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

Adaptation to room acoustics and its effect of speech understanding
Pavel ZAHORIK1
1

University of Louisville, USAABSTRACT
Effective sound transmission between source and receiver is essential for good communication and soundquality in learning spaces. Of course, sound transmission can be significantly affected by the acoustics ofthe space. Under many circumstances, however, these acoustical effects have relatively minor perceptual
consequences. This may be explained, in part, by listener adaptation to the acoustics of the listening
environment. Here, evidence that room adaptation improves speech understanding is summarized. Theadaptation, is rapid (around 1 second), strongest for rooms with reverberation times between 0.4 and 1 second,and observable for a variety of speech materials. It also appears to depend critically on the amplitude
modulation characteristic of the signal reaching the ear. A better understanding of room adaptation effects
can inform and contribute to methods for improving speech understanding and sound quality in rooms forboth normally-hearing and hearing-impaired listeners.
Keywords: Speech Intelligibility, Room Acoustics, Virtual Auditory Space

1. BACKGROUND
It has long been known that speech understanding is degraded by reverberation (1). The
degradation stems primarily from temporal distortion of the speech signal caused by the reverberation(2, 3), and is known to scale with the amount of reverberation (1, 4). Because most everyday
communication situations involve sound transmission within a reverberant sound field, it is criticallyimportant to understand how and by what mechanisms speech understanding is impacted byreverberation. This is especially true, given that the negative effects of reverberation on speech
understanding are exacerbated both by background noise (1, 5) – another ubiquitous property of
everyday listening environments, and by hearing loss (6), where poor performance in reverberation is
the most frequent complaint given by hearing aid users (7). Given these challenges, it is perhaps
remarkable that for individuals with normal hearing, few communication problems are encountered ineveryday reverberant environments. This suggests that processing in the normally functioning
auditory system must effectively counteract the deleterious effects of reflected sound andreverberation, even though acoustically, these effects are clearly measurable and specific to a given
listening environment and the spatial configuration of components in the communication chain.
The auditory system has a number of mechanisms that can immediately assist with speech
understanding in reverberation, including the binaural system (8) and mechanisms related to theprecedence effect (see 9, and 10 for reviews). Beyond these immediate effects, there is now emerging
evidence that prior listening exposure to reverberation can provide an environmental listening context
that renders speech as perceptually less reverberant (11, 12) and can result in objective improvementsin speech intelligibility (13). This suggests that the processing of sound in reverberant space within
the normally functioning auditory system may involve processes more complicated than previouslythought. The goal of the present study is to determine the extent to which this environmental context
effect depends on the acoustics of the listening space.
Watkins (11, 12) was the first to demonstrate an effect of listening context on speech perception inreverberation. He used target speech signals on an 11-point continuum from “sir” to “stir” embedded
in a carrier phrase, and noted the point at which the speech percept changed from “sir” to “stir” – acategorical perception task. When both target and carrier phrase were presented in minimal
reverberation, the change point was near the center of the continuum. When the target was presented
in moderate reverberation, but the carrier phrase remained in minimal reverberation, the change point
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pavel.zahorik@louisville.edu

shifted toward “sir”. This can be explained by reverberant energy filling in the temporal gap
following the stop consonant in “stir”, causing it to be perceived as more like “sir”. When both thetarget and the carrier phrase were presented in reverberation, the change point shifted back to where it
was observed when both target and carrier were presented in minimal reverberation. This suggeststhat the reverberant carrier phrase provides contextual information that allows the auditory system tocompensate for the effects of the reverberation on the target word. Watkins and his colleagues haveinterpreted this result as being consistent with a type of high-level perceptual constancy, similar toother well-known perceptual constancies in vision, such as brightness constancy or color constancy.
They have also demonstrated the effect with additional speech continua (14) and non-speech contexts(15, 16), and have shown that the effect is driven primarily by the amplitude envelope of the speech
signal reaching the listener (17). This latter result is appealing; because of its potential links to themodulation transfer function concept, which forms the basis for standard methods of predicting speech
intelligibility in rooms, such as the speech-transmission index (STI, 18) and speech-intelligibilityindex (SII, 19).
Brandewie and Zahorik (13) reported context effects similar to those identified by Watkins and hiscolleagues, but using different methods. In their study, Brandewie and Zahorik (13) compared speech
reception thresholds (SRTs) using the coordinate response measure (CRM, 20) in a background of
spatially separate noise within a simulated reverberant room. Two different listening conditions weretested. In one condition, listeners were provided with consistent listening exposure to the same
reverberant room, both within and across trials. In a second condition, consistent exposure to theroom was disrupted by removing the CRM carrier phrase and by changing the room from trial to trial.
SRTs were found to be 2-3 dB lower, on average, in the consistent exposure condition. This suggeststhat consistent environmental listening context in a reverberant room can facilitate speech
understanding. Similar context effects were not observed when the test room was anechoic,
suggesting that the context effects are specific to reverberant sound fields. Additional work has
demonstrated that the effect generalizes to highly heterogeneous sentence materials (21), is fully
activated by approximately 1 s of listening exposure (22), is strongest for rooms with moderate
reverberation times (0.4 ≤ RT60 ≤ 1 s, 23), and is relatively independent of location within the room(24). The importance of the amplitude envelope in the room context effect has also been
demonstrated using similar methods (25).
In addition to speech understanding, similar short-term adaptation to aspects of room acoustics has
been observed in listening tasks such as: echo detection (26), sound localization (27, 28), judgments ofloudspeaker sound quality (29), and amplitude modulation detection (30). In all cases, the adaptation
results in what can be considered improvements in listening performance, thus clearly demonstrating
the importance of this effect for natural, everyday listening that contains indirect sound. Althoughthese effects may be specific examples of perceptual priming (31) in the auditory modality, it is atpresent not clearly understood what mechanisms or basic auditory capabilities underlie the adaptation.
This is a critical gap in knowledge that is preventing further progress in this area.

2. NEXT STEPS
2.1 Theoretical Framework
Now that the existence of room adaptation effects have been clearly demonstrated fornormal-hearing listeners, and certain basic characteristics of the effects have been identified, there is a
critical need to reveal the basic auditory functions that underlie these effects and how they may
interact with other known effects (e.g. Binaural Squelch) to facilitate improved speech understanding
in reverberation. It is hypothesized that both monaural and binaural processing of the amplitude
modulation (AM) information underlies these room adaptation effects through the conceptual
framework shown in Figure 1. The basis for this framework is the modulation transfer function(MTF) concept (3), which, along with its binaural extension (32), has been shown to be highly
effective at predicting speech intelligibility in complex listening situations including reverberant
rooms with background noise. To extend the MTF-concept to explain room adaptation, we
hypothesize an additional stage that provides compensation for the MTF of the room. Through thecombination of room MTF estimation and compensation, information about the AM characteristics ofthe sound source may be gained over time, which is independent from the acoustical effects of theroom. In general, this is a type of process that seeks inference regarding distal source properties in
the face of degraded or underspecified proximal sensory input. Such processes are thought to support5891

a variety of perceptual phenomenon, including the constancies of brightness (33), size (34), andloudness (35). Neural evidence demonstrating the reverberation-resistant coding of AM in theinferior colliculus (36, 37) supports the ideas of MTF estimation and compensation in our framework,
as does the fact that room adaptation effects on speech understanding and perception have been shownto depend specifically on the time-varying amplitude envelope of the speech signal (17, 25). Further
work will be needed to test hypotheses stemming from this conceptual framework.

Room Adaptation
ESIR Mechanism

Estimate
Room MTF

Compensate
Representationof source AM
characteristic

Auditory Input(monaural/binaural)

Figure 1 - Proposed conceptual framework to explain room adaptation.
2.2 Monaural Versus Binaural Contributions
A fundamental issue that must be addressed is the role of monaural versus binaural input. Work byWatkins (11) has clearly demonstrated monaural compensation for reverberation, and monaural cells
in the inferior colliculus have been identified that are reverberation-resistant (36, 38). On the otherhand, Brandewie and Zahorik (13) have concluded that room adaptation appears to require binauralinput. This conclusion may be premature for at least two reasons, however. 1) Two of 14 listeners still
showed adaptation effects, even for monaural input, and 2) monaural input was always tested for using
the ear contralateral to the noise source (the “better” ear, in terms of signal-to-noise ratio), which may
have influenced the results. Additional studies are therefore needed to clarify the role of monauralversus binaural input in the observed room adaptation effects.
2.3 Room MTF Development over TimeWe have operationalized the conceptual framework described above for the case of monaural room
impulse response (RIR) input by first passing this input through a gammatone filterbank (39) tosimulate peripheral auditory filtering and then computing the MTF in each band from the RIR using amethod described by Schroeder (5). We then examined the effect of exposure time by varying thelength of the RIR used for analysis, ranging from the first 5 ms (near anechoic) to the entire RIR, which
was approximately 1.8 s in this case. Results of this analysis are shown in Figure 2, where a clear
development of the room MTF characteristic is evident over time. By 1000 ms of exposure, the room
MTF is fully developed in all but the lowest modulation frequencies. We suggest that this strategy ofanalyzing the developmental time course of MTF estimates may help explain time-course results forroom adaptation effects, where enhancement for a similar room was found to increase as a function ofexposure time up to about 1 s and then plateau (22). This suggestion is based on the idea thatcompensation in our hypothesized framework would be incomplete without an accurate estimate of theroom MTF, which is only available after sufficient exposure time. These preliminary data haveimportant implications, because they suggest that the room adaptation effects might be explainable
based on acoustical properties of the MTF that develop over time, and on relatively low-level auditory
processes that encode these properties. This approach will be especially relevant because like the STI(3), it can be adapted to take running speech as an input. Further scientific study will be needed todefinitively determine if such temporal development of the MTF is related to the effects of room
adaptation on speech understanding.

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Full IR…

…

Modulation Gain (dB)Frequency (Hz)IR Analysis Duration

Modulation Frequency (Hz)Figure 2 - Room modulation transfer functions (MTFs) as a function of auditory filter center frequency
and analysis window length. MTFs were derived from the impulse response (IR) at the left ear for afrontal sound source (1.4 m distance) in a simulated reverberant room (broadband RT60 = 1.8 s).
2.4 Impact of Hearing Loss
Results of a preliminary study (40) using testing methods identical to those used by Brandewie andZahorik (13) found that some hearing impaired users may also exhibits room adaptation effects related
to speech understanding, although large individual differences were observed. These differenceswere not clearly related to the large differences in pure-tone audiometric thresholds for the sample oflisteners tested. Therefore, additional study will be required to fully understand how hearing loss
may impact room adaptation effects.

3. IMPLICATIONS FOR ROOM DESIGN/COMFORT
Although most natural listening situations would allow for listening experience with an acousticalenvironment of sufficient time to activate the described room adaptation effects, there are a number ofimportant complexities related to this adaptation that are not yet known. For example, how might thepsychophysical effects of room adaptation be impacted by other devices in the communication chain,such as sound reinforcement systems, sound assistive devices (e.g. hearing aids), or teleconferencing
systems? As pointed out in the STI standard (18), such devices in the communication chain affect theMTF. To the extend that the MTF is involved in psychophysical room adaptation, such devices may
impact adaptation. There is also the issue of different or conflicting room acoustic information
potentially being conveyed through the device. This may also impact room adaptation beyond theMTF. Additional research will be needed to explore these implications.

ACKNOWLEDGEMENTS
This work was supported in part by the University of Louisville and the NIH (R01DC008168).

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