Some effects of background noise on modulation detection interference.

Modulation thresholds were obtained for a 2000-Hz signal carrier modulated at a rate of 10 Hz. Thresholds were obtained without a masker carrier and in the presence of a masker carrier that was either unmodulated or modulated at a rate of 10 Hz and a depth of 100% (m(m) = 1.0). Of primary interest was whether the amount of interference caused by the masker was influenced by the frequency proximity of the masker to the signal, and whether background noise had an influence on that proximity effect. In general, for masker carriers higher in frequency than the 2000-Hz signal carrier, there was a tendency for the interference to decline as the masker was moved farther away from the signal for masker carriers lower than 2000 Hz, there was little or no proximity effect. Broadband noise eliminated the proximity effect obtained with an unmodulated masker, but not that obtained with a modulated masker. Results with a narrowband noise suggest that the broadband noise has its effect by masking the high-frequency side of the signal's excitation pattern. These results, as well as the results of an excitation pattern analysis, suggest that the proximity effect with all unmodulated masker may be mediated via a peripheral, within-channel interaction, whereas that with a modulated masker may be mediated via a central, across-channel interaction.

Factors influencing temporal effects with notched-noise maskers.

Temporal effects in simulataneous masking were studied by measuring the reduction in the amount of masking produced by a gated masker when that masker was preceded by a 400-ms noise (the precursor) that was usually spectrally identical to the masker. The signal frequency (fs) was 1.0 or 4.0 kHz. Experiment 1 revealed a temporal effect only when there was a spectral notch (centered at fs) in the masker and precursor. For a relative notchwidth of 0.4 fs, the temporal effect was larger at 4.0 than at 1.0 kHz. In experiment 2. where the masker and precursor both consisted of two bands of noise separated by a spectral notch of 0.4 fs, the size of the temporal effect remained essentially constant as the bandwidth of these noise bands increased from 0.2-0.8 kHz. The results from experiment 3 indicated that the temporal effect was largest when the level fo the precursor was equal to the level of the masker. Finally, the results from experiment 4 suggested that the temporal effect may depend upon the frequency region below as well as above fs, but that the frequency region above fs is probably more important.

A case study of monaural diplacusis.

A detailed examination of a case of monaural diplacusis is reported. Low-intensity pure tones presented within a certain frequency range do not sound "pure"; instead, the percept is that of "roughness", "multiple tones" or "beats". In addition, an aftertone is heard upon the cessation of certain tones. Psychophysical experiments (e.g., simultaneous masking, best beats and pitch matching) suggest that the monaural diplacusis results from an interaction between the external tone and an internal tone. The internal tone, however, does not appear to be manifest as a spontaneous oto-acoustic emission.

Temporal effects in simultaneous pure-tone masking: effects of signal frequency, masker/signal frequency ratio, and masker level.

The effect of the temporal relationship between a pure-tone masker and a pure-tone signal in simultaneous masking was investigated in three experiments. The experiments extend previous work by: studying the temporal effect over a wide range of signal frequencies, studying the change in masking over time for several masker/signal frequency ratios, and studying the growth of masking for a brief signal at different temporal positions within a longer duration masker. In the first experiment, threshold was measured for a 20-ms signal temporally centered in a masker whose duration ranged from 20 ms to continuous. Signal frequency (fs) was 0.5, 1.0, 2.0, 4.0, or 8.0 kHz; masker frequency (fm) was 1.2 fs. For all signal frequencies, the amount of masking decreased as masker duration increased. In the second experiment, threshold was measured for a 20-ms, 1.0-kHz signal as a function of the signal's temporal position within a 400-ms masker whose frequency ranged from 1.0 to 1.25 kHz. For all but the 1.0-kHz masker, for which threshold was almost independent of the signal's temporal position, threshold decreased as signal onset was delayed relative to masker onset, but then increased slightly as the signal approached masker offset. In the final experiment, growth-of-masking functions were measured for a 20-ms, 1.0-kHz signal positioned at the beginning, at the temporal center, or at the end of a 400-ms masker whose frequency was 1.20 or 1.25 kHz. The masking functions generally were steepest for a signal at the onset of the masker and, for a given temporal position, steepest for the 1.20-kHz masker.

Effect of low-frequency gain reduction on speech recognition and its relation to upward spread of masking.

Speech recognition was measured in listeners with normal hearing and in listeners with sensorineural hearing loss under conditions that simulated hearing aid processing in a low-pass and speech-shaped background noise. Differing amounts of low-frequency gain reduction were applied during a high-frequency monosyllable test and a sentence level test to simulate the frequency responses of some commercial hearing aids. The results showed an improvement in speech recognition with low-frequency gain reduction in the low-pass noise, but not in the speech-shaped background noise. Masking patterns also were obtained with the two background noises at 70 and 80 dB SPL to compare with the speech results. There was no correlation observed between the masking results and the improvement in speech recognition with low-frequency gain reduction.

The effects of hearing loss and noise masking on the masking release for speech in temporally complex backgrounds.

Speech recognition was measured in three groups of listeners: those with sensorineural hearing loss of (presumably) cochlear origin (HL), those with normal hearing (NH), and those with normal hearing who listened in the presence of a spectrally shaped noise that elevated their pure-tone thresholds to match those of individual listeners in the HL group (NM). Performance was measured in four backgrounds that differed only in their temporal envelope: steady-state (SS) speech-shaped noise, speech-shaped noise modulated by the envelope of multi-talker babble (MT), speech-shaped noise modulated by the envelope of single-talker speech (ST), and speech-shaped noise modulated by a 10-Hz square wave (SQ). Threshold signal-to-noise ratios (SNRs) were typically best in the ST and especially the SQ conditions, indicating a masking release in those modulated backgrounds. SNRs in the SS and MT conditions were essentially identical to one another. The masking release was largest in the listeners in the NH group, and it tended to decrease as hearing loss increased. In 5 of the 11 listeners in the HL group, the masking release was nearly identical to that obtained in the NM group matched to those listeners; in the other 6 listeners, the release was smaller than that in the NM group. The reduced masking release was simulated best in those HL listeners for whom the masking release was relatively large. These results suggest that reduced masking release for speech in listeners with sensorineural hearing loss can only sometimes be accounted for entirely by reduced audibility.

Effect of masker level on overshoot.

Overshoot refers to the phenomenon where signal detectability improves for a short-duration signal as the onset of that signal is delayed relative to the onset of a longer duration masker. A popular explanation for overshoot is that it reflects short-term adaptation in auditory-nerve fibers. In this study, overshoot was measured for a 10-ms, 4-kHz signal masked by a broadband noise. In the first experiment, masker duration was 400 ms and signal onset delay was 1 or 195 ms; masker spectrum level ranged from - 10-50 dB SPL. Overshoot was negligible at the lowest masker levels, grew to about 10-15 dB at the moderate masker levels, but declined and approached 0 dB at the highest masker levels. In the second experiment, the masker duration was reduced to 100 ms, and the signal was presented with a delay of 1 or 70 ms; masker spectrum level was 10, 30, or 50 dB SPL. Overshoot was about 10 dB for the two lower masker levels, but about 0 dB at the highest masker level. The results from the second experiment suggest that the decline in overshoot at high masker levels is probably not due to auditory fatigue. It is suggested, instead, that the decline may be attributable to the neural response at high levels being dominated by those auditory-nerve fibers that do not exhibit short-term adaptation (i.e., those with low spontaneous rates and high thresholds).

Effects of pure-tone forward masker duration on psychophysical measures of frequency selectivity.

The effects of forward masker duration on psychophysical measures of frequency selectivity were investigated in two experiments. In both experiments, masker duration was 50 or 400 ms, signal duration was 20 ms, and there was no delay between masker offset and signal onset. In the first experiment, growth-of-masking functions were measured for a masker whose frequency was below, at, or above the 1000-Hz signal frequency. From those data, input filter patterns (IFPs) were plotted for masker levels from 40-90 dB SPL. In the second experiment, masking patterns (MPs) were measured for a 1000-Hz masker presented at 50, 70, and 90 dB SPL. Both measures of frequency selectivity (IFPs and MPs) indicate that frequency selectivity is greater for the 400-ms masker. These data suggest that there may be a sharpening of frequency selectivity with time at a stage prior to the adaptation observed in forward masking.

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.

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."

Intensity discrimination, increment detection, and magnitude estimation for 1-kHz tones.

Intensity difference limens (DLs) were measured over a wide intensity range for 200-ms, 1-kHz gated tones and for 200-ms increments in continuous 1-kHz tones. Magnitude estimates also were obtained for the gated tones over a comparable intensity range. The discrimination data are in general agreement with those from earlier studies but they extend them by showing: (1) good discrimination for gated tones over at least a 115-dB dynamic range; (2) a slight increase in the relative DL (delta I/I) as intensity increases above 95 dB SPL; (3) smaller DLs for increments than for gated tones, with the difference approximately independent of intensity; (4) negligible "negative masking" when thresholds are expressed as intensity differences (delta I). For two of the three subjects, magnitude estimates do not conform to a single-exponent power law for suprathreshold intensities. Over the middle range of intensities where a single exponent is appropriate, the value of the exponent is less than 0.1 for all subjects.

The modulated-unmodulated difference: effects of signal frequency and masker modulation depth.

The masked threshold for a signal is often times lower when the masker is modulated than when it is unmodulated. The difference in masked thresholds is referred to as the modulated-unmodulated difference, or MUD. The purpose of the present study was to follow up on the results of a previous study [Bacon et al., J. Acoust. Soc. Am. 101, 1600-1610 (1997)] which showed that the MUD is larger for high than for low signal frequencies, both when the masker is no wider than a critical band (and the processing is solely within channel) and when it is broadband (and the processing may be both within and across channel). The present results indicate that the effects of signal frequency primarily exist only when the modulated masker is modulated at a depth greater than about 0.75, and that at these large depths, thresholds in the presence of the modulated masker are governed largely by forward masking. By far, the effect of signal frequency is larger with the broadband masker than with the critical-band masker, suggesting that there may be an across-channel process whose contribution is greater at high than at low signal frequencies. It is argued here that this across-channel process may be related to psychophysical suppression.

Amplitude modulation depth discrimination of a sinusoidal carrier: effect of stimulus duration.

Discrimination of the change in depth of sinusoidal amplitude modulation (AM) was investigated as a function of stimulus duration. The carrier frequency was 4000 Hz, the standard modulation depth (m) was either 0.1, 0.18, or 0.3, and the modulation rate was either 10, 20, 40, or 80 Hz. For all standard depths and modulation rates, threshold (delta m) decreased by more than a factor o two as stimulus duration doubled from the shortest duration used up to a certain duration (critical duration), beyond which the threshold decreased only slightly or remained constant. The critical duration corresponded to about four cycles of modulation. Psychometric functions were measured for different stimulus durations to examine the extent to which a multiple-looks model could explain the present data. This model provided a reasonable prediction of the change in AM depth discrimination threshold as a function of stimulus duration.

Modulation detection, modulation masking, and speech understanding in noise in the elderly.

Temporal processing of suprathreshold sounds was examined in a group of young normal-hearing subjects (mean age of 26.0 years), and in three groups of older subjects (mean ages of 54.3, 64.8, and 72.2 years) with normal hearing or mild sensorineural hearing loss. Three experiments were performed. In the first experiment (modulation detection), subjects were asked to detect sinusoidal amplitude modulation (SAM) of a broadband noise, for modulation frequencies ranging from 2-1024 Hz. In the second experiment (modulation masking), the task was to detect a SAM signal (modulation frequency of 8 Hz) in the presence of a 100%-modulated SAM masker. Masker modulation frequency ranged from 2-64 Hz. In the final experiment, speech understanding was measured as a function of signal-to-noise ratio in both an unmodulated background noise and in a SAM background noise that had a modulation frequency of 8 Hz and a modulation depth of 100%. Except for a very modest correlation between age and modulation detection sensitivity at low modulation frequencies, there were no significant effects of age once the effect of hearing loss was taken into account. The results of the experiments suggest, however, that subjects with even a mild sensorineural hearing loss may have difficulty with a modulation masking task, and may not understand speech as well as normal-hearing subjects do in a modulated noise background.

Fringe effects in modulation masking.

Modulation detection thresholds (20 log ms) for a sinusoidally amplitude-modulated (SAM) noise were measured in the presence of a SAM noise masker with a modulation depth (mm) of 1.0 and a modulation frequency of 16 or 64 Hz. The signal and masker carriers were presented continuously, and the signal was modulated during one of the two 500-ms observation intervals. The masker was modulated during both observation intervals and, in some conditions, for a certain amount of time before and after signal modulation. The duration of this "fringe" ranged from 62.5 ms to continuous (masker modulated throughout the thresholds estimate). The first experiment showed that a 500-ms fringe could reduce masked thresholds by 4-6 dB, but only at low signal modulation frequencies (2-8 Hz). In the second and third experiments, it was found that the fringe had to have a duration of 500 ms and a depth of about 0.75 to be maximally effective. A final, supplementary experiment indicated that the fringe effect is not due solely to the fringe that occurs prior to the observation intervals. The results are discussed in terms of both peripheral and central auditory processing.

Modulation detection in subjects with relatively flat hearing losses.

Modulation detection thresholds were measured as a function of modulation frequency in 5 normal-hearing subjects and in 8 subjects with relatively flat, slight-to-moderate hearing losses. The carrier was a broadband noise that was sinusoidally amplitude modulated (SAM) in one of two observation intervals. The spectrum level of the carrier ranged from -10 to 50 dB SPL, and, for a given carrier level, modulation frequency varied from 2 to 1024 Hz. The temporal modulation transfer functions (TMTFs) were fitted very well with a simple equation describing a low-pass filter function. The TMTFs from the normal-hearing subjects were relatively independent of carrier level, although the derived time constant tended to increase slightly with decreases in carrier level, from an average value of 2.5 msec at 30 dB SPL to 6.0 msec at -10 dB SPL. In addition, sensitivity to amplitude modulation (AM) decreased by about 4 dB as the pressure spectrum level of the carrier was decreased from 0 to -10 dB SPL. The TMTFs from 7 of the 8 hearing-impaired subjects were similar to those from the normal-hearing subjects when the carriers were presented at equal SPLs, except that the derived time constants were slightly larger in the subjects with hearing impairment. When comparisons were made at comparable sensation levels (SLs), however, the TMTFs from the two groups of subjects were quantitatively similar, with the exception that at the lowest SL (20 dB), hearing-impaired subjects typically were more sensitive to AM than normal-hearing subjects, and the derived time constants from their TMTFs were somewhat smaller. These results, taken together with previously published results, suggest that a broad listening bandwidth is important for normal performance on a temporal resolution task. That the time constant from one of the hearing-impaired subjects was significantly longer than normal, regardless of whether the comparisons were made at equal SPL or equal SL, indicates that other factors can also be important.

Overshoot in normal-hearing and hearing-impaired subjects.

Overshoot was measured in both ears of four subjects with normal hearing and in five subjects with permanent, sensorineural hearing loss (two with a unilateral loss). The masker was a 400-ms broadband noise presented at a spectrum level of 20, 30, or 40 dB SPL. The signal was a 10-ms sinusoid presented 1 or 195 ms after the onset of the masker. Signal frequency was 1.0 or 4.0 kHz, which placed the signal in a region of normal (1.0 kHz) or impaired (4.0 kHz) absolute sensitivity for the impaired ears. For the normal-hearing subjects, the effects of signal frequency and masker level were similar to those published previously. In particular, overshoot was larger at 4.0 than at 1.0 kHz, and overshoot at 4.0 kHz tended to decrease with increasing masker level. At 4.0 kHz, overshoot values were significantly larger in the normal ears: Maximum values ranged from about 7-26 dB in the normal ears, but were always less than 5 dB in the impaired ears. The smaller overshoot values resulted from the fact that thresholds in the short-delay condition were considerably better in the hearing-impaired subjects than in the normal-hearing subjects. At 1.0 kHz, overshoot values for the two groups of subjects more or less overlapped. The results suggest that permanent, sensorineural hearing loss disrupts the mechanisms responsible for a large overshoot effect.

Auditory processing of vowels by normal-hearing and hearing-impaired listeners.

"Masking" patterns were obtained in normal-hearing and hearing-impaired listeners using the pulsation-threshold procedure. Synthesized vowel maskers /a/ and /ae/ were presented at various sound pressure levels and alternated witha sinusoidal probe of a given frequency at a repetition rate of 4 Hz, T = 250 msec. Simultaneous-masking patterns also were obtained in one normal-hearing listener. The vowel pulsation patterns (VPPs) for the normal-hearing listeners closely resembled the acoustic spectra of the vowel maskers. Neither the pulsation patterns for the hearing-impaired listeners nor the simultaneous-masking patterns for the normal-hearing listener preserved the contour of the vowel spectra. These results are interpreted in terms of suppression effects, which have been observed only with nonsimultaneous-masking paradigms in normal-hearing listeners and have not been observed in listeners with a cochlear hearing loss. The effects of increased masker level were the same for both groups of listeners and are discussed in terms of upward spread of masking and a masking function with a slope less than 1.0.

Forward masking by enhanced components in harmonic complexes.

The ability of a given target component in certain spectral complexes can be considerably increased by exposure to the complex with the target component deleted. This "enhancement effect" can be observed under a wide variety of conditions and presumably reflects frequency-specific adaptation: the frequency region around the target frequency is not adapted during the exposure and hence is relatively more sensitive. Data from the present study indicate that an enhanced component in a harmonic complex produces more forward masking of a sinusoidal probe than when that component is not enhanced, i.e., an enhanced component behaves as if it were physically more intense. This suggests that the adaptation process underlying the enhancement effect produces an increase in gain in the unadapted frequency region. This increase might result from a decrease, due to adaptation, of suppression of the unadapted region.

Intensity discrimination and increment detection at 16 kHz.

When presented for several seconds, a very high-frequency tone can decay to inaudibility in subjects with normal hearing. The purpose of the present study was to determine how such a tone behaves once it is inaudible. Intensity difference limens (DLs) at 16 kHz were measured for gated (audible) and continuous (inaudible) pedestals over a range of pedestal sensation levels from about 0-60 dB, and were compared with those obtained in the same two subjects at 1 kHz [N. F. Viemeister and S. P. Bacon, J. Acoust, Soc. Am. 84, 172-178 (1988)]. The results at the two frequencies were remarkably similar, indicating, among other things, that a continuous 16-kHz pedestal--despite being inaudible-behaves as if it were audible. In addition, the results suggest that there is little or no relationship between high-frequency tone decay and intensity DLs. The locus of this long-term adaptation effect is presumably peripheral to the site where binaural interactions occur, and may be at the hair cell or auditory nerve. The intensity DLs are more consistent with a multiplicative model of (long-term) adaptation than with a subtractive model, suggesting that the nature of this adaptation is different from that which characterizes short-term adaptation.