Effects of fluctuating noise and interfering speech on the speech-reception threshold for impaired and normal hearing

The speech-reception threshold (SRT) for sentences presented in a fluctuating interfering background sound of 80 dBA SPL is measured for 20 normal-hearing listeners and 20 listeners with sensorineural hearing impairment. The interfering sounds range from steady-state noise, via modulated noise, to a single competing voice. Two voices are used, one male and one female, and the spectrum of the masker is shaped according to these voices. For both voices, the SRT is measured as well in noise spectrally shaped according to the target voice as shaped according to the other voice. The results show that, for normal-hearing listeners, the SRT for sentences in modulated noise is 4-6 dB lower than for steady-state noise; for sentences masked by a competing voice, this difference is 6-8 dB. For listeners with moderate sensorineural hearing loss, elevated thresholds are obtained without an appreciable effect of masker fluctuations. The implications of these results for estimating a hearing handicap in everyday conditions are discussed. By using the articulation index (AI), it is shown that hearing-impaired individuals perform poorer than suggested by the loss of audibility for some parts of the speech signal. Finally, three mechanisms are discussed that contribute to the absence of unmasking by masker fluctuations in hearing-impaired listeners. The low sensation level at which the impaired listeners receive the masker seems a major determinant. The second and third factors are: reduced temporal resolution and a reduction in comodulation masking release, respectively.     

Contributions of talker characteristics and spatial location to auditory streaming

To examine whether auditory streaming contributes to unmasking, intelligibility of target sentences against two competing talkers was measured using the coordinate response measure (CRM) [Bolia et al., J. Acoust. Soc. Am. 107, 1065-1066 (2007)] corpus. In the control condition, the speech reception threshold (50% correct) was measured when the target and two maskers were collocated straight ahead. Separating maskers from the target by +/-30 degrees resulted in spatial release from masking of 12 dB. CRM sentences involve an identifier in the first part and two target words in the second part. In experimental conditions, masking talkers started spatially separated at +/-30 degrees but became collocated with the target before the scoring words. In one experiment, one target and two different maskers were randomly selected from a mixed-sex corpus. Significant unmasking of 4 dB remained despite the absence of persistent location cues. When same-sex talkers were used as maskers and target, unmasking was reduced. These data suggest that initial separation may permit confident identification and streaming of the target and masker speech where significant differences between target and masker voice characteristics exist, but where target and masker characteristics are similar, listeners must rely more heavily on continuing spatial cues.     

Tactual display of consonant voicing as a supplement to lipreading

This research is concerned with the development and evaluation of a tactual display of consonant voicing to supplement the information available through lipreading for persons with profound hearing impairment. The voicing cue selected is based on the envelope onset asynchrony derived from two different filtered bands (a low-pass band and a high-pass band) of speech. The amplitude envelope of each of the two bands was used to modulate a different carrier frequency which in turn was delivered to one of the two fingers of a tactual stimulating device. Perceptual evaluations of speech reception through this tactual display included the pairwise discrimination of consonants contrasting voicing and identification of a set of 16 consonants under conditions of the tactual cue alone (T), lipreading alone (L), and the combined condition (L + T). The tactual display was highly effective for discriminating voicing at the segmental level and provided a substantial benefit to lipreading on the consonant-identification task. No such benefits of the tactual cue were observed, however, for lipreading of words in sentences due perhaps to difficulties in integrating the tactual and visual cues and to insufficient training on the more difficult task of connected-speech reception.     

Informational and energetic masking effects in the perception of two simultaneous talkers

Although most recent multitalker research has emphasized the importance of binaural cues, monaural cues can play an equally important role in the perception of multiple simultaneous speech signals. In this experiment, the intelligibility of a target phrase masked by a single competing masker phrase was measured as a function of signal-to-noise ratio (SNR) with same-talker, same-sex, and different-sex target and masker voices. The results indicate that informational masking, rather than energetic masking, dominated performance in this experiment. The amount of masking was highly dependent on the similarity of the target and masker voices: performance was best when different-sex talkers were used and worst when the same talker was used for target and masker. Performance did not, however, improve monotonically with increasing SNR. Intelligibility generally plateaued at SNRs below 0 dB and, in some cases, intensity differences between the target and masking voices produced substantial improvements in performance with decreasing SNR. The results indicate that informational and energetic masking play substantially different roles in the perception of competing speech messages.     

Precedence-based speech segregation in a virtual auditory environment

When a masking sound is spatially separated from a target speech signal, substantial releases from masking typically occur both for speech and noise maskers. However, when a delayed copy of the masker is also presented at the location of the target speech (a condition that has been referred to as the front target, right-front masker or F-RF configuration), the advantages of spatial separation vanish for noise maskers but remain substantial for speech maskers. This effect has been attributed to precedence, which introduces an apparent spatial separation between the target and masker in the F-RF configuration that helps the listener to segregate the target from a masking voice but not from a masking noise. In this study, virtual synthesis techniques were used to examine variations of the F-RF configuration in an attempt to more fully understand the stimulus parameters that influence the release from masking obtained in that condition. The results show that the release from speech-on-speech masking caused by the addition of the delayed copy of the masker is robust across a wide variety of source locations, masker locations, and masker delay values. This suggests that the speech unmasking that occurs in the F-RF configuration is not dependent on any single perceptual cue and may indicate that F-RF speech segregation is only partially based on the apparent left-right location of the RF masker.     

Effects of periodic masker interruption on the intelligibility of interrupted speech

When listeners hear a target signal in the presence of competing sounds, they are quite good at extracting information at instances when the local signal-to-noise ratio of the target is most favorable. Previous research suggests that listeners can easily understand a periodically interrupted target when it is interleaved with noise. It is not clear if this ability extends to the case where an interrupted target is alternated with a speech masker rather than noise. This study examined speech intelligibility in the presence of noise or speech maskers, which were either continuous or interrupted at one of six rates between 4 and 128 Hz. Results indicated that with noise maskers, listeners performed significantly better with interrupted, rather than continuous maskers. With speech maskers, however, performance was better in continuous, rather than interrupted masker conditions. Presumably the listeners used continuity as a cue to distinguish the continuous masker from the interrupted target. Intelligibility in the interrupted masker condition was improved by introducing a pitch difference between the target and speech masker. These results highlight the role that target-masker differences in continuity and pitch play in the segregation of competing speech signals.     

Speech detection in spatial and nonspatial speech maskers

The effect of perceived spatial differences on masking release was examined using a 4AFC speech detection paradigm. Targets were 20 words produced by a female talker. Maskers were recordings of continuous streams of nonsense sentences spoken by two female talkers and mixed into each of two channels (two talker, and the same masker time reversed). Two masker spatial conditions were employed: "RF" with a 4 ms time lead to the loudspeaker 60 degrees horizontally to the right, and "FR" with the time lead to the front (0 degrees ) loudspeaker. The reference nonspatial "F" masker was presented from the front loudspeaker only. Target presentation was always from the front loudspeaker. In Experiment 1, target detection threshold for both natural and time-reversed spatial maskers was 17-20 dB lower than that for the nonspatial masker, suggesting that significant release from informational masking occurs with spatial speech maskers regardless of masker understandability. In Experiment 2, the effectiveness of the FR and RF maskers was evaluated as the right loudspeaker output was attenuated until the two-source maskers were indistinguishable from the F masker, as measured independently in a discrimination task. Results indicated that spatial release from masking can be observed with barely noticeable target-masker spatial differences.     

Cochlear implant speech recognition with speech maskers

Speech recognition performance was measured in normal-hearing and cochlear-implant listeners with maskers consisting of either steady-state speech-spectrum-shaped noise or a competing sentence. Target sentences from a male talker were presented in the presence of one of three competing talkers (same male, different male, or female) or speech-spectrum-shaped noise generated from this talker at several target-to-masker ratios. For the normal-hearing listeners, target-masker combinations were processed through a noise-excited vocoder designed to simulate a cochlear implant. With unprocessed stimuli, a normal-hearing control group maintained high levels of intelligibility down to target-to-masker ratios as low as 0 dB and showed a release from masking, producing better performance with single-talker maskers than with steady-state noise. In contrast, no masking release was observed in either implant or normal-hearing subjects listening through an implant simulation. The performance of the simulation and implant groups did not improve when the single-talker masker was a different talker compared to the same talker as the target speech, as was found in the normal-hearing control. These results are interpreted as evidence for a significant role of informational masking and modulation interference in cochlear implant speech recognition with fluctuating maskers. This informational masking may originate from increased target-masker similarity when spectral resolution is reduced.     

Effect of masker type and age on speech intelligibility and spatial release from masking in children and adults

Speech recognition in noisy environments improves when the speech signal is spatially separated from the interfering sound. This effect, known as spatial release from masking (SRM), was recently shown in young children. The present study compared SRM in children of ages 5-7 with adults for interferers introducing energetic, informational, and/or linguistic components. Three types of interferers were used: speech, reversed speech, and modulated white noise. Two female voices with different long-term spectra were also used. Speech reception thresholds (SRTs) were compared for: Quiet (target 0 degrees front, no interferer), Front (target and interferer both 0 degrees front), and Right (interferer 90 degrees right, target 0 degrees front). Children had higher SRTs and greater masking than adults. When spatial cues were not available, adults, but not children, were able to use differences in interferer type to separate the target from the interferer. Both children and adults showed SRM. Children, unlike adults, demonstrated large amounts of SRM for a time-reversed speech interferer. In conclusion, masking and SRM vary with the type of interfering sound, and this variation interacts with age; SRM may not depend on the spectral peculiarities of a particular type of voice when the target speech and interfering speech are different sex talkers.     

Spatial unmasking of birdsong in human listeners: energetic and informational factors

Spatial unmasking describes the improvement in the detection or identification of a target sound afforded by separating it spatially from simultaneous masking sounds. This effect has been studied extensively for speech intelligibility in the presence of interfering sounds. In the current study, listeners identified zebra finch song, which shares many acoustic properties with speech but lacks semantic and linguistic content. Three maskers with the same long-term spectral content but different short-term statistics were used: (1) chorus (combinations of unfamiliar zebra finch songs), (2) song-shaped noise (broadband noise with the average spectrum of chorus), and (3) chorus-modulated noise (song-shaped noise multiplied by the broadband envelope from a chorus masker). The amount of masking and spatial unmasking depended on the masker and there was evidence of release from both energetic and informational masking. Spatial unmasking was greatest for the statistically similar chorus masker. For the two noise maskers, there was less spatial unmasking and it was wholly accounted for by the relative target and masker levels at the acoustically better ear. The results share many features with analogous results using speech targets, suggesting that spatial separation aids in the segregation of complex natural sounds through mechanisms that are not specific to speech.     

Effect of target-masker similarity on across-ear interference in a dichotic cocktail-party listening task

Similarity between the target and masking voices is known to have a strong influence on performance in monaural and binaural selective attention tasks, but little is known about the role it might play in dichotic listening tasks with a target signal and one masking voice in the one ear and a second independent masking voice in the opposite ear. This experiment examined performance in a dichotic listening task with a target talker in one ear and same-talker, same-sex, or different-sex maskers in both the target and the unattended ears. The results indicate that listeners were most susceptible to across-ear interference with a different-sex within-ear masker and least susceptible with a same-talker within-ear masker, suggesting that the amount of across-ear interference cannot be predicted from the difficulty of selectively attending to the within-ear masking voice. The results also show that the amount of across-ear interference consistently increases when the across-ear masking voice is more similar to the target speech than the within-ear masking voice is, but that no corresponding decline in across-ear interference occurs when the across-ear voice is less similar to the target than the within-ear voice. These results are consistent with an "integrated strategy" model of speech perception where the listener chooses a segregation strategy based on the characteristics of the masker present in the target ear and the amount of across-ear interference is determined by the extent to which this strategy can also effectively be used to suppress the masker in the unattended ear.     

Variability and uncertainty in masking by competing speech

This study investigated the role of uncertainty in masking of speech by interfering speech. Target stimuli were nonsense sentences recorded by a female talker. Masking sentences were recorded from ten female talkers and combined into pairs. Listeners' recognition performance was measured with both target and masker presented from a front loudspeaker (nonspatial condition) or with a masker presented from two loudspeakers, with the right leading the front by 4 ms (spatial condition). In Experiment 1, the sentences were presented in blocks in which the masking talkers, spatial configuration, and signal-to-noise (S-N) ratio were fixed. Listeners' recognition performance varied widely among the masking talkers in the nonspatial condition, much less so in the spatial condition. This result was attributed to variation in effectiveness of informational masking in the nonspatial condition. The second experiment increased uncertainty by randomizing masking talkers and S-N ratios across trials in some conditions, and reduced uncertainty by presenting the same token of masker across trials in other conditions. These variations in masker uncertainty had relatively small effects on speech recognition.     

Intermodal timing relations and audio-visual speech recognition by normal-hearing adults

Audio-visual identification of sentences was measured as a function of audio delay in untrained observers with normal hearing; the soundtrack was replaced by rectangular pulses originally synchronized to the closing of the talker's vocal folds and then subjected to delay. When the soundtrack was delayed by 160 ms, identification scores were no better than when no acoustical information at all was provided. Delays of up to 80 ms had little effect on group-mean performance, but a separate analysis of a subgroup of better lipreaders showed a significant trend of reduced scores with increased delay in the range from 0-80 ms. A second experiment tested the interpretation that, although the main disruptive effect of the delay occurred on a syllabic time scale, better lipreaders might be attempting to use intermodal timing cues at a phonemic level. Normal-hearing observers determined whether a 120-Hz complex tone started before or after the opening of a pair of liplike Lissajou figures. Group-mean difference limens (70.7% correct DLs) were - 79 ms (sound leading) and + 138 ms (sound lagging), with no significant correlation between DLs and sentence lipreading scores. It was concluded that most observers, whether good lipreaders or not, possess insufficient sensitivity to intermodal timing cues in audio-visual speech for them to be used analogously to voice onset time in auditory speech perception. The results of both experiments imply that delays of up to about 40 ms introduced by signal-processing algorithms in aids to lipreading should not materially affect audio-visual speech understanding.     

Informational masking of speech produced by speech-like sounds without linguistic content

This study investigated whether speech-like maskers without linguistic content produce informational masking of speech. The target stimuli were nonsense Chinese Mandarin sentences. In experiment I, the masker contained harmonics the fundamental frequency (F0) of which was sinusoidally modulated and the mean F0 of which was varied. The magnitude of informational masking was evaluated by measuring the change in intelligibility (releasing effect) produced by inducing a perceived spatial separation of the target speech and masker via the precedence effect. The releasing effect was small and was only clear when the target and masker had the same mean F0, suggesting that informational masking was small. Performance with the harmonic maskers was better than with a steady speech-shaped noise (SSN) masker. In experiments II and III, the maskers were speech-like synthesized signals, alternating between segments with harmonic structure and segments composed of SSN. Performance was much worse than for experiment I, and worse than when an SSN masker was used, suggesting that substantial informational masking occurred. The similarity of the F0 contours of the target and masker had little effect. The informational masking effect was not influenced by whether or not the noise-like segments of the masker were synchronous with the unvoiced segments of the target speech.     

Beneficial acoustic speech cues for cochlear implant users with residual acoustic hearing

This study investigated which acoustic cues within the speech signal are responsible for bimodal speech perception benefit. Seven cochlear implant (CI) users with usable residual hearing at low frequencies in the non-implanted ear participated. Sentence tests were performed in near-quiet (some noise on the CI side to reduce scores from ceiling) and in a modulated noise background, with the implant alone and with the addition, in the hearing ear, of one of four types of acoustic signals derived from the same sentences: (1) a complex tone modulated by the fundamental frequency (F0) and amplitude envelope contours; (2) a pure tone modulated by the F0 and amplitude contours; (3) a noise-vocoded signal; (4) unprocessed speech. The modulated tones provided F0 information without spectral shape information, whilst the vocoded signal presented spectral shape information without F0 information. For the group as a whole, only the unprocessed speech condition provided significant benefit over implant-alone scores, in both near-quiet and noise. This suggests that, on average, F0 or spectral cues in isolation provided limited benefit for these subjects in the tested listening conditions, and that the significant benefit observed in the full-signal condition was derived from implantees' use of a combination of these cues.     

The spatial unmasking of speech: evidence for within-channel processing of interaural time delay

Across-frequency processing by common interaural time delay (ITD) in spatial unmasking was investigated by measuring speech reception thresholds (SRTs) for high- and low-frequency bands of target speech presented against concurrent speech or a noise masker. Experiment 1 indicated that presenting one of these target bands with an ITD of +500 micros and the other with zero ITD (like the masker) provided some release from masking, but full binaural advantage was only measured when both target bands were given an ITD of + 500 micros. Experiment 2 showed that full binaural advantage could also be achieved when the high- and low-frequency bands were presented with ITDs of equal but opposite magnitude (+/- 500 micros). In experiment 3, the masker was also split into high- and low-frequency bands with ITDs of equal but opposite magnitude (+/-500 micros). The ITD of the low-frequency target band matched that of the high-frequency masking band and vice versa. SRTs indicated that, as long as the target and masker differed in ITD within each frequency band, full binaural advantage could be achieved. These results suggest that the mechanism underlying spatial unmasking exploits differences in ITD independently within each frequency channel.     

The benefit of binaural hearing in a cocktail party: effect of location and type of interferer

The "cocktail party problem" was studied using virtual stimuli whose spatial locations were generated using anechoic head-related impulse responses from the AUDIS database [Blauert et al., J. Acoust. Soc. Am. 103, 3082 (1998)]. Speech reception thresholds (SRTs) were measured for Harvard IEEE sentences presented from the front in the presence of one, two, or three interfering sources. Four types of interferer were used: (1) other sentences spoken by the same talker, (2) time-reversed sentences of the same talker, (3) speech-spectrum shaped noise, and (4) speech-spectrum shaped noise, modulated by the temporal envelope of the sentences. Each interferer was matched to the spectrum of the target talker. Interferers were placed in several spatial configurations, either coincident with or separated from the target. Binaural advantage was derived by subtracting SRTs from listening with the "better monaural ear" from those for binaural listening. For a single interferer, there was a binaural advantage of 2-4 dB for all interferer types. For two or three interferers, the advantage was 2-4 dB for noise and speech-modulated noise, and 6-7 dB for speech and time-reversed speech. These data suggest that the benefit of binaural hearing for speech intelligibility is especially pronounced when there are multiple voiced interferers at different locations from the target, regardless of spatial configuration; measurements with fewer or with other types of interferers can underestimate this benefit.     

Relative contribution of target and masker temporal fine structure to the unmasking of consonants in noise

The present study assessed the relative contribution of the "target" and "masker" temporal fine structure (TFS) when identifying consonants. Accordingly, the TFS of the target and that of the masker were manipulated simultaneously or independently. A 30 band vocoder was used to replace the original TFS of the stimuli with tones. Four masker types were used. They included a speech-shaped noise, a speech-shaped noise modulated by a speech envelope, a sentence, or a sentence played backward. When the TFS of the target and that of the masker were disrupted simultaneously, consonant recognition dropped significantly compared to the unprocessed condition for all masker types, except the speech-shaped noise. Disruption of only the target TFS led to a significant drop in performance with all masker types. In contrast, disruption of only the masker TFS had no effect on recognition. Overall, the present data are consistent with previous work showing that TFS information plays a significant role in speech recognition in noise, especially when the noise fluctuates over time. However, the present study indicates that listeners rely primarily on TFS information in the target and that the nature of the masker TFS has a very limited influence on the outcome of the unmasking process.     

Comodulation masking release in speech identification with real and simulated cochlear-implant hearing

For normal-hearing (NH) listeners, masker energy outside the spectral region of a target signal can improve target detection and identification, a phenomenon referred to as comodulation masking release (CMR). This study examined whether, for cochlear implant (CI) listeners and for NH listeners presented with a "noise vocoded" CI simulation, speech identification in modulated noise is improved by a co-modulated flanking band. In Experiment 1, NH listeners identified noise-vocoded speech in a background of on-target noise with or without a flanking narrow band of noise outside the spectral region of the target. The on-target noise and flanker were either 16-Hz square-wave modulated with the same phase or were unmodulated; the speech was taken from a closed-set corpus. Performance was better in modulated than in unmodulated noise, and this difference was slightly greater when the comodulated flanker was present, consistent with a small CMR of about 1.7 dB for noise-vocoded speech. Experiment 2, which tested CI listeners using the same speech materials, found no advantage for modulated versus unmodulated maskers and no CMR. Thus although NH listeners can benefit from CMR even for speech signals with reduced spectro-temporal detail, no CMR was observed for CI users.     

Spatial unmasking of nearby speech sources in a simulated anechoic environment

Spatial unmasking of speech has traditionally been studied with target and masker at the same, relatively large distance. The present study investigated spatial unmasking for configurations in which the simulated sources varied in azimuth and could be either near or far from the head. Target sentences and speech-shaped noise maskers were simulated over headphones using head-related transfer functions derived from a spherical-head model. Speech reception thresholds were measured adaptively, varying target level while keeping the masker level constant at the "better" ear. Results demonstrate that small positional changes can result in very large changes in speech intelligibility when sources are near the listener as a result of large changes in the overall level of the stimuli reaching the ears. In addition, the difference in the target-to-masker ratios at the two ears can be substantially larger for nearby sources than for relatively distant sources. Predictions from an existing model of binaural speech intelligibility are in good agreement with results from all conditions comparable to those that have been tested previously. However, small but important deviations between the measured and predicted results are observed for other spatial configurations, suggesting that current theories do not accurately account for speech intelligibility for some of the novel spatial configurations tested.