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.     

A new procedure for measuring peripheral compression in normal-hearing and hearing-impaired listeners.

Forward-masking growth functions for on-frequency (6-kHz) and off-frequency (3-kHz) sinusoidal maskers were measured in quiet and in a high-pass noise just above the 6-kHz probe frequency. The data show that estimates of response-growth rates obtained from those functions in quiet, which have been used to infer cochlear compression, are strongly dependent on the spread of probe excitation toward higher frequency regions. Therefore, an alternative procedure for measuring response-growth rates was proposed, one that employs a fixed low-level probe and avoids level-dependent spread of probe excitation. Fixed-probe-level temporal masking curves (TMCs) were obtained from normal-hearing listeners at a test frequency of 1 kHz, where the short 1-kHz probe was fixed in level at about 10 dB SL. The level of the preceding forward masker was adjusted to obtain masked threshold as a function of the time delay between masker and probe. The TMCs were obtained for an on-frequency masker (1 kHz) and for other maskers with frequencies both below and above the probe frequency. From these measurements, input/output response-growth curves were derived for individual ears. Response-growth slopes varied from >1.0 at low masker levels to <0.2 at mid masker levels. In three subjects, response growth increased again at high masker levels (>80 dB SPL). For the fixed-level probe, the TMC slopes changed very little in the presence of a high-pass noise masking upward spread of probe excitation. A greater effect on the TMCs was observed when a high-frequency cueing tone was used with the masking tone. In both cases, however, the net effects on the estimated rate of response growth were minimal.     

Comparisons of frequency selectivity in simultaneous and forward masking for subjects with unilateral cochlear impairments

Two experiments are described in which frequency selectivity was estimated, in simultaneous and forward masking, for each ear of subjects with moderate (25-60 dB HL) unilateral cochlear hearing losses. In both experiments, the signal level was fixed for a given ear and type of masking (simultaneous or forward), and the masker level was varied to determine threshold, using an adaptive, two-alternative forced-choice procedure. In experiment I, the masker was a noise with a spectral notch centered at the signal frequency (either 1.0 or 1.5 kHz); threshold was determined as a function of notch width. Signal levels were chosen so that the noise level required at threshold for a notch width of zero was similar for the normal and impaired ear of each subject in both simultaneous and forward masking. The function relating threshold to notch width had a steeper slope for the normal ear than for the impaired ear of each subject. For the normal ears, these functions were steeper in forward masking than in simultaneous masking. This difference was interpreted as resulting from suppression. For the impaired ears, significant differences in the same direction were observed for three of the five subjects, but the differences were smaller. In experiment II, psychophysical tuning curves (PTCs) were determined in the presence of a fixed notched noise centered at the signal frequency (1.0 kHz). For the normal ears, the PTCs were sharper in forward masking than in simultaneous masking. For the impaired ears, the PTCs were similar in simultaneous and forward masking, but those in forward masking tended to be sharper at masker frequencies far removed from the signal frequency. Overall, the results suggest that suppression is reduced, but not completely absent in cases of moderate cochlear hearing loss.     

Speech reception in quiet and in noisy conditions by individuals with noise-induced hearing loss in relation to their tone audiogram.

Tone thresholds and speech-reception thresholds were measured in 200 individuals (400 ears) with noise-induced hearing loss. The speech-reception thresholds were measured in a quiet condition and in noise with a speech spectrum at levels of 35, 50, 65, and 80 dBA. The tone audiograms could be described by three principal components: hearing loss in the regions above 3 kHz, from 1 to 3 kHz and below 1 kHz; the speech thresholds could be described by two components: speech reception in quiet and speech reception in noise at 50-80 dBA. Hearing loss above 1 kHz was related to speech reception in noise; hearing loss at and below 1 kHz to speech reception in quiet. The correlation between the speech thresholds in quiet and in noise was only R = 0.45. An adequate predictor of the speech threshold in noise, the primary factor in the hearing handicap, was the pure-tone average at 2 and 4 kHz (PTA2,4, R = 0.72). The minimum value of the prediction error for any tone-audiometric predictor of this speech threshold was 1.2 dB (standard deviation). The prediction could not be improved by taking into account the critical ratio for low-frequency noise nor by its upward spread of masking. The prediction error is due to measurement error and to a factor common to both ears. The latter factor is ascribed to cognitive skill in speech reception. Hearing loss above 10 to 15 dB HL (hearing level) already shows an effect on the speech threshold in noise, a noticeable handicap is found at PTA2,4 = 30 dB HL.     

Detection and intensity discrimination of a sinusoid

Intensity discrimination thresholds were measured for gated 100-ms, 1000-Hz tones. Discrimination thresholds were measured at several intensities near absolute threshold as well as at 30, 60, and 90 dB SPL. Psychometric functions were obtained for several of these discrimination conditions, and for detection of the signal in quiet. The results showed that Weber's law is approximately valid for standards as low as 0 dB SL. Small amounts of negative masking were observed even when the data were expressed in terms of increment energy. The psychometric functions for the discrimination conditions had a similar form and were shallower than the psychometric function for the detection of a signal in quiet. A similar set of conditions was run in the presence of a continuous, broadband noise. The results were generally in agreement with those obtained in quiet, but slight differences suggested that the variability which limits performance in the two conditions is different. The results are discussed in terms of the effects of nonlinear transduction, the effects of uncertainty, and contrast mechanisms as proposed by Laming [Sensory Analysis (Academic, London, 1986)].     

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.     

Shielding against noise interfering with quantitation of insect infestations by acoustic detection systems in grain elevators

Acoustic devices used to detect hidden insect infestations must be shielded from noise in most practical applications. One device developed specifically for use in a noisy environment, the Acoustic Location ‘Fingerprinting’ Insect Detector (ALFID), counts the numbers of insects present in grain samples from shipments being graded for export at commercial grain elevators. This report considers the performance of ALFID's noise-shielding components, which include an enclosure for passive reduction of ambient noise, and an electronic system for active detection and masking of sounds originating outside the grain sample container. Sound pressure levels (SPLs) of ambient noise are reduced inside the enclosure by 60–90 dB at frequencies between 1 and 10 kHz, with a reduction of ~6.5 dB per octave (frequency doubling). The active noise-masking system protects ALFID from loud ambient sounds not sufficiently attenuated by the enclosure. If the output from one of four sensors mounted on the outside of the grain sample container rises above a preset amplitude threshold, a signal is triggered that inhibits acquisition of insect sound data from sensors inside the container. In tests of the complete ALFID system at a grain elevator with ambient noise of 73 ± W dB re 20 μPa SPL, the mean rate of noise-mask triggering was 5.5 s⁻¹, inhibiting acquisition of insect sounds for only 3.9% of the total testing period. This level of performance is sufficient to enable successful operation of ALFID under such noise conditions.     

Effects of signal delay on auditory filter shapes derived from psychophysical tuning curves and notched-noise data obtained in simultaneous masking

Psychophysical tuning curves (PTCs) measured in simultaneous masking usually sharpen as a short duration signal is moved from the onset to the temporal center of a longer duration masker. Filter shapes derived from notched-noise maskers have not consistently shown this effect. One possible explanation for this difference is that the signal level is fixed in the PTC paradigm, whereas the masker level is usually fixed in the notched-noise paradigm. In the present study, the signal level was fixed at 10 dB SL in both paradigms. The signal was 20 ms in duration, and presented at the onset or temporal center of the 400-ms masker. The masker was a pure tone presented in quiet (PTC) or in the presence of a pure-tone "restrictor" intended to limit off-frequency listening (PTCr), or it was a noise with a spectral notch placed symmetrically or asymmetrically about the 2-kHz signal frequency. Filter shapes were derived from the PTC, PTCr, and notched-noise data using the roex (p, w, t) model. The effects of signal delay and masking paradigm on filter bandwidth were analyzed with a two-factor repeated-measures ANOVA. There was a significant effect of signal delay (the filters sharpened with time) and masking paradigm (the filters derived from the notched-noise data were significantly wider than those derived from either of the PTC measurements, which did not differ from one another). Although the interaction between delay and paradigm was not significant, the filter derived from the notched-noise data sharpened more with time than did the other filters, and thus the bandwidth of the filters from the three paradigms were more similar at the longer delay than at the shorter delay. It is likely that the tuning-curve and notched-noise paradigms measure the same underlying filtering, but that various other factors contribute differentially to the derived filter shapes.     

Basilar-membrane nonlinearity estimated by pulsation threshold

The pulsation threshold technique was used to estimate the basilar-membrane (BM) response to a tone at characteristic frequency (CF). A pure-tone signal was alternated with a pure-tone masker. The frequency of the masker was 0.6 times that of the signal. For signal levels from around 20 dB above absolute threshold to 85 dB SPL, the masker level was varied to find the level at which a transition occurred between the signal being perceived as "pulsed" or "continuous" (the pulsation threshold). The transition is assumed to occur when the masker excitation is somewhat greater than the signal excitation at the place on the BM tuned to the signal. If it is assumed further that the response at this place to the lower-frequency masker is linear, then the shape of the masking function provides an estimate of the BM response to the signal. Signal frequencies of 0.25, 0.5, 1, 2, 4, and 8 kHz were tested. The mean slopes of the masking functions for signal levels between 50 and 80 dB SPL were 0.76, 0.50, 0.34, 0.32, 0.35, and 0.41, respectively. The results suggest that compression on the BM increases between CFs of 0.25 and 1 kHz and is roughly constant for frequencies of 1 kHz and above. Despite requiring a subjective criterion, the pulsation threshold measurements had a reasonably low variability. However, the estimated compression was less than in an earlier study using forward masking. The smaller amount of compression observed here may be due to the effects of off-frequency listening.     

Speech-reception threshold for sentences as a function of age and noise level.

For 140 male subjects (20 per decade between the ages 20 and 89) and 72 female subjects (20 per decade between 60 and 89, and 12 for the age interval 90-96), the monaural speech-reception threshold (SRT) for sentences was investigated in quiet and at four noise levels (22.2, 37.5, 52.5, and 67.5 dBA noise with long-term average speech spectra). The median SRT as well as the quartiles are given as a function of age. The data are described in terms of a model published earlier [J. Acoust. Soc. Am. 63, 533-549 (1978)]. According to this model every hearing loss for speech (SHL) is interpreted as the sum of a loss class A (attenuation), characterized by a reduction of the levels of both speech signal and noise, and a loss class D (distortion), comparable with a decrease in signal-to-noise ratio. Both SHLA+D (hearing loss in quiet) and SHLD (hearing loss at high noise levels) increase progressively above the age of 50 (reaching typical values of 30 and 6 dB, respectively, at age 85). The spread of SHLD as a function of SHLA+D for the individual ears is so large (sigma = 2.7 dB) that subjects with the same hearing loss for speech in quiet may differ considerably in their ability to understand speech in noise. The data confirm that the hearing handicap of many elderly subjects manifests itself primarily in a noisy environment. Acceptable noise levels in rooms used by the aged must be 5 to 10 dB lower than those for normal-hearing subjects.     

Frequency specificity of human auditory brainstem responses as revealed by pure-tone masking profiles

The frequency specificity of the auditory brainstem response (ABR) was examined by means of pure-tone masking profiles using click, 4000-Hz, and 1000-Hz filtered-click stimuli. Simultaneous pure-tone maskers were presented at one-half octave intervals around stimulus center frequency. Masking profiles at two intensities (60 and 40 dB SL) were obtained by measuring both latency and amplitude shifts in wave V as a result of the discrete-frequency maskers. Both latency and amplitude analyses showed masking profiles at 40 dB SL that were narrow and centered around stimulus frequency, whereas profiles at 60 dB SL showed high-frequency spread of the cochlear excitation area.     

Psychophysical correlates of contralateral efferent suppression. I. The role of the medial olivocochlear system in "central masking" in nonhuman primates

An extensive physiological literature, including experimental and clinical studies in humans, demonstrates that activation of the medial olivocochlear (MOC) efferent system, by either contralateral sound or electrical stimulation, can produce significant alterations in cochlear function and suggests a role for the MOC system in influencing the auditory behavior of binaural hearing. The present data are from psychophysical studies in nonhuman primates which seek to determine if the noted physiological changes in response to contralateral acoustic stimulation have a perceptual counterpart. Four juvenile Japanese macaques were trained to respond to the presence of 1-s sinusoids, presented to the test ear, in an operant reinforcement paradigm. Thresholds were compared for frequencies ranging from 1.0 to 4.0 kHz in quiet, with thresholds measured when continuous, two octave-band noise, centered on the test tone frequency, was presented in the contralateral ear. Contralateral noise was presented at levels of 10-60 dB above detection threshold for the test-tone frequency. While some variability was evident across subjects, both in the frequency distribution and magnitude (as a function of contralateral noise level), all subjects exhibited an increase, or suppression of thresholds in the presence of contralateral noise. On average, thresholds increased systematically with contralateral noise level, to a peak of 7 dB. In one subject, the threshold increase seen with contralateral noise was significantly reduced when the MOC was surgically sectioned on the floor of the IVth ventricle. The characteristics of the measured shifts in behavioral thresholds, in the presence of contralateral noise reported here, are qualitatively and quantitatively similar to both efferent physiological suppression effects and psychophysical central masking threshold shifts which have been reported previously. These data suggest that at least some aspects of "central masking" are efferent-mediated peripheral processes, and that the term "central masking" may be incorrect.     

Level dependence of auditory filters in nonsimultaneous masking as a function of frequency

Auditory filter bandwidths were measured using nonsimultaneous masking, as a function of signal level between 10 and 35 dB SL for signal frequencies of 1, 2, 4, and 6 kHz. The brief sinusoidal signal was presented in a temporal gap within a spectrally notched noise. Two groups of normal-hearing subjects were tested, one using a fixed masker level and adaptively varying signal level, the other using a fixed signal level and adaptively varying masker level. In both cases, auditory filters were derived by assuming a constant filter shape for a given signal level. The filter parameters derived from the two paradigms were not significantly different. At 1 kHz, the equivalent rectangular bandwidth (ERB) decreased as the signal level increased from 10 to 20 dB SL, after which it remained roughly constant. In contrast, at 6 kHz, the ERB increased consistently with signal levels from 10 to 35 dB SL. The results at 2 and 4 kHz were intermediate, showing no consistent change in ERB with signal level. Overall, the results suggest changes in the level dependence of the auditory filters at frequencies above 1 kHz that are not currently incorporated in models of human auditory filter tuning.     

Detection of interaural phase shift between a subaudible and an audible tone

Can a shift in interaural phase between a subthreshold signal and an audible contralateral probe tone affect perception of the probe? To obtain an answer, an 800-Hz tone was presented to both ears. The tone was presented continuously to one ear (-25 to + 10 dB SL) and in a sequence of four bursts per trial to the other ear (+ 10 dB SL). Interaural phase was reversed for either the second or the fourth burst in a 2 AFC task. Interaural phase-shift detection threshold (65% correct) varied with the intensity of the continuous signal; across subjects, this threshold varied from -21 to + 1 dB SL. When a 300-or 500-Hz masking tone was added to the ear with the continuous signal, phase-shift detection accuracy depended primarily upon the sensation level of the signal rather than its sound pressure level. These findings demonstrate temporal encoding at signal levels well below hearing threshold.     

Excess masking among listeners with a sensorineural hearing loss

Three experiments were conducted to determine whether listeners with a sensorineural hearing loss exhibited greater than normal amounts of masking at frequencies above the frequency of the masker. Excess masking was defined as the difference (in dB) between the masked thresholds actually obtained from a hearing-impaired listener and the expected thresholds calculated for the same individual. The expected thresholds were the power sum of the listener's thresholds in quiet and the average masked thresholds obtained from a group of normal-hearing subjects at the test frequency. Hearing-impaired listeners, with thresholds in quiet ranging from approximately 35-70 dB SPL (at test frequencies between 500-3000 Hz), displayed approximately 12-15 dB of maximum excess masking. The maximum amount of excess masking occurred in the region where the threshold in quiet of the hearing-impaired listener and the average normal masked threshold were equal. These findings indicate that listeners with a sensorineural hearing loss display one form of reduced frequency selectivity (i.e., abnormal upward spread of masking) even when their thresholds in quiet are taken into account.     

Formant discrimination in noise for isolated vowels

Formant discrimination for isolated vowels presented in noise was investigated for normal-hearing listeners. Discrimination thresholds for F1 and F2, for the seven American English vowels /i, I, epsilon, ae, [symbol see text], a, u/, were measured under two types of noise, long-term speech-shaped noise (LTSS) and multitalker babble, and also under quiet listening conditions. Signal-to-noise ratios (SNR) varied from -4 to +4 dB in steps of 2 dB. All three factors, formant frequency, signal-to-noise ratio, and noise type, had significant effects on vowel formant discrimination. Significant interactions among the three factors showed that threshold-frequency functions depended on SNR and noise type. The thresholds at the lowest levels of SNR were highly elevated by a factor of about 3 compared to those in quiet. The masking functions (threshold vs SNR) were well described by a negative exponential over F1 and F2 for both LTSS and babble noise. Speech-shaped noise was a slightly more effective masker than multitalker babble, presumably reflecting small benefits (1.5 dB) due to the temporal variation of the babble.     

Effects of background noise level on behavioral estimates of basilar-membrane compression

Hearing-impaired (HI) listeners often show poorer performance on psychoacoustic tasks than do normal-hearing (NH) listeners. Although some such deficits may reflect changes in suprathreshold sound processing, others may be due to stimulus audibility and the elevated absolute thresholds associated with hearing loss. Masking noise can be used to raise the thresholds of NH to equal the thresholds in quiet of HI listeners. However, such noise may have other effects, including changing peripheral response characteristics, such as the compressive input-output function of the basilar membrane in the normal cochlea. This study estimated compression behaviorally across a range of background noise levels in NH listeners at a 4 kHz signal frequency, using a growth of forward masking paradigm. For signals 5 dB or more above threshold in noise, no significant effect of broadband noise level was found on estimates of compression. This finding suggests that broadband noise does not significantly alter the compressive response of the basilar membrane to sounds that are presented well above their threshold in the noise. Similarities between the performance of HI listeners and NH listeners in threshold-equalizing noise are therefore unlikely to be due to a linearization of basilar-membrane responses to suprathreshold stimuli in the NH listeners.     

Speech recognition in noise by individuals with mild hearing impairments

The study was designed to test the validity of the American Academy of Ophthalmology and Otolaryngology's (AAOO) 26-dB average hearing threshold level at 500, 1000, and 2000 Hz as a predictor of hearing handicap. To investigate this criterion the performance of a normal-hearing group was compared with that of two groups, categorized according to the AAOO [Trans. Am. Acad. Ophthal. Otolaryng. 63, 236-238 (1959)] guidelines as having no handicap. The latter groups, however, had significant hearing losses in the frequencies above 2000 Hz. Mean hearing threshold levels for 3000, 4000, and 6000 Hz were 54 dB for group II and 63 dB for group III. Two kinds of speech stimuli were presented at an A-weighted sound level of 60 dB in quiet and in three different levels of noise. The resulting speech recognition scores were significantly lower for the hearing-impaired groups than for the normal-hearing group on both kinds of speech materials and in all three noise conditions. Mean scores for group III were significantly lower than those of the normal-hearing group, even in the quiet condition. Speech recognition scores showed significantly better correlation with hearing levels for frequency combinations including frequencies above 2000 Hz than for the 500-, 1000-, and 2000-Hz combination. On the basis of these results the author recommends that the 26-dB fence should be somewhat lower, and that frequencies above 2000 Hz should be included in any scheme for evaluating hearing handicap.     

Transient masking and the temporal course of simultaneous tone-on-tone masking

Previous studies have shown that threshold for a signal in tone-on-tone simultaneous masking is sometimes lower when the masker is continuous than when it is gated. Threshold may also decline as signal onset is delayed relative to the onset of a longer duration masker, though it may increase again near masker offset. In the present study, the level of a 1250-Hz sinusoidal masker was found which would just mask a 20-ms, 1000-Hz sinusoid presented at 10-dB sensation level (SL). Masker duration was 20 or 400 ms; in the latter case, the signal was presented in one of three temporal positions within the masker. The level of the 1250-Hz masker necessary to mask the signal was reduced, sometimes by as much as 20-25 dB, by a 20-ms, 500-Hz sinusoid (transient masker) presented at the times when the signal might occur, but at a level 30 dB below that at which it would mask the 10-dB SL signal. This suggests that, in the earlier studies, at least some of the elevation in threshold in the presence of a short-duration masker or at the beginning (or end) of a longer duration masker may have been due to the transient responses to the masker affecting detection of the signal, but not necessarily masking the signal in terms of excitation in the signal "channel."     

Signal transmission in noisy environments: auditory masking in the tympanic nerve of the bushcricket Metaballus litus (Orthoptera: Tettigoniinae)

Physiological responses of the auditory leg nerve were recorded in the tettigoniid Metaballus litus to suprathreshold tone pulses of 12.45 kHz, which is close to the carrier frequency of the male's call. This stimulus tone frequency was determined by characterizing the polar response of the foreleg. Physiological threshold of the receptors was calculated from intensity input/output curves, and the experimental stimulus was set at 40 dB above this threshold value. There was low variance in threshold values between preparations. Continuous octave filtered white noise centered on the stimulus frequency was presented at the same time as the tone pulse at increasing intensities. The summed action potentials (SAPs) of the whole leg nerve were averaged over 256 stimulus presentations and the magnitude of the response was calibrated to dB values. The range of noise levels was set between that inducing no decrease in the SAP response to the tone pulse stimulus, up to a masking intensity where the response to the tone pulse was only just observable. Decrement in SAP magnitude was linear, and complete masking occurred when the noise level was 20-25 dB above the initial level of zero masking. This final level was comparable in magnitude to the sound-pressure level of the tone pulse and within the natural range of the insect's auditory behavior. Following the cessation of the noise signal, the SAPs were monitored over intervals of 2 min until the SAP asymptoted to the preexperimental condition. The reduction in SAP magnitude during noise presentation was attributed to a loss in synchrony from the individual tympanic receptors.