Using HDC to evaluate signal similarity for information masking

In the many studies done on informational masking, interfering speech reduces speech intelligibility. This effect is often used to secure privacy in public spaces. These applications require estimates of how much masking is required. In general, masking effects are estimated by using spectrum information as excitation patterns. However, estimates of informational masking can hardly be obtained by only using spectrum information. Therefore, we estimated the effects of informational masking using time-domain information. Then, we calculated the cepstra of the envelopes’ magnitude histograms. If these cepstra are different between the target and the masker, the signals are not similar in the time-domain. Furthermore, the effect of informational masking would be low. Therefore, we considered the histograms’ cepstra distances (HCD) to estimate signal similarities. The signal similarities in our first experiment were estimated using five maskers by utilizing the HCD. These maskers were random noise, music, female speech, male speech, and target speaker’s speech. Male and female speech were more similar to the target speech than music and noise. Also, the same speaker’s speech was the most similar in the set of maskers. A listening test was carried out in the second experiment to verify the HCD. A double masker was used in this experiment as an effective informational masker. It has similar characteristics to reversal speech. The listening test results suggest the double-masker’s masking effects has the same relation with HCD. This suggests informational masking can be estimated by signal similarity using the HCD.     

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.     

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.     

The effects of spatial separation in distance on the informational and energetic masking of a nearby speech signal

Although many studies have shown that intelligibility improves when a speech signal and an interfering sound source are spatially separated in azimuth, little is known about the effect that spatial separation in distance has on the perception of competing sound sources near the head. In this experiment, head-related transfer functions (HRTFs) were used to process stimuli in order to simulate a target talker and a masking sound located at different distances along the listener's interaural axis. One of the signals was always presented at a distance of 1 m, and the other signal was presented 1 m, 25 cm, or 12 cm from the center of the listener's head. The results show that distance separation has very different effects on speech segregation for different types of maskers. When speech-shaped noise was used as the masker, most of the intelligibility advantages of spatial separation could be accounted for by spectral differences in the target and masking signals at the ear with the higher signal-to-noise ratio (SNR). When a same-sex talker was used as the masker, the intelligibility advantages of spatial separation in distance were dominated by binaural effects that produced the same performance improvements as a 4-5-dB increase in the SNR of a diotic stimulus. These results suggest that distance-dependent changes in the interaural difference cues of nearby sources play a much larger role in the reduction of the informational masking produced by an interfering speech signal than in the reduction of the energetic masking produced by an interfering noise source.     

Speech perception and talker segregation: effects of level, pitch, and tactile support with multiple simultaneous talkers

Speech intelligibility was investigated by varying the number of interfering talkers, level, and mean pitch differences between target and interfering speech, and the presence of tactile support. In a first experiment the speech-reception threshold (SRT) for sentences was measured for a male talker against a background of one to eight interfering male talkers or speech noise. Speech was presented diotically and vibro-tactile support was given by presenting the low-pass-filtered signal (0-200 Hz) to the index finger. The benefit in the SRT resulting from tactile support ranged from 0 to 2.4 dB and was largest for one or two interfering talkers. A second experiment focused on masking effects of one interfering talker. The interference was the target talker's own voice with an increased mean pitch by 2, 4, 8, or 12 semitones. Level differences between target and interfering speech ranged from -16 to +4 dB. Results from measurements of correctly perceived words in sentences show an intelligibility increase of up to 27% due to tactile support. Performance gradually improves with increasing pitch difference. Louder target speech generally helps perception, but results for level differences are considerably dependent on pitch differences. Differences in performance between noise and speech maskers and between speech maskers with various mean pitches are explained by the effect of informational masking.     

Speech competition effects on synthetic stop-vowel perception by normal and hearing-impaired listeners

A triadic comparisons task and an identification task were used to evaluate normally hearing listeners' and hearing-impaired listeners' perceptions of synthetic CV stimuli in the presence of competition. The competing signals included multitalker babble, continuous speech spectrum noise, a CV masker, and a brief noise masker shaped to resemble the onset spectrum of the CV masker. All signals and maskers were presented monotically. Interference by competition was assessed by comparing Multidimensional Scaling solutions derived from each masking condition to that derived from the baseline (quiet) condition. Analysis of the effects of continuous maskers revealed that multitalker babble and continuous noise caused the same amount of change in performance, as compared to the baseline condition, for all listeners. CV masking changed performance significantly more than did brief noise masking, and the hearing-impaired listeners experienced more degradation in performance than normals. Finally, the velar CV maskers (g epsilon and k epsilon) caused significantly greater masking effects than the bilabial CV maskers (b epsilon and p epsilon), and were most resistant to masking by other competing stimuli. The results suggest that speech intelligibility difficulties in the presence of competing segments of speech are primarily attributable to phonetic interference rather than to spectral masking. Individual differences in hearing-impaired listeners' performances are also discussed.     

Recognition of filtered words in noise at higher-than-normal levels: decreases in scores with and without increases in masking

To examine spectral effects on declines in speech recognition in noise at high levels, word recognition for 18 young adults with normal hearing was assessed for low-pass-filtered speech and speech-shaped maskers or high-pass-filtered speech and speech-shaped maskers at three speech levels (70, 77, and 84 dB SPL) for each of three signal-to-noise ratios (+8, +3, and -2 dB). An additional low-level noise produced equivalent masked thresholds for all subjects. Pure-tone thresholds were measured in quiet and in all maskers. If word recognition was determined entirely by signal-to-noise ratio, and was independent of signal levels and the spectral content of speech and maskers, scores should remain constant with increasing level for both low- and high-frequency speech and maskers. Recognition of low-frequency speech in low-frequency maskers and high-frequency speech in high-frequency maskers decreased significantly with increasing speech level when signal-to-noise ratio was held constant. For low-frequency speech and speech-shaped maskers, the decline was attributed to nonlinear growth of masking which reduced the "effective" signal-to-noise ratio at high levels, similar to previous results for broadband speech and speech-shaped maskers. Masking growth and reduced "effective" signal-to-noise ratio accounted for some but not all the decline in recognition of high-frequency speech in high-frequency maskers.     

Spectral and temporal changes to speech produced in the presence of energetic and informational maskers

Talkers change the way they speak in noisy conditions. For energetic maskers, speech production changes are relatively well-understood, but less is known about how informational maskers such as competing speech affect speech production. The current study examines the effect of energetic and informational maskers on speech production by talkers speaking alone or in pairs. Talkers produced speech in quiet and in backgrounds of speech-shaped noise, speech-modulated noise, and competing speech. Relative to quiet, speech output level and fundamental frequency increased and spectral tilt flattened in proportion to the energetic masking capacity of the background. In response to modulated backgrounds, talkers were able to reduce substantially the degree of temporal overlap with the noise, with greater reduction for the competing speech background. Reduction in foreground-background overlap can be expected to lead to a release from both energetic and informational masking for listeners. Passive changes in speech rate, mean pause length or pause distribution cannot explain the overlap reduction, which appears instead to result from a purposeful process of listening while speaking. Talkers appear to monitor the background and exploit upcoming pauses, a strategy which is particularly effective for backgrounds containing intelligible speech.     

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.     

Combining energetic and informational masking for speech identification

This study examined combinations of energetic and informational maskers in speech identification. Speech targets and maskers (speech or noise) were processed and filtered into sets of 15 narrow frequency bands. The target was the sum of eight randomly selected bands. More masking occurred for speech maskers than for spectrally matched noise maskers regardless of whether the masker bands overlapped the target bands. The greater effect of the speech maskers was interpreted as due to informational masking. When the masker was comprised of nonoverlapping bands of speech, the addition of bands of noise overlapping the speech masker, but not the speech target, reduced the overall amount of masking. Surprisingly, presenting the noise to the ear contralateral to the target and masker produced an even greater release from masking. The contralateral noise was apparently sufficient to cause a slight change in the image of the ipsilateral speech masker, possibly pulling it away from the target enough to allow the focus of attention on the target. This finding is consistent with the interpretation that in some conditions small binaural differences may be sufficient to cause, or significantly strengthen, the perceptual segregation of sounds.     

Informational masking caused by contralateral stimulation

Although informational masking is thought to reflect central mechanisms, the effects are generally much stronger when the target and masker are presented to the same ear than when they are presented to different ears. However, the results of a recent study by Brungart and Simpson [J. Acoust. Soc. Am. 112, 2985-2995 (2002)] indicated that a speech masker that is presented contralateral to a speech signal can produce substantial amounts of informational masking when a second speech masker is played simultaneously in the same ear as the signal. In this study, we conducted a series of experiments that paralleled those of Brungart and Simpson but used a pure-tone signal and multitone informational maskers in a detection task. Both the signal and the maskers were played as sequences of short bursts in each observation interval. The maskers were arranged in two types of spectrotemporal patterns. One type of pattern, called "multiple-bursts same" (MBS), has previously been shown to produce very large amounts of informational masking while the other type of pattern, called "multiple-bursts different" (MBD), has been shown to produce very small amounts of informational masking. Several conditions of ipsilateral, contralateral, and combined presentation of these maskers were tested. The results showed that presentation of the MBS masker in the contralateral ear produced a substantial amount of informational masking when the MBD masker was simultaneously presented to the ipsilateral ear. The results supported the earlier findings of Brungart and Simpson indicating that listeners are unable to selectively focus their attention on a single ear in some complex dichotic listening conditions. These results suggest that this contralateral masking effect is not restricted to speech and may reflect more general limitations on processing capacity. Further, it was concluded that the magnitude of the contralateral masking effect was related both to the informational masking value of the contralateral masker and the complexity of the stimulus and/or task in the ear in which the signal was presented.     

Effect of masker type on native and non-native consonant perception in noise

Spoken communication in a non-native language is especially difficult in the presence of noise. This study compared English and Spanish listeners' perceptions of English intervocalic consonants as a function of masker type. Three maskers (stationary noise, multitalker babble, and competing speech) provided varying amounts of energetic and informational masking. Competing English and Spanish speech maskers were used to examine the effect of masker language. Non-native performance fell short of that of native listeners in quiet, but a larger performance differential was found for all masking conditions. Both groups performed better in competing speech than in stationary noise, and both suffered most in babble. Since babble is a less effective energetic masker than stationary noise, these results suggest that non-native listeners are more adversely affected by both energetic and informational masking. A strong correlation was found between non-native performance in quiet and degree of deterioration in noise, suggesting that non-native phonetic category learning can be fragile. A small effect of language background was evident: English listeners performed better when the competing speech was Spanish.     

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.     

The extent to which a position-based explanation accounts for binaural release from informational masking

Detection was measured for a 500 Hz tone masked by noise (an "energetic" masker) or sets of ten randomly drawn tones (an "informational" masker). Presenting the maskers diotically and the target tone with a variety of interaural differences (interaural amplitude ratios and/or interaural time delays) resulted in reduced detection thresholds relative to when the target was presented diotically ("binaural release from masking"). Thresholds observed when time and amplitude differences applied to the target were "reinforcing" (favored the same ear, resulting in a lateralized position for the target) were not significantly different from thresholds obtained when differences were "opposing" (favored opposite ears, resulting in a centered position for the target). This irrelevance of differences in the perceived location of the target is a classic result for energetic maskers but had not previously been shown for informational maskers. However, this parallellism between the patterns of binaural release for energetic and informational maskers was not accompanied by high correlations between the patterns for individual listeners, supporting the idea that the mechanisms for binaural release from energetic and informational masking are fundamentally different.     

Spectral and threshold effects on recognition of speech at higher-than-normal levels

To examine spectral and threshold effects for speech and noise at high levels, recognition of nonsense syllables was assessed for low-pass-filtered speech and speech-shaped maskers and high-pass-filtered speech and speech-shaped maskers at three speech levels, with signal-to-noise ratio held constant. Subjects were younger adults with normal hearing and older adults with normal hearing but significantly higher average quiet thresholds. A broadband masker was always present to minimize audibility differences between subject groups and across presentation levels. For subjects with lower thresholds, the declines in recognition of low-frequency syllables in low-frequency maskers were attributed to nonlinear growth of masking which reduced "effective" signal-to-noise ratio at high levels, whereas the decline for subjects with higher thresholds was not fully explained by nonlinear masking growth. For all subjects, masking growth did not entirely account for declines in recognition of high-frequency syllables in high-frequency maskers at high levels. Relative to younger subjects with normal hearing and lower quiet thresholds, older subjects with normal hearing and higher quiet thresholds had poorer consonant recognition in noise, especially for high-frequency speech in high-frequency maskers. Age-related effects on thresholds and task proficiency may be determining factors in the recognition of speech in noise at high levels.     

Auditory masking of a 10 kHz tone with environmental, comodulated, and Gaussian noise in bottlenose dolphins (Tursiops truncatus)

The pattern of auditory masking derived from Gaussian noise is often cited and used to predict the detrimental effects of masking noise on marine mammals. However, environmental noise (both anthropogenic and natural) may not always be Gaussian distributed. Some noise sources are highly structured with complex amplitude fluctuations that extend across frequency regions, which are often termed comodulated noise. Recent evidence with bottlenose dolphins using comodulated noise demonstrated a significant release from masking compared to Gaussian maskers of the same bandwidth and pressure spectral density level, a result known as comodulation masking release. The present study demonstrates a pattern of masking where both temporally fluctuating comodulated noise and environmental noise produce lower masked thresholds compared to Gaussian noise of the same spectral density level and bandwidth. Furthermore, a threshold reduction or "masking release" occurred when the environmental noise bandwidth increased beyond a critical band. These results provide further evidence that conventional models of auditory masking using Gaussian maskers (i.e., the power spectrum model) do not fully describe the masking effects that occur in realistic environments.     

Stimulus factors influencing spatial release from speech-on-speech masking

This study examined spatial release from masking (SRM) when a target talker was masked by competing talkers or by other types of sounds. The focus was on the role of interaural time differences (ITDs) and time-varying interaural level differences (ILDs) under conditions varying in the strength of informational masking (IM). In the first experiment, a target talker was masked by two other talkers that were either colocated with the target or were symmetrically spatially separated from the target with the stimuli presented through loudspeakers. The sounds were filtered into different frequency regions to restrict the available interaural cues. The largest SRM occurred for the broadband condition followed by a low-pass condition. However, even the highest frequency bandpass-filtered condition (3-6 kHz) yielded a significant SRM. In the second experiment the stimuli were presented via earphones. The listeners identified the speech of a target talker masked by one or two other talkers or noises when the maskers were colocated with the target or were perceptually separated by ITDs. The results revealed a complex pattern of masking in which the factors affecting performance in colocated and spatially separated conditions are to a large degree independent.     

Similarity, uncertainty, and masking in the identification of nonspeech auditory patterns

This study examined whether increasing the similarity between informational maskers and signals would increase the amount of masking obtained in a nonspeech pattern identification task. The signals were contiguous sequences of pure-tone bursts arranged in six narrow-band spectro-temporal patterns. The informational maskers were sequences of multitone bursts played synchronously with the signal tones. The listener's task was to identify the patterns in a 1-interval 6-alternative forced-choice procedure. Three types of multitone maskers were generated according to different randomization rules. For the least signal-like informational masker, the components in each multitone burst were chosen at random within the frequency range of 200-6500 Hz, excluding a "protected region" around the signal frequencies. For the intermediate masker, the frequency components in the first burst were chosen quasirandomly, but the components in successive bursts were constrained to fall in narrow frequency bands around the frequencies of the components in the initial burst. Within the narrow bands the frequencies were randomized. This masker was considered to be more similar to the signal patterns because it consisted of a set of narrow-band sequences any one of which might be mistaken for a signal pattern. The most signal-like masker was similar to the intermediate masker in that it consisted of a set of synchronously played narrow-band sequences, but the variation in frequency within each sequence was sinusoidal, completing roughly one period in a sequence. This masker consisted of discernible patterns but not patterns that were part of the set of signals. In addition, masking produced by Gaussian noise bursts--thought to produce primarily peripherally based "energetic masking"--was measured and compared to the informational masking results. For the three informational maskers, more masking was produced by the maskers comprised of narrow-band sequences than for the masker in which the frequencies were not constrained to narrow bands. Also, the slopes of the performance-level functions for the three informational maskers were much shallower than for the Gaussian noise masker or for no masker. The findings provided qualified support for the hypothesis that increasing the similarity between signals and maskers, or parts of the maskers, causes greater informational masking. However, it is also possible that the greater masking was a consequence of increasing the number of perceptual "streams" that had to be evaluated by the listener.     

Effects of simulated cochlear-implant processing on speech reception in fluctuating maskers

This study investigated the effects of simulated cochlear-implant processing on speech reception in a variety of complex masking situations. Speech recognition was measured as a function of target-to-masker ratio, processing condition (4, 8, 24 channels, and unprocessed) and masker type (speech-shaped noise, amplitude-modulated speech-shaped noise, single male talker, and single female talker). The results showed that simulated implant processing was more detrimental to speech reception in fluctuating interference than in steady-state noise. Performance in the 24-channel processing condition was substantially poorer than in the unprocessed condition, despite the comparable representation of the spectral envelope. The detrimental effects of simulated implant processing in fluctuating maskers, even with large numbers of channels, may be due to the reduction in the pitch cues used in sound source segregation, which are normally carried by the peripherally resolved low-frequency harmonics and the temporal fine structure. The results suggest that using steady-state noise to test speech intelligibility may underestimate the difficulties experienced by cochlear-implant users in fluctuating acoustic backgrounds.     

The role of visual speech cues in reducing energetic and informational masking

Two experiments compared the effect of supplying visual speech information (e.g., lipreading cues) on the ability to hear one female talker's voice in the presence of steady-state noise or a masking complex consisting of two other female voices. In the first experiment intelligibility of sentences was measured in the presence of the two types of maskers with and without perceived spatial separation of target and masker. The second study tested detection of sentences in the same experimental conditions. Results showed that visual cues provided more benefit for both recognition and detection of speech when the masker consisted of other voices (versus steady-state noise). Moreover, visual cues provided greater benefit when the target speech and masker were spatially coincident versus when they appeared to arise from different spatial locations. The data obtained here are consistent with the hypothesis that lipreading cues help to segregate a target voice from competing voices, in addition to the established benefit of supplementing masked phonetic information.