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PubMed1980

Influence of auditory fatigue on masked speech intelligibility.

Intelligibility of PB word lists embedded in simultaneous masking noise was evaluated before and after fatiguing-noise exposure. Intelligibility was determined by observing the number of words correctly repeated during a shadowing task. Both the speech signal and the masking noise were filtered to a 2825-3185 Hz band. Masking-noise levels were varied from 0- 90-dB SL. Fatique was produced by a 1500-3000-Hz octave band of noise at 115 dB (re 20 muPa) presented continuously for 5 min. The results of three experiments indicated that speech intelligibility was reduced when the speech was presented against a background of silence but that the fatiguing-noise exposure had no effect on intelligibility when the speech was made more intense and embedded in masking noise of 40-90-dB SL. These observations are interpreted by considering the recruitment produced by fatique and masking noise.

Auditory Fatigue↗
PubMed1981

Anomalous auditory filter bandwidth derived from direct masking of complex signals.

Psychoacoustical thresholds were determined for complex signals (triangular spectral envelope and 2000-Hz center frequency) masked by broadband noise. Variation of the fundamental frequency and of the phase relation between the components had no influence on the masked thresholds, although the temporal envelopes of the complexes were completely different. The bandwidth of the signals was varied by changing the slope of the spectral envelopes. From the dependence of the masked threshold on signal bandwidth the auditory filter bandwidth was estimated to be about 100 Hz at 2000 Hz. Masked thresholds determined for complexes with flat spectral envelope resulted in the same auditory filter bandwidth. These values are extremely narrow compared to the value of about 320 Hz often observed in other measures. It is suggested that the masked thresholds are influenced by temporal interactions.

Acoustics↗
PubMed1983

ABR measurements in the cat using a forward-masking paradigm.

Probe-elicited wave V amplitudes of the auditory brainstem response (ABR) were measured using a forward-masking paradigm. Subjects were anesthetized cats. For individual experiments, probe frequency and intensity were fixed and masker frequencies and intensities were varied. For each masker frequency, the extent to which the probe-elicited wave V amplitude was reduced by the preceding masker was plotted as a function of masker intensity. The rising segments of the masking functions were fitted with straight lines, using a least-squares procedure, to obtain estimates of their slopes. Masking grew most rapidly for masker frequencies below probe frequency, becoming progressively less steep as masker frequency increased. ABR tuning curves were constructed by using the linear fits to define the masker intensity that caused a 50% reduction in probe-elicited wave V amplitude. The shapes of these tuning curves were comparable to whole-nerve action potential (AP) tuning curves obtained under similar stimulus conditions. These results indicate that ABR amplitude measurements in a forward-masking paradigm can be used to estimate the growth of response to masking stimuli and frequency selectivity in a manner similar to AP amplitude measurements.

Action Potentials↗
PubMed1983

Additivity of simultaneous masking.

Simultaneous masking functions (signal level at threshold versus masker level) were obtained for equally intense maskers presented individually and in pairs. The signal was a 2.0-kHz sinusoid. The pairs of maskers were (1) two sinusoids with frequencies 1.9 and 2.1 kHz, (2) two narrow bands of noise (50 Hz wide) centered at 1.9 and 2.1 kHz, (3) two narrow bands of noise (50 Hz wide) centered at 1.8 and 1.9 kHz, and (4) the 1.9-kHz sinusoid combined with the narrow band of noise centered at 2.1 kHz. The pairs of maskers produced anywhere from 10 to 17 dB of masking beyond that predicted from the simple sum of the masking produced by the individual maskers. The amount of this "additional masking" was independent of masker level. Adding a continuous low level background noise reduced the amount of additional masking only slightly (approximately 5 dB). The data are consistent with a model in which the effects of the maskers are summed after undergoing independent compressive transformations.

Acoustic Stimulation↗
PubMed1983

Simultaneous masking and unmasking with bandlimited noise.

This study examines how simultaneous masking of a tone by bandlimited noise may be affected by nonlinear interactions among spectral components of the noise. Simultaneous masking patterns (signal threshold versus signal frequency) were obtained with three types of maskers: (A) a narrow-band noise, 50 Hz wide with variable center frequency fv, (B) pairs of narrow-band noises, each band 50 Hz wide with center frequencies fl and fu, and (C) wide-band noise formed by filling the spectral gap between the two bands of (B). The variable frequency fv was set to 1.0, 1.1, 1.2, and 1.3 kHz: fl was fixed at 1.0 kHz, and fu had values of 1.1, 1.2, and 1.3 kHz. In most conditions, the two-band maskers and the wideband maskers produced more masking than would be predicted from the masking produced by the single narrow-band maskers. For certain signal frequencies below the maskers, adding noise to fill the spectral gap of the two-band masker actually resulted in a 3- to 15-dB release from masking. These results reveal factors that may operate to confound modern measures of frequency selectivity.

Acoustic Stimulation↗
PubMed1983

Susceptibility to intraspeech spread of masking in listeners with sensorineural hearing loss.

Previous research with speechlike signals has suggested that upward spread of masking from the first formant (F 1) may interfere with the identification of place of articulation information signaled by changes in the upper formants. This suggestion was tested by presenting two-formant stop consonant--vowel syllables varying along a/ba--/da/--/ga/ continuum to hearing-impaired listeners grouped according to etiological basis of the disorder. The syllables were presented monaurally at 80 dB and 100 dB SPL when formant amplitudes were equal and when F 1 amplitude was reduced by 6, 12, and 18 dB. Noise-on-tone masking patterns were also generated using narrow bands of noise at 80 and 100 dB SPL to assess the extent of upward spread of masking. Upward spread of masking could be demonstrated in both speech and nonspeech tasks, irrespective of the subject's age, audiometric configuration, or etiology of hearing impairment. Attenuation of F 1 had different effects on phonetic identification in different subject groups: While listeners with noise-induced hearing loss showed substantial improvement in identifying place of articulation, upward spread of masking did not consistently account for poor place identification in other types of sensorineural hearing impairment.

Adult↗
PubMed1986

The effect of across-frequency differences in masking level on spectro-temporal pattern analysis.

Masking noise well separated in frequency from the signal may improve the detectability of the signal if the masking noise is modulated. This effect is referred to as co-modulation masking release (CMR). The present experiments examine the effect of across-frequency differences in masking noise level on CMR. Three experiments were performed, each using a different method to create modulated noise stimuli having across-frequency differences in the spectrum level. All stimulation was monaural. Experiment I used a notched noise method (selectively reducing the level for the critical band centered on the signal). Experiment II used a method in which the level of a 100-Hz-wide masker centered on the signal was varied, and flanking noise bands were of constant level. Experiment III used a method in which flanking noise bands were varied in level, and the 100-Hz-wide masker centered on the signal was of constant level. The signal was a 1000-Hz, 300-ms pure tone. The CMR effect was negated by small spectral notches centered on the signal (experiment I). However, CMR proved to be relatively robust to across-frequency level differences in experiments II and III (a CMR effect occurred for across-frequency differences in spectrum level as great as 20 dB). Low CMR's obtained in experiment I were probably due to relatively poor correlation of across-frequency modulation pattern which occurred with notched noise. The results of experiments II and III suggest that the fluctuation pattern is of primary importance in providing release from masking, and that information on absolute levels, coded across frequency, is of less importance.

Adult↗
PubMed1987

Correlation between time functions of sound pressure, masking, and OAE suppression.

Periodic sound-pressure time functions with frequency components below 50 Hz were used to measure within their period (a) the temporal course of masking, called a masking-period pattern (MPP), and (b) the temporal course of suppression of delayed evoked otoacoustic emissions, called a suppression-period pattern (SPP). Three different time functions were used: an alternating Gaussian impulse, its first integral, and its second integral. In each case, the course of the SPP is a mirror image of that of the MPP: Small masking corresponds to small suppression, while strong masking coincides with almost total suppression. Since otoacoustic emissions are assumed to have their origin in the inner ear, it can be argued that simultaneous masking, an effect including central processing, is very strongly based on peripheral processes located in the cochlea. Both MPP and SPP are closely related to the second derivative of the sound-pressure time function.

Humans↗
PubMed1987

Growth of masking as a measure of response growth in hearing-impaired listeners.

Growth-of-masking functions were obtained from 19 normal and 5 hearing-impaired listeners using a simultaneous-masking paradigm. When masker and probe frequency are identical, the slope of masking approximates 1.0 for both normal-hearing and impaired listeners. For masker frequencies less than or greater than probe frequency, the slopes for impaired listeners are shallower than those of normals. These findings are consistent with previously reported physiological data (single-fiber rate versus level and AP masking functions) for animals with induced cochlear lesions. Results are discussed in terms of a potential masking technique to estimate the growth of response in normal and impaired ears.

Adolescent↗
PubMed1988

Noise power fluctuations and the masking of sine signals.

This article is concerned with fluctuations in noise power and with the role that such fluctuations play in the masking of sine signals by noise. Several measures of noise fluctuations are discussed: the fourth moment of the waveform, the fourth moment of the envelope, and the crest factor. Relationships among these quantities are found for cases of equal-amplitude random-phase noise and Rayleigh-distributed-amplitude noise. Of particular interest is a special non-Gaussian noise called low-noise noise in which the fluctuations are small by any of our measures. The results of frozen-noise masking experiments are reported, where the noise waveform was fixed for all stimulus presentations. In separate experiments, equal-amplitude random-phase Gaussian noise, with typical fluctuations, and low-noise noise, with almost no fluctuations were used. The data show that for a noise bandwidth less than the critical bandwidth, the masked threshold is about 5 dB lower for low-noise noise than for Gaussian noise. When the noise bandwidth is larger than the critical bandwidth, the masked threshold is the same for both kinds of noise. It is concluded that noise power fluctuations increase masked threshold by about 5 dB and that filtering by the auditory system reintroduces fluctuations into broadband low-noise noise.

Acoustic Stimulation↗
PubMed1988

Absence of overshoot in a dichotic masking condition.

Brief tonal signals presented soon after the onset of a masking noise are known to be less detectable than signals delayed by several hundred milliseconds. This difference in detectability is known as the "overshoot." Signals of two sorts were studied here--either interaurally in phase (S o) or interaurally out of phase by 180 degrees (S pi). When S omicron signals of 750 Hz and about 14 ms in duration were presented 4 ms after the onset of a diotic, broadband masking noise (N o), detectability was about 6 dB worse than when the signal was presented 325 ms after onset. By contrast, there was no such overshoot when S pi signals were presented at varying times after masker onset; detectability was about the same for all values of signal delay. Accordingly, the difference in performance between N o S o and N o S pi--the masking-level difference or MLD--was large (about 16 dB) with the shortest delays used and diminished (to about 9 dB) as the delay was increased. This absence of overshoot with the S pi signals is in accord with the well-established view that detectability in the dichotic masking conditions is based upon different stimulus information from that used in the diotic masking conditions. Specifically, the evidence confirms the common view that detectability in the diotic conditions is based more or less directly on neural firing rate, whereas, in the dichotic conditions, it is based upon interaural time differences encoded in the periodicity of neural firings.

Adult↗
PubMed1988

Auditory filter shape derived from binaural masking experiments.

The shape of the auditory filter was calculated from binaural masking experiments. Two different types of maskers were used in the study, a masker that was interaurally in phase at all frequencies (No), and a masker with an interaural phase difference of 0 below 500 Hz and of pi above 500 Hz. The test-signal frequency varied between 200 and 800 Hz, and the test signal was presented either monaurally (Sm) or binaurally in antiphase (S pi). By comparing the masked thresholds from the two experimental conditions, the following conclusion can be drawn: The threshold of the test signal is only affected by the masker phase within a narrow frequency range around the test frequency. Thus, for test-signal frequencies well above or below 500 Hz, no influence of the phase transition on the BMLD is observed, and normal masked thresholds for No and N pi maskers are obtained. For test frequencies around 500 Hz, the step in interaural phase difference leads to a decrease in the interaural correlation of the masker within the critical band around the test-signal frequency. This results in strong threshold changes for both monaural and binaural signals. A calculation of the auditory filter shape from the masked threshold values was performed under the assumption that the masked threshold is only dependent on the interaural cross correlation of the masker within the filter band. Using the formula of the EC theory for the relation between masker correlation and BMLD, the experimental data are well described by a trapezoidal filter with an equivalent rectangular bandwidth of 80 to 84 Hz.

Adult↗
PubMed1989

The temporal evolution of masking and frequency selectivity in the goldfish (Carassius auratus).

The temporal evolution of masking and frequency selectivity was studied in the goldfish using classical respiratory conditioning and a tracking psychophysical procedure. The temporal position of a brief tonal signal within a longer duration, tonal masker has little or no effect on signal detectability when the frequency of the masker is less than or equal to that of the signal. For masker frequencies above that of the signal, signal detectability improves as the signal onset is delayed relative to that of the masker. These patterns of tone-on-tone masking are quite similar to those observed for humans. These temporal masking patterns are qualitatively similar in shape to the peristimulus-time histogram profiles of the low-frequency saccular fibers thought to be used in this task. Frequency- and time-dependent changes in signal detectability result in specific changes in the sharpness of psychophysical tuning curves (PTC). In general, PTCs determined for signals occurring at masker onset are the most broadly tuned, and PTCs determined in forward masking are the most sharply tuned. The PTCs for signals temporally centered in the masker are intermediate. These results suggest that temporal tone-on-tone masking patterns and the temporal evolution of psychophysical tuning curves result from the response properties of peripheral auditory-nerve fibers.

Acoustic Stimulation↗
PubMed1989

Reverberant overlap- and self-masking in consonant identification.

Two effects of reverberation on the identification of consonants were evaluated for ten normal-hearing subjects: (1) the overlap of energy of a preceding consonant on the following consonant, called "overlap-masking"; and (2) the internal temporal smearing of energy within each consonant, called "self-masking." The stimuli were eight consonants/p,t,k,f,m,n,l,w/. The consonants were spoken in /s-at/context (experiment 1) and generated by a speech synthesizer in /s-at/ and/-at/contexts (experiment 2). In both experiments, identification of consonants was tested in four conditions: (1) quiet, without degradations; (2) with a babble of voices; (3) with noise that was shaped like either natural or synthetic/s/ for the two experiments, respectively; and (4) with room reverberation. The results for the natural and synthetic syllables indicated that the effect of reverberation on identification of consonants following/s/ was not comparable to masking by either the /s/ -spectrum-shaped noise or the babble. In addition, the results for the synthetic syllables indicated that most of the errors in reverberation for the /s-at/context were similar to a sum of errors in two conditions: (1) with /s/-shaped noise causing overlap masking; and (2) with reverberation causing self-masking within each consonant.

Adult↗
PubMed1990

Neural correlates of psychophysical release from masking.

Responses of chinchilla auditory-nerve fibers were measured for stimulus conditions analogous to those in which psychophysical release from masking has been observed in humans. The maskers were two equal power, narrow-band noise stimuli with different amplitude envelopes. The neurons in the sample fell into three groups that resolved the maskers' envelopes with varying degrees of accuracy. The boundaries of these groups were not sharply delineated by characteristic frequency (CF) but were dependent on the relationship between the masker level and the neurons' thresholds at the masker frequency. For the neurons that best preserved the maskers' envelope fluctuations, a neural release from masking was observed; rate-based neural masked thresholds were higher for the masker with the least fluctuating envelope. The results suggest that neural and psychophysical release from masking arises because the probe evokes larger rate changes, relative to the background response to the masker, during periods of low masker energy. Between two otherwise equivalent maskers, the one with the periods of lowest energy will produce the lower masked thresholds because rate changes are larger and more detectable.

Animals↗
PubMed1991

The processing of envelope information in comodulation masking release (CMR) and envelope discrimination.

In comodulation masking release (CMR), thresholds for a signal masked by a narrow-band noise are reduced when additional noise is present. To demonstrate CMR, the additional noise must have similar amplitude envelope fluctuations over time as the primary noise band masking the signal. The specific source of information that provides the reduction in masked threshold remains unclear, although it is the focus of recent interest. The envelope of the on-frequency masking band (OFB) changes with the addition of the signal at threshold: in the present work, the importance of these changes in providing the CMR has been investigated. First, CMR thresholds were collected in sinusoidally amplitude-modulated noises which varied either in the phase relationship of their envelopes or in their modulation depth. Then envelope discrimination (ED) thresholds were collected for changes in envelope phase disparity (i.e., envelope correlation) and for changes in modulation depth per se. The patterns of CMR thresholds with envelope phase disparity and with modulation depth are not similar to those of the ED thresholds. A computer simulation was conducted in which the stimulus waveforms were processed through an auditory model which comprised bandpass auditory filters, a square-law nonlinearity, and a sliding temporal window. The envelopes were then extracted from the processed waveforms to determine whether the envelope changes that occur in the signal intervals in the CMR and ED tasks may be similar. The results of this analysis indicate that discriminability of envelope correlation due to the addition of the signal at threshold in the CMR task was insufficient to explain CMR. However, the discriminability of changes in modulation depth due to the addition of the signal is in agreement with thresholds obtained from the CMR task.

Adult↗
PubMed1992

Effects of temporal separation and masker level on binaural analysis in forward masking.

In the present study detection under diotic (NoSo) and dichotic (NoS pi) listening conditions in a forward masking paradigm was investigated. Both the level of a noise masker and the temporal separation between the masker and a 250-Hz tone burst served as independent variables. Results showed that most of the variance in the data could be accounted for by the amount of masking in the NoSo condition, independent of the value of the temporal parameter, which itself accounted for only 1.4% of the variance that remained. Once the data were corrected for NoSo masking effectiveness, the MLD was found to decrease by only 1.4 dB as temporal separation increased from 5-100 ms, which is consistent with a very long time constant for the binaural system. Consistent with this finding, it was shown that slope changes of the growth of masking functions, for simultaneous as compared to forward masking, were similar for both the NoSo and NoS pi conditions.

Auditory Threshold↗
PubMed1992

Comparison of frequency selectivity and consonant recognition among hearing-impaired and masked normal-hearing listeners.

Frequency selectivity and consonant recognition were determined for normal-hearing and hearing-impaired listeners using techniques that facilitate comparisons of performance among listeners whose absolute thresholds vary in magnitude and configuration. First, for each of six subjects with cochlear hearing loss, masked thresholds in notched noise and narrow-band-noise maskers were obtained and compared to results for three normal-hearing listeners whose thresholds were precisely matched to the impaired listeners' by masking with spectrally shaped broadband noise. Second, for hearing-impaired listeners and their masked normal-hearing controls, measurements of consonant recognition were obtained at several speech-presentation levels selected on the basis of articulation-index predictions to assure equal speech-spectrum audibility across listeners. The results suggest that frequency selectivity is poorer for hearing-impaired listeners than for masked normal-hearing listeners, even when thresholds among subjects are equated, but the deviation from normal frequency selectivity is smaller than estimated from comparisons with normal-hearing listeners in quiet. Critical ratios for hearing-impaired listeners are equivalent to normal. Although frequency selectivity is reduced, there is no consistent difference in consonant recognition between hearing-impaired subjects and masked normal-hearing subjects, when performance is assessed under conditions that assure equal speech-spectrum audibility across subjects.

Adolescent↗