Free-field release from masking

Free-field release from masking was studied as a function of the spatial separation of a signal and masker in a two-interval, forced-choice (2IFC) adaptive paradigm. The signal was a 250-ms train of clicks (100/s) generated by filtering 50-microseconds pulses with a TDH-49 speaker (0.9 to 9.0 kHz). The masker was continuous broadband (0.7 to 11 kHz) white noise presented at a level of 44 dBA measured at the position of the subject's head. In experiment I, masked and absolute thresholds were measured for 36 signal source locations (10 degree increments) along the horizontal plane as a function of seven masking source locations (30 degree increments). In experiment II, both absolute and masked thresholds were measured for seven signal locations along three vertical planes located at azimuthal rotations of 0 degrees (median vertical plane), 45 degrees, and 90 degrees. In experiment III, monaural absolute and masked thresholds were measured for various signal-masker configurations. Masking-level differences (MLDs) were computed relative to the condition where the signal and mask were in front of the subjects after using absolute thresholds to account for differences in the signal's sound-pressure level (SPL) due to direction. Maximum MLDs were 15 dB along the horizontal plane, 8 dB along the vertical, and 9 dB under monaural conditions.     

Physiological mechanisms of psychophysical masking: observations from auditory-nerve fibers

Masking might be due either to the spread of the excitation produced by the masker to the place of the tone signal along the cochlea or to the suppression of the response to the signal by the masker. In order to identify the contributions of these two mechanisms to tone-on-tone masking, masked thresholds of auditory-nerve fibers were measured in anesthetized cats using the same stimulus paradigms and detection criteria as in psychophysics. Suppressive masking was identified by comparing thresholds for simultaneous masking with those for a nonsimultaneous masking technique resembling pulsation thresholds. These nonsimultaneous thresholds do not include the contribution of suppression to masking because suppression only occurs for stimuli that overlap in time. For each masker and signal frequency, the fibers with the lowest (or "best") masked thresholds had characteristic frequencies (CF) slightly on the opposite side of the masker frequency with respect to the signal frequency, consistent with the psychophysical phenomenon of off-frequency listening. Patterns of best masked thresholds against signal frequency resembled psychophysical masking patterns in that they showed a maximum for signal frequencies close to the masker, and a skew toward high frequencies. Masking was found to be both excitatory and suppressive, with the relative contribution of the two mechanisms depending on the frequency separation between signal and masker. Suppressive masking was large for signal frequencies well above the masker. For these conditions, simultaneous thresholds grew more rapidly with masker level than did nonsimultaneous thresholds, suggesting that the upward spread of masking is largely due to the growth of suppression rather than to that of excitation.     

Across-channel masking and comodulation masking release

These experiments on across-channel masking (ACM) and comodulation masking release (CMR) were designed to extend the work of Grose and Hall [J. Acoust. Soc. Am. 85, 1276-1284 (1989)] on CMR. They investigated the effect of the temporal position of a brief 700-Hz signal relative to the modulation cycle of a 700-Hz masker 100% sinusoidally amplitude modulated (SAM) at a 10-Hz rate, which was either presented alone (reference masker) or formed part of a masker consisting of the 3rd to 11th harmonics of a 100-Hz fundamental. In the harmonic maskers, each harmonic was either SAM with the same 10-Hz modulator phase (comodulated masker) or with a shift in modulator phase of 90 degrees for each successive harmonic (phase-incoherent masker). When the signal was presented at the dips of the envelope of the 700-Hz component, the comodulated masker gave lower thresholds than the reference masker, while the phase-incoherent masker gave higher thresholds, i.e., a CMR was observed. No CMR was found when the signal was presented at the peaks of the envelope. In experiment 1, we replicated the experiment of Grose and Hall, but with an additional condition in which the 600- and 800-Hz components were removed from the masker, in order to investigate the role of within-channel masking effects. The results were similar to those of Grose and Hall. In experiment 2, the signal was added at the peaks of the envelope of the 700-Hz component, but in antiphase to the carrier of that component and at a level chosen to transform the peaks into dips. No CMR was found. Rather, performance was worse for both the comodulated and phase-incoherent maskers than for the reference masker. This was true even when the flanking components in the maskers were all remote in frequency from 700 Hz. In experiment 3, the masker components were all 50% SAM and the signal was added in antiphase at a dip of the envelope of the 700-Hz component, thus making the dip deeper. Performance was worse for the phase-incoherent than for the reference masker and was worse still for the comodulated masker. The results of all three experiments indicate strong ACM effects. CMR was found only when the signal was placed in the dips of the masker envelope and when it produced an increase in level relative to that in adjacent bands.     

Models of auditory masking: a molecular psychophysical approach

Gilkey et al. [J. Acoust. Soc. Am. 78, 1207-1219 (1985)] measured hit proportions and false alarm proportions for detecting a 500-Hz tone at each of four starting phase angles in each of 25 reproducible noise samples. In the present paper, their results are modeled by fitting the general form of the electrical analog model of Jeffress [J. Acoust. Soc. Am. 48, 480-488 (1967)] to the diotic data. The best-fitting configurations of this model do not correspond to energy detectors or to envelope detectors. A detector composed of a 50-Hz-wide single-tuned filter, followed by a half-wave rectifier and an integrator with an integration time of 100 to 200 ms fits the data of all four subjects relatively well. Linear combinations of the outputs of several detectors that differ in center frequency or integration window provide even better fits to the data. These linear combinations assign negative weights to some frequencies or to some time intervals, suggesting that a subject's decision is based on a comparison of information in different spectral or temporal portions of the stimulus.     

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.     

Spatial release from masking of aerial tones in pinnipeds

In most masking experiments, target signals and sound intended to mask are located in the same position. Spatial release from masking (SRM) occurs when signals and maskers are spatially separated, resulting in detection improvement relative to when they are spatially co-located. In this study, SRM was investigated in a harbor seal, who naturally lacks pinnae, and California sea lion, who possesses reduced pinnae. Subjects had to detect aerial tones at 1, 8, and 16 kHz in the presence of octave bands of white noise centered at the tone frequency. While the masker occurred in front of the subject (0 degree), the tone occurred at 0, 45, or 90 degrees in the horizontal plane. Unmasked thresholds were also measured at these angles to determine sensitivity differences based on source azimuth. Compared to when signal and masker where co-located, masked thresholds were lower by as much as 19 and 12 dB in the harbor seal and sea lion, respectively, when signal and masker were separated. Masked threshold differences of the harbor seal were larger than those previously measured under water. Performance was consistent with some measurements collected on terrestrial animals but differences between subjects at the highest frequency likely reflect variations in pinna anatomy.     

Temporal decline of masking and comodulation masking release.

Masking sounds can be continuously present, gated simultaneously with the signal, or gated somewhat prior to the signal. This continuum of relative onset times was explored using waveforms of the sort commonly employed in studies of comodulation masking release (CMR). There was a 50-Hz masker band centered on the 1250-Hz tonal signal, and four 50-Hz flanker bands centered at 850, 1050, 1450, and 1650 Hz. In some conditions, all four flanker bands had the same temporal envelope, and the masker band either had that same envelope (correlated presentations) or a different envelope (uncorrelated presentations). In other conditions, all five bands had different temporal envelopes (all-uncorrelated presentations). The masker band and/or the four flanker bands were either gated nearly simultaneously with the signal (burst conditions) or were gated prior to the signal by a duration that was systematically varied (fringed conditions). The eight listeners could be partitioned into three groups on the basis of their response to these fringing manipulations. Two listeners (the large fringers) showed a gradual improvement in detectability with increasing fringe duration (called a temporal decline of masking), while three others (the small fringers) showed little improvement in detectability. For the remaining three subjects, there was evidence of a "learning" effect that changed them from large fringers to small fringers over a 10-week period of listening. When present, the temporal decline of masking was greater for the correlated than for the uncorrelated comodulation condition; as a consequence, the difference in detectability between them (the comodulation masking release or CMR) increased with fringe duration. By fringing the masker and flanker bands separately and in combination, it was revealed that the temporal declines of masking were primarily attributable to the fringing of the flanker bands. In contrast, large CMRs required long fringes on both the masker and flanker bands. The above results were obtained with 50-ms signals, but generally similar data were obtained with a signal duration of 240 ms. The difficulties raised for experimentalists and theorists by such long-term practice effects are discussed.     

Temporal effects in masking and their influence on psychophysical tuning curves

Psychophysical tuning curves (PTCs) were obtained in simultaneous and forward masking for a 20-ms, 1000-Hz signal presented at 10 dB SL. The signal was presented at the beginning of, at the temporal center of, at the end of, or immediately following a 400-ms masker. The first experiment was done in quiet; the second experiment was done in the presence of two bands of noise on either side of 1000 Hz. The results were similar in quiet and in noise. In simultaneous masking, the PTCs were broadest for the signal at masker onset, and generally sharpest for the signal at temporal center; the differences were largest on the high-frequency side. In most cases, there was virtually no difference in Q10 between the forward-masking PTC and the simultaneous-masking PTC with the signal temporally centered, although the high-frequency slope was always steeper in forward masking. These results indicate that, at least for brief signals, frequency selectivity measured with simultaneous-masking PTCs and the degree of sharpening revealed in forward-masking PTCs depend upon the temporal position of the signal within the simultaneous masker.     

Spatial release from speech-on-speech masking in the median sagittal plane

Several studies have described a release from speech-on-speech masking associated with separation of target and masker sources in the median sagittal plane. Some have excluded the possibility that small differences between target and masker interaural time disparities can fully account for this release. This study explored the mechanisms underlying the spatial release from speech-on-speech masking that can be obtained in the absence of such differences. In one condition, interaural time disparities were removed from the nominal median-sagittal-plane, head-related impulse responses used to generate the virtual auditory space within which competing sentences were presented. In other conditions, interaural level and spectral disparities also were manipulated by presenting competing sentences monaurally or diotically after convolution with one ear's head-related impulse responses. It was found that substantial spatial release from masking can be obtained in the absence of any interaural disparities and that such disparities probably make a relatively minor contribution to spatial release from speech-on-speech masking in the median sagittal plane. It is argued that this release from masking is driven primarily by a reduction in informational masking that occurs when monaural information at one, or both, of the listener's ears facilitates differentiation of competing sentences that emanate from spatially separated sources.     

Speech intelligibility and spatial release from masking in young children

Children between the ages of 4 and 7 and adults were tested in free field on speech intelligibility using a four-alternative forced choice paradigm with spondees. Target speech was presented from front (0 degrees); speech or modulated speech-shaped-noise competitors were either in front or on the right (90 degrees). Speech reception thresholds were measured adaptively using a three-down/one-up algorithm. The primary difference between children and adults was seen in elevated thresholds in children in quiet and in all masked conditions. For both age groups, masking was greater with the speech-noise versus speech competitor and with two versus one competitor(s). Masking was also greater when the competitors were located in front compared with the right. The amount of masking did not differ across the two age groups. Spatial release from masking was similar in the two age groups, except for in the one-speech condition, when it was greater in children than adults. These findings suggest that, similar to adults, young children are able to utilize spatial and/or head shadow cues to segregate sounds in noisy environments. The potential utility of the measures used here for studying hearing-impaired children is also discussed.     

Combined monaural and binaural masking release

Stimulus conditions were examined where both across-frequency [comodulation masking release (CMR)] and across-ear [binaural masking-level difference (BMLD)] cues were available, as well as conditions where only one of these cue types was available. The main goal of the study was to determine how the two types of cues combine. The effects of comodulation were assessed either by modulating a masking noise and manipulating its bandwidth (experiment 1) or by using two comodulated narrow bands of noise separated in frequency (experiment 2). The masker was always No, and the 500-Hz pure-tone signal was either So or S pi. The effect of the frequency of modulation was examined either by changing the frequency of the modulating stimulus (experiment 1) or by changing the bandwidth of the comodulated narrow-band noise (experiment 2). Four of six subjects showed greater masking release when both BMLD and CMR cues were available than for either type of cue alone, whereas the other two subjects did not show an ability to combine the two cues for additional advantage. For the subjects who were able to combine the two types of cue, the additional advantage was greater for low frequencies of modulation. The results indicate that one component of CMR may be based upon across-frequency envelope comparisons at a stage of processing after binaural analysis.     

Vibrotactile forward masking: psychophysical evidence for a triplex theory of cutaneous mechanoreception

Threshold shifts for the detection of vibrotactile test stimuli were determined as a function of the intensity of a masker. A 50-ms sinusoidal test stimulus was applied to the thenar eminence of the hand 25 ms after the termination of a 700-ms sinusoidal masker applied to the same site. The frequency of the test stimulus and the frequency of the masker were varied. To eliminate the influence of the Pacinian receptor system, stimuli were delivered through a 0.01-cm2 contactor. The results support the hypothesis that the detection of vibration delivered through a small contactor is determined by two separate populations of non-Pacinian receptors. The study constitutes a psychophysical demonstration of the existence of three receptor systems responsible for the detection of vibration.     

Comodulation masking release and auditory grouping

The detectability of a pure-tone signal masked by a band of noise centered on the signal can be improved by the addition of flanking noise bands, provided that the temporal envelopes of the flanking bands are correlated with that of the on-signal band. This phenomenon is referred to as comodulation masking release (CMR). The present study examined CMR in conditions in which some flanking noise bands were comodulated with the on-signal band, but other flanking bands (termed "deviant" bands) were not. Past research has indicated that CMR is often substantially reduced when deviant bands are present at spectral locations close to the signal frequency. An investigation was undertaken to determine whether the disruptive effects of such bands could be reduced by factors related to auditory grouping. The signal frequency was 100 Hz. In one condition, only 20-Hz-wide comodulated bands, centered on 400, 600, 800, 1000, 1200, 1400, and 1600 Hz, were present. The CMR for this condition, referenced to threshold for the on-signal band only, was approximately 15 dB. In a second condition, two deviant bands were added at 900 and 1100 Hz; their presence reduced the CMR to only 3-4 dB. The number of deviant bands was then increased progressively, from two to eight bands. Deviant bands either shared a common envelope (codeviant), or had unique envelopes (multideviant). The number of bands that were comodulated with the on-signal band was held constant at six.(ABSTRACT TRUNCATED AT 250 WORDS)     

Comodulation masking release in consonant recognition

Comodulation masking release (CMR) refers to an improvement in the detection threshold of a signal masked by noise with coherent amplitude fluctuation across frequency, as compared to noise without the envelope coherence. The present study tested whether such an advantage for signal detection would facilitate the identification of speech phonemes. Consonant identification of bandpass speech was measured under the following three masker conditions: (1) a single band of noise in the speech band ("on-frequency" masker); (2) two bands of noise, one in the on-frequency band and the other in the "flanking band," with coherence of temporal envelope fluctuation between the two bands (comodulation); and (3) two bands of noise (on-frequency band and flanking band), without the coherence of the envelopes (noncomodulation). A pilot experiment with a small number of consonant tokens was followed by the main experiment with 12 consonants and the following masking conditions: three frequency locations of the flanking band and two masker levels. Results showed that in all conditions, the comodulation condition provided higher identification scores than the noncomodulation condition, and the difference in score was 3.5% on average. No significant difference was observed between the on-frequency only condition and the comodulation condition, i.e., an "unmasking" effect by the addition of a comodulated flaking band was not observed. The positive effect of CMR on consonant recognition found in the present study endorses a "cued-listening" theory, rather than an envelope correlation theory, as a basis of CMR in a suprathreshold task.     

A release from masking by continuous, random, notched noise

Thresholds for 10-ms sinusoids simultaneously masked by bursts of bandpass noise centered on the signal frequency were measured for a wide range of signal frequencies and noise levels. Thresholds were defined as the signal power relative to the masker power at the output of an auditory filter centered on the signal frequency. It was found that the presentation of a continuous random noise, with a spectral notch centered on the signal frequency, produced a reduction in signal thresholds of up to 11 dB. A notched noise spectrum level of 0-5 dB above that of the masker proved most effective in producing a masking release, as measured by a reduction in masked threshold. A release from masking of up to 7 dB could be obtained with a continuous bandpass noise. The most effective spectrum level of this noise was 5 dB below that of the masker. The effect of the continuous notched noise was to reduce signal-to-masker ratios at threshold to about 0 dB, regardless of the threshold in the absence of continuous noise. Thus the greatest release from masking occurred when "unreleased" thresholds were highest. The release from masking is almost complete within 320 ms of notched noise onset, and persists for about 160 ms after notched noise offset, regardless of notched noise level. The phenomenon is similar in many ways to the "overshoot" effect reported by Zwicker [J. Acoust. Soc. Am. 37, 653-663 (1965)]. It is argued that both effects can be largely attributed to peripheral short-term adaptation, a mechanism which is also believed to be involved in forward masking.     

母语为汉语的听者听英语时的空间去掩蔽现象研究

通过心理声学实验研究了来自不同方向具有不同信噪比的两种干扰声条件下,母语为汉语的听者对英语的空间去掩蔽现象。在消声室指定位置布放扬声器,发出目标声和干扰声,通过听者对目标声进行听音识别,得到听者识别的正确率。实验结果显示:只在正前方播放目标语音时,识别正确率大于99%,当目标和干扰语音都位于听者正前方时,正确率为57%;当目标和干扰语音随机位于士60°时,正确率为96%;特别地,当目标语音和干扰信号都位于听者正前方时,若干扰为噪声,随着信噪比从0 dB降低到-12 dB,正确率从96%降低到34%,而当干扰为语音时,随着信噪比从0 dB降低到-12 dB,正确率先是下降,随后有平均幅度为27%的明显上升,在此之后又是下降的趋势;当噪声干扰和语音干扰位于60°时,随着信噪比从-4 dB降低到-16 dB,正确率分别从99%降低到80%和从98%降低到91%。研究表明:空间分离对于母语为汉语的听者的英语语音可懂度有明显增益;大多数情况下英语语音的正确率都随着信噪比的降低而下降。这和对母语为其他语言的相关研究结论一致。