Steady Masker Research Articles

Masking Release for Speech in Modulated Maskers: Electrophysiological and Behavioral Measures.

The purpose of this study was to obtain an electrophysiological analog of masking release using speech-evoked cortical potentials in steady and modulated maskers and to relate this masking release to behavioral measures for the same stimuli. The hypothesis was that the evoked potentials can be tracked to a lower stimulus level in a modulated masker than in a steady masker and that the magnitude of this electrophysiological masking release is of the same order as that of the behavioral masking release for the same stimuli. Cortical potentials evoked by an 80-ms /ba/ stimulus were measured in two steady maskers (30 and 65 dB SPL), and in a masker that modulated between these two levels at a rate of 25 Hz. In each masker, a level series was undertaken to determine electrophysiological threshold. Behavioral detection thresholds were determined in the same maskers using an adaptive tracking procedure. Masking release was defined as the difference between signal thresholds measured in the steady 65-dB SPL masker and the modulated masker. A total of 23 normal-hearing adults participated. Electrophysiological thresholds were uniformly elevated relative to behavioral thresholds by about 6.5 dB. However, the magnitude of masking release was about 13.5 dB for both measurement domains. Electrophysiological measures of masking release using speech-evoked cortical auditory evoked potentials correspond closely to behavioral estimates for the same stimuli. This suggests that objective measures based on electrophysiological techniques can be used to reliably gauge aspects of temporal processing ability.

Amplitude fluctuations in a masker influence lexical segmentation in cochlear implant users.

Normal-hearing listeners show masking release, or better speech understanding in a fluctuating-amplitude masker than in a steady-amplitude masker, but most cochlear implant (CI) users consistently show little or no masking release even in artificial conditions where masking release is highly anticipated. The current study examined the hypothesis that the reduced or absent masking release in CI users is due to disruption of linguistic segmentation cues. Eleven CI subjects completed a sentence keyword identification task in a steady masker and a fluctuating masker with dips timed to increase speech availability. Lexical boundary errors in their responses were categorized as consistent or inconsistent with the use of the metrical segmentation strategy (MSS). Subjects who demonstrated masking release showed greater adherence to the MSS in the fluctuating masker compared to subjects who showed little or no masking release, while both groups used metrical segmentation cues similarly in the steady masker. Based on the characteristics of the segmentation cues, the results are interpreted as evidence that CI listeners showing little or no masking release are not reliably segregating speech from competing sounds, further suggesting that one challenge faced by CI users listening in noisy environments is a reduction of reliable segmentation cues.

Modulation masking release using the Brazilian-Portuguese HINT: Psychometric functions and the effect of speech time compression

Objective: The Brazilian-Portuguese hearing in noise test (HINT) was used to investigate the benefit to speech recognition of listening in a fluctuating background. The goal was to determine whether modulation masking release varied as a function of the speech-to-masker ratio at threshold. Speech-to-masker ratio at threshold was manipulated using the novel approach of adjusting the time-compression of the speech. Design: Experiment 1 measured performance-intensity functions in both a steady speech-shaped noise masker and a 10-Hz square-wave modulated masker. Experiment 2 measured speech-to-masker ratios at threshold as a function of time-compression of the speech (0, 33, and 50%) in both maskers. Study sample: Participants were normal-hearing adults who were native speakers of Brazilian Portuguese (Experiment 1: N = 10; Experiment 2: N = 30). Results: The slope of the performance-intensity function was shallower in the modulated masker than in the steady masker for both words and sentences. Thresholds increased with increasing time-compression in both maskers, but more markedly in the modulated masker, resulting in reduced modulation masking release with increasing time-compression. Conclusions: Speech-to-masker ratio at threshold varies with time-compression of speech. The results are relevant to the issue of whether degree of masker modulation benefit depends on speech-to-masker ratio at threshold.

The monaural temporal window based on masking period pattern data in school-aged children and adults

Several lines of evidence indicate that auditory temporal resolution improves over childhood, whereas other data implicate the development of processing efficiency. The present study used the masking period pattern paradigm to examine the maturation of temporal processing in normal-hearing children (4.8 to 10.7 yrs) compared to adults. Thresholds for a brief tone were measured at 6 temporal positions relative to the period of a 5-Hz quasi-square-wave masker envelope, with a 20-dB modulation depth, as well as in 2 steady maskers. The signal was a pure tone at either 1000 or 6500 Hz, and the masker was a band of noise, either spectrally wide or narrow (21.3 and 1.4 equivalent rectangular bandwidths, respectively). Masker modulation improved thresholds more for wide than narrow bandwidths, and adults tended to receive more benefit from modulation than young children. Fits to data for the wide maskers indicated a change in window symmetry with development, reflecting relatively greater backward masking for the youngest listeners. Data for children >6.5 yrs of age appeared more adult-like for the 6500- than the 1000-Hz signal. Differences in temporal window asymmetry with listener age cannot be entirely explained as a consequence of a higher criterion for detection in children, a form of inefficiency.

Contribution of Consonant Landmarks to Speech Recognition in Simulated Acoustic-Electric Hearing

The purpose of this study is to assess the contribution of information provided by obstruent consonants (e.g., stops and fricatives) to speech intelligibility in simulated acoustic-electric hearing. As a secondary objective, this study examines the performance of an objective measure that can potentially be used for predicting the intelligibility of vocoded speech. Noise-corrupted sentences are used in experiment 1 in which the noise-corrupted obstruent consonants are replaced with clean obstruent consonants, while leaving the sonorant sounds (vowels, semivowels, and nasals) corrupted. In one condition, listeners have only access to the low-frequency (<600 Hz) acoustic portion of the clean consonant spectra, in other condition, listeners have only access to the higher frequency (>600 Hz) portion (vocoded) of the clean consonant spectra, and in the third condition, they have access to both. In experiment 2, we investigate a speech-coding strategy that selectively attenuates the low-frequency portion of the consonant spectra while leaving the vocoded portion corrupted by noise. Finally, using the data collected from experiments 1 and 2, we evaluate the performance of an objective measure in terms of predicting intelligibility of vocoded speech. This measure was originally designed to predict speech quality and has never been evaluated with vocoded speech. Significant improvements (about 30 percentage points) in intelligibility were noted in experiment 1 in steady and two-talker masker conditions when the listeners had access to the clean obstruent consonants in both the acoustic and the vocoded portions of the spectrum. The improvement was more evident in the low signal to noise ratio levels (-5 and 0 dB). Further analysis indicated that it was access to the vocoded portion of the consonant spectra, rather than access to the low-frequency acoustic portion of the consonant spectra that contributed the most to the large improvements in performance. In experiment 2, a small (14 percentage points) but statistically significant improvement in performance was obtained at 0 dB signal to noise ratio (steady masker) when the obstruent consonants were selectively attenuated in the low-frequency acoustic portion alone (the vocoded portion was left noise corrupted). The examined objective measure predicted with a relatively high correlation (r = 0.92 to 0.94) [corrected] the intelligibility of vocoded speech improved in both steady and two-talker masking conditions. Providing access to the clean obstruent spectra can yield substantial improvements in intelligibility relative to the simulated acoustic-electric condition. Much of this improvement can be attributed to the listeners having access to the clean vocoded portion of the obstruent consonants. The large contribution of obstruent consonants in speech recognition in simulated acoustic-electric hearing stems from the fact that these consonants provide reliable acoustic landmarks which in turn enable listener to integrate effectively pieces of the message glimpsed over temporal gaps into one coherent speech stream. It is argued that these landmarks are smeared in existing cochlear implant systems, including the bimodal systems, owing to envelope compression, and the fact that the obstruent consonants are probably the first to be masked by background noise. Overall, the outcomes from this study suggest that the obstruent consonants need to be treated differently for improved speech recognition in noise.

Masking release for words in amplitude-modulated noise as a function of modulation rate and task

For normal-hearing listeners, masked speech recognition can improve with the introduction of masker amplitude modulation. The present experiments tested the hypothesis that this masking release is due in part to an interaction between the temporal distribution of cues necessary to perform the task and the probability of those cues temporally coinciding with masker modulation minima. Stimuli were monosyllabic words masked by speech-shaped noise, and masker modulation was introduced via multiplication with a raised sinusoid of 2.5-40 Hz. Tasks included detection, three-alternative forced-choice identification, and open-set identification. Overall, there was more masking release associated with the closed than the open-set tasks. The best rate of modulation also differed as a function of task; whereas low modulation rates were associated with best performance for the detection and three-alternative identification tasks, performance improved with modulation rate in the open-set task. This task-by-rate interaction was also observed when amplitude-modulated speech was presented in a steady masker, and for low- and high-pass filtered speech presented in modulated noise. These results were interpreted as showing that the optimal rate of amplitude modulation depends on the temporal distribution of speech cues and the information required to perform a particular task.

The contribution of temporal fine structure to the intelligibility of speech in steady and modulated noise

Speech reception thresholds were measured with steady and amplitude-modulated noise maskers for signals processed to contain variable amounts of temporal fine structure (TFS) information. Subjects benefited more from TFS information for the modulated than for the steady masker. For both maskers, addition of TFS information up to 548 Hz improved performance, though the improvement was greater for the modulated masker. The addition of TFS information at higher frequencies improved performance further for the modulated masker only. These results are consistent with the idea that TFS information is important for listening in the dips of a fluctuating masker.

The contribution of temporal fine structure information to the intelligibility of speech in noise

Temporal fine structure (TFS) information in speech may be particularly useful when listening to a target in a background that contains temporal 'dips'. A change in TFS may allow identification of signal portions where the target‐to‐background ratio is high. This hypothesis was tested. Speech reception thresholds were measured with steady and an 8‐Hz amplitude‐modulated noise background for signals processed to contain variable amounts of TFS information. Signals were filtered into channels and channel signals for channel numbers above a 'cut off channel' (CO) were tone‐vocoded to remove TFS information, while channel signals with channel numbers of CO and below were left unprocessed. Signals from all channels were combined. Five values of CO were tested for each noise type, with ten normal‐hearing subjects. Subjects benefited more from TFS information when listening in the modulated masker than the steady masker. For steady noise, addition of TFS information above 548 Hz did not improve performance, whereas for modulated noise, addition of TFS at high frequencies did improve performance. The greater benefit from TFS information when listening in modulated noise is consistent with the idea that TFS information is important for listening in the dips of a fluctuating masker.

Consonant identification under maskers with sinusoidal modulation: masking release or modulation interference?

The present study investigated the effect of envelope modulations in a background masker on consonant recognition by normal hearing listeners. It is well known that listeners understand speech better under a temporally modulated masker than under a steady masker at the same level, due to masking release. The possibility of an opposite phenomenon, modulation interference, whereby speech recognition could be degraded by a modulated masker due to interference with auditory processing of the speech envelope, was hypothesized and tested under various speech and masker conditions. It was of interest whether modulation interference for speech perception, if it were observed, could be predicted by modulation masking, as found in psychoacoustic studies using nonspeech stimuli. Results revealed that masking release measurably occurred under a variety of conditions, especially when the speech signal maintained a high degree of redundancy across several frequency bands. Modulation interference was also clearly observed under several circumstances when the speech signal did not contain a high redundancy. However, the effect of modulation interference did not follow the expected pattern from psychoacoustic modulation masking results. In conclusion, (1) both factors, modulation interference and masking release, should be accounted for whenever a background masker contains temporal fluctuations, and (2) caution needs to be taken when psychoacoustic theory on modulation masking is applied to speech recognition.

Masking of short probe sounds by tone bursts with a sweeping frequency

The masked threshold of a short probe tone or a short one‐third octave probe noise appears to increase if the frequency of a tonal masker is swept. Frequency sweeps were exponential (octave/s) and unidirectional. Probe sounds were presented in the time center of the masker at the center frequency of the masker. The sweep gradient appeared to be an important parameter, masker duration was much less important. For 10‐ms probes, 100‐ms maskers, and upward sweeps increase of the masked threshold appeared to be maximal at a sweep gradient of 30 octave/s and the masked threshold was 21 dB higher than the masked threshold found for the steady masker (0 octave/s). Above 30 octave/s the masked threshold decreased. For downward sweeps masking was maximal at 20 octave/s and the threshold was 15 dB higher. The results are inconsistent with current models of masking: sweeping the frequency, the masked threshold increases whereas the energy within the critical band at the probe frequency decreases. [Research supported by the Netherlands Organization for the Advancement of Pure Research, ZWO.]

Auditory Threshold Shifts Produced by Simultaneously Pulsed Contralateral Stimuli

Evidence from studies of tactual sensitivity indicates that transient suprathreshold stimuli presented simultaneously with and contralateral to test signals will mask such signals more effectively than do steady stimuli. The present report presents evidence that when pulsed masking sounds are presented simultaneously with pulsed auditory signals, two major effects are observed: First, a significantly greater amount of contralateral masking is obtained with the pulsed masker than with the steady masker, and, second, the pulsed masker is more effective in the monaural condition also. Furthermore, the masking effect of pulsed maskers increases with the increase of signal frequency, but not with increase of masker intensity. The frequency of the masker must be near that of the signal to be effective, and if pure-tone maskers are used with pure-tone signals, the masking effect shifts continuously to the localization effect as the signal frequency approaches the masker frequency. Some explanations of binaural and contralateral masking are discussed.