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.     

Detection and F0 discrimination of harmonic complex tones in the presence of competing tones or noise

Normal-hearing listeners' ability to "hear out" the pitch of a target harmonic complex tone (HCT) was tested with simultaneous HCT or noise maskers, all bandpass-filtered into the same spectral region (1200-3600 Hz). Target-to-masker ratios (TMRs) necessary to discriminate fixed fundamental-frequency (F0) differences were measured for target F0s between 100 and 400 Hz. At high F0s (400 Hz), asynchronous gating of masker and signal, presenting the masker in a different F0 range, and reducing the F0 rove of the masker, all resulted in improved performance. At the low F0s (100 Hz), none of these manipulations improved performance significantly. The findings are generally consistent with the idea that the ability to segregate sounds based on cues such as F0 differences and onset/offset asynchronies can be strongly limited by peripheral harmonic resolvability. However, some cases were observed where perceptual segregation appeared possible, even when no peripherally resolved harmonics were present in the mixture of target and masker. A final experiment, comparing TMRs necessary for detection and F0 discrimination, showed that F0 discrimination of the target was possible with noise maskers at only a few decibels above detection threshold, whereas similar performance with HCT maskers was only possible 15-25 dB above detection threshold.     

Center frequency modulation detection for harmonic complexes resembling vowel formants and its interference by off-frequency maskers.

Vowels are characterized by peaks in their spectral envelopes: the formants. To gain insight into the perception of speech as well as into the basic abilities of the ear, sensitivity to modulations in the positions of these formants is investigated. Frequency modulation detection thresholds (FMTs) were measured for the center frequency of formantlike harmonic complexes in the absence and in the presence of simultaneous off-frequency formants (maskers). Both the signals and the maskers were harmonic complexes which were band-pass filtered with a triangular spectral envelope, on a log-log scale, into either a LOW (near 500 Hz), a MID (near 1500 Hz), or a HIGH region (near 3000 Hz). They had a duration of 250 ms, and either an 80- or a 240-Hz fundamental. The modulation rate was 5 Hz for the signals and 10 Hz for the maskers. A pink noise background was presented continuously. In a first experiment no maskers were used. The measured FMTs were roughly two times larger than previously reported just-noticeable differences for formant frequency. In a second experiment, no significant differences were found between the FMTs in the absence of maskers and those in the presence of stationary (i.e., nonfrequency modulated) maskers. However, under many conditions the FMTs were increased by the presence of simultaneous modulated maskers. These results indicate that frequency modulation detection interference (FMDI) can exist for formantlike complex tones. The FMDI data could be divided into two groups. For stimuli characterized by a steep (200-dB/oct) slope, it was found that the size of the FMDI depended on which cues were used for detecting the signal and masker modulations. For stimuli with shallow (50-dB/oct) slopes, the FMDI was reduced when the signal and the masker had widely differing fundamentals, implying that the fundamental information is extracted before the interference occurs.     

Forward-masking properties of multicomponent signals in normal and hearing-impaired subjects

The forward-masking properties of inharmonic complex stimuli were measured both for normal and hearing-impaired subjects. The signal threshold for a 1000-Hz pure-tone probe was obtained for six different maskers, which varied in the number of pure-tone components. The masking stimuli consisted of 1, 3, 5, 7, 9, or 11 components, logarithmically spaced in frequency surrounding the signal and presented at a fixed level of 80 dB SPL per component. In most normal-hearing subjects, the threshold for the probe decreased as the number of masking components was increased, demonstrating that stimuli with more components tended to be less effective maskers. Results from hearing-impaired subjects showed no decrease in threshold with increasing number of masking components. Instead, the thresholds increased as more components were added to the first masker. These results appear to be consistent with suppression effects within the multicomponent maskers for the normal subjects and a lack of suppression effects for the hearing-impaired subjects. The results from the normal-hearing subjects are also consistent with "across-channel" cuing.     

Stimulus parameters governing confusion effects in forward masking

In forward masking, performance may be affected by confusion, that is, by the difficulty of discriminating a suprathreshold signal from the preceding masker. This study investigated confusion effects for forward maskers composed of repeated bursts of a 100-Hz sinusoid followed by sinusoidal signals; such "pulsing" maskers produce confusion when the properties of the signal are identical to those of an individual masker "pulse." The level, frequency, and duration of the signal relative to an individual masker pulse, as well as offset-onset delay, were varied to determine the minimum change necessary to eliminate confusion. For maskers composed of 20-ms pulses, confusion was eliminated by changes in signal level of 5 dB or changes in signal frequency of 30 to 40 Hz. For maskers composed of 10-, 20-, or 40-ms pulses, confusion was eliminated by signal delays of 8 to 16 ms or by signal durations less than half or greater than twice the masker-pulse duration. Results with adaptive procedures designed to measure confusion-free or confusion-determined thresholds suggest that confusion effects can be minimized or avoided by extensive listener training with a procedure in which the signal and masker are not presented at similar intensities.     

Additivity of simultaneous masking, revisited

Lutfi [J. Acoust. Soc. Am. 73, 262-267 (1983)] compared simultaneous masking functions (signal threshold versus masker level) for individual sinusoidal and narrow-band noise maskers, and for those maskers presented in pairs. Lutfi found that the pairs of maskers produced 10-17 dB "excess" masking over that predicted from the linear sum of their individual masking and explained the results in terms of a model in which the effects of the maskers are summed after undergoing independent compressive transformations. This paper describes experiments similar to those of Lutfi, and presents evidence suggesting that Lutfi's results may have been influenced by two factors: (1) combination-product detection, and (2) the use of different detection cues for single maskers and for pairs of maskers. Experiment I showed that when the stimulus conditions were chosen so as to minimize the likelihood of combination-product detection, "excess" masking was only 3-5 dB. Experiment II supported the idea that for a single narrow-band noise masker, subjects make use of the relatively slow envelope fluctuations to enhance performance. When two independent narrow-band noise maskers are added, the effectiveness of this cue is reduced, and between 3 and 9 dB of "excess" masking occurs. When the two noises are derived from the same source, and have correlated envelope fluctuations, no "excess" masking occurs. The results indicate that Lufti's compressive-nonlinearity model clearly fails in some situations.     

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.     

Comodulation masking release using SAM tonal complex maskers: effects of modulation depth and signal position

The purpose of this investigation was to examine two stimulus parameters that were reasoned to be of importance to comodulation masking release (CMR). The first was the degree of fluctuation, or depth of modulation, in the masker bands, and the second was the temporal position of the signal with respect to the modulations of the masker. The investigation began by demonstrating the efficacy of sinusoidally amplitude-modulated (SAM) tonal complex maskers in eliciting CMR. "Nine-band" maskers, 650 ms in duration, were constructed by adding together nine SAM tones spaced at 100-Hz intervals from 300 to 1100 Hz. The rate of modulation for each SAM tone was 10 Hz, and the depth of modulation was 100%. Using such maskers, it was shown that when the on-frequency SAM tone had a modulation depth of 100%, the threshold for a 250-ms, 700-Hz tone improved monotonically as the modulation depths of the flanking SAM tones increased from 0% to 100%. When the on-frequency SAM tone had a modulation depth of 63%, some listeners performed optimally when the flanking SAM tones also exhibited a modulation depth of 63%, whereas others performed best when the flankers had modulation depths of 100%. With regard to signal position, a typical CMR effect was observed when the signal, consisting of a train of three 50-ms, 700-Hz tone bursts, was placed in the dips of the on-frequency masker. However, when the signal was placed at the peaks of the envelope, an increase in masking was observed for a comodulated masker.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Confusion effects with sinusoidal and narrow-band noise forward maskers

In some forward-masking conditions, signal thresholds may be elevated by the listener's inability to distinguish the signal from the preceding masker. In this study, such "confusion" effects are investigated for both sinusoidal and narrow-band noise forward maskers combined with sinusoidal signals of varying duration. Results for the sinusoidal maskers show effects of off-frequency listening for brief signals and possibly small effects of confusion for longer signals. Results for the narrow-band noise maskers show a marked influence of confusion over a wide range of signal durations. This range is in good agreement with that predicted from previous work with "pulsing" maskers [D. Neff, J. Acoust. Soc. Am. 78, 1966-1976 (1985)]. These results suggest that studies using narrow-band noise forward maskers or studies of psychophysical suppression should include direct tests for confusion effects in key conditions.     

Notionally steady background noise acts primarily as a modulation masker of speech

Stone et al. [J. Acoust. Soc Am. 130, 2874-2881 (2011)], using vocoder processing, showed that the envelope modulations of a notionally steady noise were more effective than the envelope energy as a masker of speech. Here the same effect is demonstrated using non-vocoded signals. Speech was filtered into 28 channels. A masker centered on each channel was added to the channel signal at a target-to-background ratio of -5 or -10 dB. Maskers were sinusoids or noise bands with bandwidth 1/3 or 1 ERB(N) (ERB(N) being the bandwidth of "normal" auditory filters), synthesized with Gaussian (GN) or low-noise (LNN) statistics. To minimize peripheral interactions between maskers, odd-numbered channels were presented to one ear and even to the other. Speech intelligibility was assessed in the presence of each "steady" masker and that masker 100% sinusoidally amplitude modulated (SAM) at 8 Hz. Intelligibility decreased with increasing envelope fluctuation of the maskers. Masking release, the difference in intelligibility between the SAM and its "steady" counterpart, increased with bandwidth from near-zero to around 50 percentage points for the 1-ERB(N) GN. It is concluded that the sinusoidal and GN maskers behaved primarily as energetic and modulation maskers, respectively.     

A further test of the linearity of temporal summation in forward masking

An experiment tested the hypothesis that the masking effects of two nonoverlapping forward maskers are summed linearly over time. First, the levels of individual noise maskers required to mask a brief 4-kHz signal presented at 10-, 20-, 30-, or 40-dB sensation level (SL) were found. The hypothesis predicts that a combination of the first masker presented at the level required to mask the 10-dB SL signal and the second masker presented at the level required to mask the 20-dB SL signal, should produce the same amount of masking as the converse situation (i.e., the first masker presented at the level required to mask the 20-dB SL signal and the second masker presented at the level required to mask the 10-dB SL signal), and similarly for the 30- and 40-dB SL signals. The results were consistent with the predictions.     

Binaural masking experiments using noise maskers with frequency-dependent interaural phase differences. I: Influence of signal and masker duration

In this paper previous experiments on auditory filter shapes in binaural masking experiments [A. Kohlrausch, J. Acoust. Soc. Am. 84, 573-583 (1988)] are extended to a wider range of masker and signal durations. The masker was a dichotic broadband noise with frequency-dependent interaural parameters. The interaural phase difference of the masker was 0 below 500 Hz and pi above 500 Hz. Signal frequency varied between 200 and 800 Hz, and the signal was presented either monaurally (Sm) or binaurally in antiphase (S pi). In the first experiment, the masker duration was fixed at 500 ms and signals of 250 and 20 ms were used. In the second experiment, the signal duration was fixed at 20 ms, and the masker duration was reduced to 25 ms. The results from both experiments are consistent with studies using No or N pi maskers: The binaural masking level difference (BMLD) increases slightly for shorter test signals and decreases strongly for short maskers. The BMLD patterns of the first experiment are well described by the auditory-filter model derived for stationary test signals, if the additional influence of "off-frequency listening" for the short test signal is taken into account. The BMLDs resulting from the second experiment (25-ms masker), however, are much lower than predicted by this filter model This outcome supports previous observations that binaural unmasking becomes less effective for very short masker durations and indicates that this effect is even stronger for maskers with a complex structure of interaural parameters.     

The effect of signal-frequency uncertainty on comodulation masking release

The aim of this study was to examine whether the scheme of across-frequency comparison underlying comodulation masking release (CMR) is sensitive to the placement of the signal in the array of comodulating bands. This was addressed using the paradigm of signal-frequency uncertainty. In the first experiment, maskers were constructed of linearly spaced sinusoidally amplitude-modulated tones, and the signal was a pure tone presented at one of five frequencies. A small uncertainty effect was observed for the noncomodulated masker, but no significant effect was observed for the comodulated masker. In the second experiment, the maskers were constructed of logarithmically spaced noise bands, and the signal was a pure tone presented at one of seven frequencies. In these conditions, an uncertainty effect was observed in both noncomodulated and comodulated maskers, which was larger than that observed in experiment 1. The results were interpreted as indicating that the mechanism of across-frequency comparison underlying CMR is sensitive to signal location.     

Effect of masker harmonicity on informational masking

Detection thresholds for a tone in an unfamiliar tonal pattern can be greatly elevated under conditions of masker uncertainty [Neff and Green, Percept. Psychophys. 41, 409-415 (1987); Oh and Lutfi, J. Acoust. Soc. Am. 101, 3148 (1997)]. The present experiment was undertaken to determine whether harmonicity of masker tones can reduce the detrimental effect of masker uncertainty. Inharmonic maskers were comprised of m=2-49 frequency components selected at random on each presentation within 100-10000 Hz, excluding frequencies between 920-1080. Harmonic maskers were comprised of frequency components selected at random within this same range, but constrained to have a fundamental frequency of 200 Hz. For inharmonic maskers the signal was a 1000-Hz tone. For harmonic-maskers the signal was a tone whose frequency was either harmonically (1000 Hz) or inharmonically (1047 Hz) related to the masker. In all conditions the amount of masking was greatest for m = 20-40 components. At this point, harmonic maskers with harmonic signal produced an average of 9-12 dB less masking than inharmonic maskers. Harmonic maskers with inharmonic signal produced an average of 16-20 dB less masking.     

Effects of peripheral nonlinearity on psychometric functions for forward-masked tones

Psychometric functions (PFs) for forward-masked tones were obtained for conditions in which signal level was varied to estimate threshold at several masker levels (variable-signal condition), and in which masker level was varied to estimate threshold at several signal levels (variable-masker condition). The changes in PF slope across combinations of masker frequency, masker level, and signal delay were explored in three experiments. In experiment 1, a 2-kHz, 10-ms tone was masked by a 50, 70 or 90 dB SPL, 20-ms on-frequency forward masker, with signal delays of 2, 20, or 40 ms, in a variable-signal condition. PF slopes decreased in conditions where signal threshold was high. In experiments 2 and 3, the signal was a 4-kHz, 10-ms tone, and the masker was either a 4- or 2.4-kHz, 200-ms tone. In experiment 2, on-frequency maskers were presented at 30 to 90 dB SPL in 10-dB steps and off-frequency maskers were presented at 60 to 90 dB SPL in 10-dB steps, with signal delays of 0, 10, or 30 ms, in a variable-signal condition. PF slopes decreased as signal level increased, and this trend was similar for on- and off-frequency maskers. In experiment 3, variable-masker conditions with on- and off-frequency maskers and 0-ms signal delay were presented. In general, the results were consistent with the hypothesis that peripheral nonlinearity is reflected in the PF slopes. The data also indicate that masker level plays a role independent of signal level, an effect that could be accounted for by assuming greater internal noise at higher stimulus levels.     

Minimum spectral contrast for vowel identification by normal-hearing and hearing-impaired listeners

To determine the minimum difference in amplitude between spectral peaks and troughs sufficient for vowel identification by normal-hearing and hearing-impaired listeners, four vowel-like complex sounds were created by summing the first 30 harmonics of a 100-Hz tone. The amplitudes of all harmonics were equal, except for two consecutive harmonics located at each of three "formant" locations. The amplitudes of these harmonics were equal and ranged from 1-8 dB more than the remaining components. Normal-hearing listeners achieved greater than 75% accuracy when peak-to-trough differences were 1-2 dB. Normal-hearing listeners who were tested in a noise background sufficient to raise their thresholds to the level of a flat, moderate hearing loss needed a 4-dB difference for identification. Listeners with a moderate, flat hearing loss required a 6- to 7-dB difference for identification. The results suggest, for normal-hearing listeners, that the peak-to-trough amplitude difference required for identification of this set of vowels is very near the threshold for detection of a change in the amplitude spectrum of a complex signal. Hearing-impaired listeners may have difficulty using closely spaced formants for vowel identification due to abnormal smoothing of the internal representation of the spectrum by broadened auditory filters.     

Comodulation masking release (CMR) as a function of masker bandwidth, modulator bandwidth, and signal duration

These experiments examine how comodulation masking release (CMR) varies with masker bandwidth, modulator bandwidth, and signal duration. In experiment 1, thresholds were measured for a 400-ms, 2000-Hz signal masked by continuous noise varying in bandwidth from 50-3200 Hz in 1-oct steps. In one condition, using random noise maskers, thresholds increased with increasing bandwidth up to 400 Hz and then remained approximately constant. In another set of conditions, the masker was multiplied (amplitude modulated) by a low-pass noise (bandwidth varied from 12.5-400 Hz in 1-oct steps). This produced correlated envelope fluctuations across frequency. Thresholds were generally lower than for random noise maskers with the same bandwidth. For maskers less than one critical band wide, the release from masking was largest (about 5 dB) for maskers with low rates of modulation (12.5-Hz-wide low-pass modulator). It is argued that this release from masking is not a "true" CMR but results from a within-channel cue. For broadband maskers (greater than 400 Hz), the release from masking increased with increasing masker bandwidth and decreasing modulator bandwidth, reaching an asymptote of 12 dB for a masker bandwidth of 800 Hz and a modulator bandwidth of 50 Hz. Most of this release from masking can be attributed to a CMR. In experiment 2, the modulator bandwidth was fixed at 12.5 Hz and the signal duration was varied. For masker bandwidths greater than 400 Hz, the CMR decreased from 12 to 5 dB as the signal duration was decreased from 400 to 25 ms.(ABSTRACT TRUNCATED AT 250 WORDS)     

Failure to obtain comodulation masking release with frequency-modulated maskers

These experiments were intended to determine whether comodulation masking release (CMR) occurs for maskers that are modulated in frequency rather than in amplitude. In experiment I, thresholds for a sinusoidal signal were measured in the presence of two continuous sinusoidal maskers: one was centered at the signal frequency (1.0 kHz), and the other was positioned at flanking frequencies ranging from 0.5 to 2.0 kHz. The two maskers were frequency modulated (FM) by the same low-pass-noise modulator (correlated condition) or by independent noise modulators (uncorrelated condition). Thresholds were the same for the correlated and uncorrelated maskers, i.e., no CMR occurred. This was also true when the flanking band was presented in the ear opposite to that containing the signal and the on-frequency masking band. In experiment II, 25-Hz-wide noise maskers were used. The on-frequency band was sinusoidally frequency modulated, while the off-frequency band either had the same FM or no FM. Thresholds were similar for the two conditions, again indicating that no CMR occurred. The results suggest that, unlike amplitude modulation, correlated FM of the masker in different frequency bands does not give rise to a release from masking.