uses brief, mundanetarget sentences superimposed on speech-shaped quasi-stationary noise, presented diotically overheadphones in an adaptive 1-up-1-down sentence-based scoring procedure. The non-stimuli-related
specifics of this procedure, as well as the SRT calculation method and program interface were left
unaltered. To embed the 60 training and 200 test speech sentences in the spherical office meetingbackground, they were convolved with impulse responses measured between the target and listener
positions by the spherical microphone array. Each sentence was individually calibrated to retain thesame unique level as that of the single-channel version, referred to the sweet spot of the loudspeakerarray in the AudioVisual Immersion Lab (AVIL) at DTU. Throughout the test, different SNRs wereaccomplished by varying speech levels with respect to the constant, continuously looped background.
10 NH and 10 HI subjects participated in the experiment. The NH subjects had a four-frequency
average hearing loss of at most 15dB, while the HI ones had an average sloping mild (N2) to moderate(N3) hearing loss (15). Figure 5a shows the individual mean left-right as well as the overall mean
audiograms with standard deviation for both groups. The NH/HI median age was 28/70 years old, withstandard deviations of 16.6/5.3 years. All HI subjects had a discrimination performance of at least 92%
for both ears, and a maximum left-right-ear difference of at most 10 dB for all frequencies.
The N2-N3 HI target criterion was intended to produce a homogenous and ecologically
representative listener group with a common, but not overly severe hearing loss. Symmetrical hearing
loss was required to avoid effects of better-ear listening in a task that allows for head movement.

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a. Audiograms of NH and HI subjects

b. 64-loudspeaker array for HOA reproduction

Figure 5 – Audiograms & loudspeaker array in the AudioVisual Immersion Lab (AVIL)
One training round and two evaluation rounds of 20 sentences each were carried out to determine
SRTs per subject in the office meeting scenario background. A target-sentence-only training round wasused to ascertain audibility, in accordance with the HINT procedure. The order of used speech material
was randomized by arranging the individual test list orders in a Latin square, and an SRT score wasdetermined for each subject as the average of the two evaluated SRTs. The reference HINT study
included 16 NH and HI subjects, and SRTs values were determined per subject by averaging evaluatedSRTs over 10 test lists. Figure 5b shows the AVIL loudspeaker array used in the experiment.

3. RESULTS
3.1 Acoustic properties of the backgroundFigure 6a and 6b respectively show the long-term average spectrum (LTAS) up to the Nyquist
frequency, and the modulation power spectrum from 1Hz exponentially up to 64Hz, for the classical
HINT background noise, the office meeting background noise, and a concatenation of all two hundred
spatialized HINT test sentences.
The LTAS were normalized to have the same broadband RMS, and the modulation power spectra
were obtained by normalizing calculated modulation power within a band by its respective bandwidth,
as well as the power in the DC component (16). The auralized HINT sentences were recorded with the
HATS inside the loudspeaker array (with the left ear spectra shown here). Several observations can be
made. The LTAS and modulation spectrum of the classical HINT noise represent its quasi -stationary
nature and lack of significant low-frequency modulation power. The modulation power is mostly
constant for all considered modulation frequencies, resulting in a flat spectral representation.

a. Long-term average spectrac. Modulation spectra

Figure 6 – Spectral properties of the SI stimuli

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The situation is very different for the office meeting noise. The LTAS is largely similar in terms of
its speech-like spectral shape, though containing fewer prominent peaks, and including the effect of
the room. On the other hand, the modulation spectrum now has significant power at modulation
frequencies up to 8Hz. The modulation spectrum of the office meeting background noise bears agreater resemblance to the auralized HINT sentences than to the classical HINT noise. The LTAS for
the HINT sentences is naturally more similar to that of the classical noise, since this noise wasoriginally constructed from the average power spectrum of one hundred HINT sentences.
Thus, the classical HINT noise takes the averaged spectral characteristics of the speech stimuli into
account, but not the low-frequency modulations. The office background noise clearly includes thesemodulations, like the HINT sentences, while its LTAS resembles that of the classical HINT noise.
3.2 Speech reception thresholds
Fig. 7 shows individual SRTs as well as their median, mean and 25th/75 th percentiles for the NH andHI subjects in both the classical, reference HINT study (Ref. - left) and the current study (VSE - right).
The means/median values for NH ref and HIref subjects are -2.52/-2.49dB and 0.09/0.25dB, respectively.
Those for NHVSE and HIVSE are -1.17/-1.32dB and 1.54/1.58dB. The NH interquartile range is larger
for the VSE condition compared to the reference condition, while the situation is reversed for the HI.

Fig. 7 – SRTs for reference and VSE

Sources

Mean squaresሺ ଴Ǥ଴ହ ሻ

Hearing (NH – HI)

86.88<0.001

Condition (Ref. – VSE)

24.06

<0.001Hearing*Condition

0.03

0.87

Table 2 – Two-way ANOVA with interaction

A two-way analysis of variance (ANOVA) in Table 2 reveals significant differences between themeans of each combination of hearing status (NH - HI) and condition (Ref. - VSE). However, there isno significant interaction effect between hearing status and condition. This indicates that thedifference in performance between the NH and HI subjects is very similar in both conditions, at
2.74dB (Ref.) and 2.90dB (VSE). Likewise, the difference in performance between the reference andVSE conditions is very similar for both the NH and HI groups, at 1.17dB (NH) and 1.33dB (HI).

4. DISCUSSION
The results show that the difficulty of understanding speech in the HINT SI task is increased
significantly by conducting it inside an auralized CSS, as SRTs were found to increase by over 1dB
compared to the classical version, while retaining a stable difference between NH and HI SI
performance. The wider interquartile range for NH subjects in the VSE condition can be explained by
the more variable levels occurring in the office background noise. The fact that the interquartile range
for the HI is largest in the reference condition is most likely due to those subjects’ more variable
hearing thresholds.
The realistic SI task as defined here differs in two major ways from existing SI procedures. First,
the background noise was constructed from a real-world in-situ recording instead of speech-shaped
stationary noise. Second, target and background stimuli were spatialized, not presented diotically.
Both modifications were intended to increasing experimental realism and ecological validity.
Considering the spectral characteristics of the stimuli, the modulation spectrum of the realistic
background noise is similarly shaped to that of the HINT sentences, as opposed to the classical HINT
noise. The presence of these modulations may lead to poorer perception of the speech stimulus in therealistic background, due to effects of modulation masking. On the other hand, using modulated
maskers in an SI task can introduce “listening-in-the-dips” (17), increasing speech intelligibility.
Therefore, the specific structure and contextual properties of the speech components in the noise must
play a role as well, for instance through informational masking (18).

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Besides the stimuli’s spectral properties, their spatialization could also decrease SI performance.
The diotic speech stimuli of the classical HINT do not contain any reverberation, a property which is
widely known to decrease speech intelligibility (19). The spectral characteristics of the single-channel
and auralized versions of the HINT sentences are very similar, implying that the spatialization
preserves the long-term average and modulation spectral properties.
A more cognitively inspired argument in support of increasing realism in SI paradigms builds onthe concept of experimental realism, as opposed to mundane realism, as defined in psychology.
Mundane realism refers to experimental conditions that mimic those of the real world as closely as
possible, whereas experimental realism indicates the extent to which subjects actually experience theirconditions as realistic (20). Therefore, to obtain meaningful results from a subject, his or her
perception of realism can be just as important as its objective realization.
Even though the VSE did not include visuals, based on a 5-point modified Likert questionnaire
given to all subjects, the reproduced environment was found to be very realistic (4), and subjective
speech understanding was very to extremely difficult for hearing-impaired subjects (4-5). Respective
reasons for these ratings were familiarity with similar situations in daily life, and the acoustic andcognitive interference of the intelligible background speech with the stimuli that needed to beattended.
However, is not clear if this increased perceptual difficulty corresponds to the actually experienced
ecological struggle of HI people. One way to relate SRTs to reported real-world SNRs is by looking at
which percentage of these SNRs are below a specific SRT. This provides a quantitative measure of the
proportion of real-world sound scenarios in which subjects’ SI would be below 50%, assuming the SI
task is representative of the real world.
Applying this strategy here shows that the mean SRTs for the VSE background intersect acumulative Gaussian distribution calculated from SNR distribution data collected by Pearsons et al.
and Smeds et al. (21) at respectively 11.8% and 9.3% for NH subjects. This means that a predicted
intelligibility of less than 50% would only occur in about one out of ten real-world scenarios. For theHI, these values are 24.0% and 14.9%. Conducting the same procedure for the reference condition
gives values of 6.8/6.1% (NH) and 10.7%/8.6% (HI), respectively. It is unclear what % correct in termsof speech intelligibility is necessary for communication in the real world, but if 50% is taken as ameasure, these results indicate that the number of situations HI people struggle can be as high as one in
four. Applying a threshold of 75% SI for proper communication raises these values by about another
5%. For SRTs obtained in an SI task to meaningfully quantify these struggles, this comparison can help
determine accurate and representative paradigms. The elevated percentages that result from a more
realistic SI task seem to be more in line with the amount of situational difficulties HI people report.
The VSE condition is limited in its acoustical accuracy by the recording and reproduction methods.
However, the application of dual-band decoding, as used here, significantly increases the spatial
aliasing frequency of an HOA system (22), and as said, the subjective perceptual impression of the
reproduction was deemed highly realistic. As said, no visual or situational context was available to thesubjects during the SI task, the presence of which should decrease SRT scores. Thus, an SI task thatwould include visuals might need to be made even more challenging acoustically to remain
representative of real-world difficulties. As such, it is likely that the SI paradigm with a realistic
background developed here could still be improved to yield higher SRTs, akin to reality.

5. CONCLUSION
In this study, a novel implementation of a speech intelligibility task was realized, driven byprinciples of ecological validity and experimental realism at every stage in its development.
It was shown that both NH and HI SRTs obtained in an HOA-reproduced office meeting CSS are, on
average, over 1dB higher compared to the classical headphone-based HINT reference condition. A
comparison of the used signals suggests that this increase in difficulty is primarily caused by the
speech-like envelope modulations and intelligibility of the SI masking noise, as well as the reverberant
properties of the spatialized stimuli. By quantitatively relating SRTs to cumulative distributions ofreal-world recorded SNRs, it followed that mild to moderate hearing impairments might lead to lessthan chance level speech intelligibility in over 20% of critical sound scenarios.
To conclude, the present work suggests that introducing controlled realism in a speech
intelligibility paradigm can produce results that are more representative of real-world conditions. Suchparadigms may be more appropriate when attempting to characterize hearing difficulties that are notexperienced in a laboratory setting, but in the real world.

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