Adam Westermann

Binaural dereverberation based on interaural coherence histograms

A binaural dereverberation algorithm is presented that utilizes the properties of the interaural coherence (IC) inspired by the concepts introduced in Allen et al. [J. Acoust. Soc. Am. 62, 912-915 (1977)]. The algorithm introduces a non-linear sigmoidal coherence-to-gain mapping that is controlled by an online estimate of the present coherence statistics. The algorithm automatically adapts to a given acoustic environment and provides a stronger dereverberation effect than the original method presented in Allen et al. [J. Acoust. Soc. Am. 62, 912-915 (1977)] in most acoustic conditions. The performance of the proposed algorithm was objectively and subjectively evaluated in terms of its impacts on the amount of reverberation and overall quality. A binaural spectral subtraction method based on Lebart et al. [Acta Acust. Acust. 87, 359-366 (2001)] and a binaural version of the original method of Allen et al. were considered as reference systems. The results revealed that the proposed coherence-based approach is most successful in acoustic scenarios that exhibit a significant spread in the coherence distribution where direct sound and reverberation can be segregated. This dereverberation algorithm is thus particularly useful in large rooms for short source-receiver distances.

The effect of spatial separation in distance on the intelligibility of speech in rooms

The influence of spatial separation in source distance on speech reception thresholds (SRTs) is investigated. In one scenario, the target was presented at 0.5 m distance, and the masker varied from 0.5 m distance up to 10 m. In a second scenario, the masker was presented at 0.5 m distance and the target distance varied. The stimuli were synthesized using convolution with binaural room impulse responses (BRIRs) measured on a dummy head in a reverberant auditorium, and were equalized to compensate for distance-dependent spectral and intensity changes. All sources were simulated directly in front of the listener. SRTs decreased monotonically when the target was at 0.5 m and the speech-masker was moved further away, resulting in a SRT improvement of up to 10 dB. When the speech masker was at 0.5 m and the target was moved away, a large variation across subjects was observed. Neither short-term signal-to-noise ratio (SNR) improvements nor cross-ear glimpsing could account for the observed improvement in intelligibility. However, the effect might be explained by an improvement in the SNR in the modulation domain and a decrease in informational masking. This study demonstrates that distance-related cues can play a significant role when listening in complex environments.

The influence of informational masking in reverberant, multi-talker environments.

The relevance of informational masking (IM) in real-world listening is not well understood. In literature, IM effects of up to 10 dB in measured speech reception thresholds (SRTs) are reported. However, these experiments typically employed simplified spatial configurations and speech corpora that magnified confusions. In this study, SRTs were measured with normal hearing subjects in a simulated cafeteria environment. The environment was reproduced by a 41-channel 3D-loudspeaker array. The target talker was 2 m in front of the listener and masking talkers were either spread throughout the room or colocated with the target. Three types of maskers were realized: one with the same talker as the target (maximum IM), one with talkers different from the target, and one with unintelligible, noise-vocoded talkers (minimal IM). Overall, SRTs improved for the spatially distributed conditions compared to the colocated conditions. Within the spatially distributed conditions, there was no significant difference between thresholds with the different- and vocoded-talker maskers. Conditions with the same-talker masker were the only conditions with substantially higher thresholds, especially in the colocated conditions. These results suggest that IM related to target-masker confusions, at least for normal-hearing listeners, is of low relevance in real-life listening.

Cochlear implant speech intelligibility outcomes with structured and unstructured binary mask errors

It has been shown that intelligibility can be improved for cochlear implant (CI) recipients with the ideal binary mask (IBM). In realistic scenarios where prior information is unavailable, however, the IBM must be estimated, and these estimations will inevitably contain errors. Although the effects of both unstructured and structured binary mask errors have been investigated with normal-hearing (NH) listeners, they have not been investigated with CI recipients. This study assesses these effects with CI recipients using masks that have been generated systematically with a statistical model. The results demonstrate that clustering of mask errors substantially decreases the tolerance of errors, that incorrectly removing target-dominated regions can be as detrimental to intelligibility as incorrectly adding interferer-dominated regions, and that the individual tolerances of the different types of errors can change when both are present. These trends follow those of NH listeners. However, analysis with a mixed effects model suggests that CI recipients tend to be less tolerant than NH listeners to mask errors in most conditions, at least with respect to the testing methods in each of the studies. This study clearly demonstrates that structure influences the tolerance of errors and therefore should be considered when analyzing binary-masking algorithms.

The effect of hearing loss on source-distance dependent speech intelligibility in rooms

Westermann and Buchholz [(2015). J. Acoust. Soc. Am. 137(2), 757-767] found substantial improvements in speech reception thresholds (SRTs) for normal hearing listeners in a reverberant auditorium when the target talker was separated in distance from a two-talker masker. This study applied similar methodology, but tested listeners with a hearing impairment. On average, the participants received a 7 dB benefit in SRTs when the target was fixed at 0.5 m and the masker was moved from 0.5 to 10 m. But when the target was moved away, the SRTs increased by 5 dB. This indicates that hearing impaired listeners have difficulties suppressing nearby maskers while focusing attention on a far target.

The effect of nearby maskers on speech intelligibility in reverberant, multi-talker environmentsa)

The extent to which informational masking (IM) is involved in real-world listening is not well understood. In the literature, IM effects of more than 8 dB are reported, but these experiments typically used simplified spatial configurations and speech materials with exaggerated confusions. Westermann and Buchholz [(2015b). J. Acoust. Soc. Am. 138, 584-593] considered a simulated cafeteria and found only substantial IM effects when the target and maskers were colocated and the same talker. The present study further investigates the relevance of IM in real-world environments, specifically distractions by nearby maskers and the effect of hearing impairment. Speech reception thresholds (SRTs) were measured with normal hearing (NH) and sensorineural hearing impaired (HI) listeners in a simulated cafeteria environment. Three different masker configurations were considered: (1) seven dialogues distributed in the cafeteria, (2) two monologues presented close to the listener with varying angular separation, and (3) a combination of (1) and (2). The contribution of IM was measured as the difference in SRTs between speech maskers and unintelligible vocoded maskers. No significant IM was found with the seven dialogues alone. Including nearby maskers resulted in substantial IM for both NH and HI listeners, suggesting that such maskers might result in IM in real-world environments.

Release from masking through spatial separation in distance in hearing impaired listeners

It is widely accepted that speech intelligibility improves as a speech signal and interfering masker are separated spatially in azimuth. In a previous study [Westermann et al. (2012), IHCON] a similarly strong improvement was found for normal hearing (NH) listeners when target and masker are separated in distance. In this study speech reception thresholds (SRTs) were measured for 16 hearing impaired (HI) listeners using the Listening in Spatialized Noise Sentences Test (LiSN-S) and the Coordinate Response Measure (CRM). Acoustic scenarios were auralized via headphones using binaural room impulse responses recorded in an auditorium. In the first scenario the target was presented at a distance of 0.5 m from the center of the listeners head and the interferer at a distance of 0.5 m or 10 m. In a second setup the interferer′s location was fixed and the target′s location was varied. HI listeners showed a substantial release from masking as target and interferer were separated in distance. This effect was consistent for both LiSN-S and CRM, but less pronounced than for NH listeners. This study suggests that distance related cues play a significant role when listening in complex environments and are also to some extent available to HI listeners.

The impact of reverberation on speech intelligibility in cochlear implant recipients

Listening to speech in an environment with reverberation can be challenging for both the normal and impaired auditory system. However, it has been shown for both normal- and impaired-hearing listeners that it is the late reflections that are responsible for degrading intelligibility, whereas early reflections actually aid intelligibility by increasing the effective signal-to-noise ratio. Contrastingly, studies conducted with cochlear implant (CI) recipients have suggested that CI recipients have almost no tolerance for reverberation and that they are negatively impacted by both early and late reflections. The main objective of the current study is to re-evaluate the influence of reverberation on speech intelligibility in CI recipients using more authentic virtual auditory environments. Unlike previous studies in this area, this study was conducted using a loudspeaker-based auralization system rather than non-individualized binaural room simulations. Speech intelligibility was measured in simulations of a range of actual physical rooms with plausible source-receiver distances, both with and without late reflections. The results show that the effect of reverberation is much smaller than previously suggested, especially with short source-receiver distances. Furthermore, the results suggest that, in contrast to previous literature, early reflections may not actually be detrimental to CI recipients.

The influence of informational masking in complex real-world environments

Spatial release from masking (SRM) is believed to be an essential auditory mechanism aiding listeners in reverberant multi-talker environments. However, SRM is often measured in simplified spatial configurations using speech corpora with exaggerated talker and/or context confusions. Besides energetic better-ear listening and binaural unmasking, the perceived spatial separation of target and masking speech signals is thought to aid listeners segregation of speech signals, resulting in a so-called release from informational masking. This study aims to estimate the amount of informational masking that is apparent in complex real-world environments. Speech reception thresholds (SRTs) were measured by presenting Bamford-Kowal-Bench (BKB) sentences in a simulated cafeteria environment recreated by a spherical array of 41 loudspeakers placed in an anechoic chamber. Three maskers with varying degree of informational masking were realized: one with talkers different from the target, one with an unintelligible noise vocoder (minimal informational masking) and one with the same talker as the target (maximum informational masking). The maskers were constructed with either two or seven two-talker conversations and were either spatially distributed in the simulated cafeteria or colocated with the target. Seven normal hearing listeners were tested. All conditions showed improved thresholds for the spatialized condition compared to the colocated condition. However there was no significant difference between the different talker speech and vocoded masker. Only the same talker masker showed increased thresholds and this was only substantial in the two conversation colocated condition. These results suggest that informational masking is of low relevance in real-life listening and is exaggerated in listening tests by target/masker similarities and the colocated spatial configuration. However, this may be different in (aided) hearing impaired listeners where spectral and spatial cues can be significantly disturbed.