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PubMed1982

The case of the missing 6 dB.

In 1933 data were reported which indicated that thresholds of hearing for frequencies apparently depended upon whether the source was an earphone (MAP) or a loudspeaker (MAF). A decade or so later the same type of discrepancy appeared when loudness balances were made at 100 Hz between an earphone source and a loudspeaker source. In both cases approximately 6 dB more sound pressure level at the eardrum was required when the earphone was the source than when the loudspeaker was the source. Later research added credence fo this paradox, namely, the ear should act as a pressure operated device, and there should be no difference between MAP and MAF; yet a difference seemed to exist. Research reported in abstract form and orally by the author in 1962 and 1963 showed that (a) the difference at threshold was due to physiological noise generated in the ear canal by the earphone-cushion-head combination (and could be eliminated with a special earphone-coupling system), and (b) the suprathreshold differences obtained with loudness balancing were due to a number of subtle procedural and experimental techniques (techniques which could be modified so as to avoid all of the problems of past experimenters). This research is reported here for the first time in full detail. A total of 15 different subjects participated in eight experimental comparisons using three to nine subjects each, with sufficient replications so that most subjects' MAF-MAP and/or loudness differences were determined within 1 or 2 dB at the 95% confidence level. It was often possible to replicate previous results using previous methods, but with the modified methods reported here the average difference across experiments was less than 0.2 dB, and no subject in any experiment exhibited more than a 1.8-dB difference averaged across trials. The case of the missing 6 dB should be considered closed.

Acoustics↗

PubMed1982

Frequency discrimination at 1200 Hz in the presence of high-frequency masking noise.

To evaluate the possible contribution of high-frequency hearing to frequency discrimination at lower frequencies, pure-tone frequency discrimination tasks at 1200 Hz, with and without high-frequency masking noise, were performed by two highly practiced normal-hearing listeners. Signal levels ranged from 10 to 80 dB SPL. Spectra of the three high-frequency masking noises ranged from 1.8 to 2.1 kHz, from 2.8 to 3.1 kHz, and from 4 to 8 kHz. The frequency difference limen (DLF) intensity functions, describing DLFs as a function of signal level which were obtained in the presence of high-frequency masking noise, were essentially the same as the DLF intensity functions obtained without masking noise. These results indicate that high-frequency hearing is not necessary for acute frequency discriminations at low and at middle frequencies.

Adult↗

PubMed1982

Forward masking of diotic and dichotic clicks by noise.

The first experiment reported here measured thresholds for clicks in forward masking as a function of the masker level and as a function of the temporal relation of a 6-kHz low-pass noise masker to the click (300-msec duration with a 20-msec temporal gap or 10-msec duration with a 5-msec gap). Also varied were the spectral content of the click (low-pass filtered at 1 or 5 kHz) and the interaural phase of the click (0 or pi). The difference in frequency content had no effect on the amount of masking for the 300-msec masker, while with the 10-msec masker greater masking was found for the 1-kHz click. This combination (1 kHz, 10 msec) was also the only one to produce Masking Level Differences (MLDs) when the click was presented dichotically. A second experiment investigated the effects of combining the maskers used in the first experiment. Additional masking (above that predicted by an energy sum) was found, as has been reported elsewhere [Penner and Shiffrin, A. Acoust. Soc. Am. 67, 617-627 (1980)]. However, the magnitude of this additional masking was decreased for certain conditions. These data conflict with the predictions of additivity of masking obtained from a model proposed by Penner and Shiffrin and suggest that modifications to that model are needed. The results of both experiments can be explained by assuming that two processes are acting in forward masking [Duifhuis, J. Acoust. Soc. Am. 54, 1471-1488 (1973)].

Acoustic Stimulation↗

PubMed1982

Detectability of time-varying interaural correlation in narrow-band noise stimuli.

In a two-interval, forced-choice task, observers discriminated a binaural noise whose interaural correlation r varied according to the function r(t) = m cos 2 pi fmt from an interaurally uncorrelated noise [NU; r(t) = 0.0]. The former stimulus produces a perceptual binaural "flutter," where the flutter rate is equal to the modulation frequency fm, and the amount of flutter corresponds to m. The stimuli were 0.4-octave-wide Gaussian noises with center frequencies of 500, 1000, 2000, or 4000 Hz. Presentation levels were 70-75 dB SPL; duration was 1.0 s. For a given modulation frequency, the peak interaural correlation m was varied in a blocked procedure, and thresholds were estimated as that value of m required for 75% discriminability. Plotting thresholds as a function of fm yields a modulation function, which can be interpreted as an "attenuation characteristic" of the binaural system. For the 500-Hz stimulus, thresholds increased from m - 0.28 at fm = 1 Hz to m - 0.80 at fm - 50 Hz. For higher frequency stimuli, discrimination was generally poorer and the modulation functions were flatter: thus, for 2000 Hz, m = 0.54 at fm = 1 Hz and m = 0.71 at fm = 20 Hz; for 4000 Hz, m = 0.71 at fm = 1 Hz and m = 0.81 at fm = 10 Hz. The modulation function from an "ideal observer" (at 500 Hz) differed in both form and absolute level from those of real observers. The data are discussed in relation to previous work on dynamic binaural processing.

Auditory Perception↗

PubMed1983

Suppression in simultaneous masking.

Suppression, i.e., the decrease of masked threshold caused by the addition of a second masker M2 to a first masker M1, is measured for the case of simultaneous masking. The magnitude of suppression decreases with increasing test tone duration; pulsed maskers elicit somewhat more suppression than continuous maskers. In comparison to suppression effects obtained in nonsimultaneous masking (post-masking, pulsation threshold) suppression in simultaneous masking is considerably smaller and was found only at the lower slopes of the two maskers. Suppression in simultaneous masking would not be predicted by those models of suppression which require nonsimultaneous presentation of maskers and test sound.

Acoustics↗

PubMed1983

Broadband masking noise and behavioral pure tone thresholds in cats.

The threshold for detection of pure tones in broadband noise was determined for three cats using an auditory reaction time procedure. Critical ratio is defined as the ratio of the signal power at the masked threshold for detection to the spectrum level of the noise. Critical ratios were obtained for 250-, 500-Hz, and 1-, 2-, 4-, 8-, and 16-kHz tones over a wide range of noise intensities. Results indicate that critical ratios increase with the frequency of the tone stimulus. At frequencies below 4 kHz, critical ratios remain constant at moderate and high noise intensities. For frequencies above 4 kHz, critical ratios increase as the level of the masking noise is raised from moderate to high levels. The difference between low- and high-frequency behavior of the level dependence of critical ratios is considered in terms of two possible mechanisms: (1) different mechanisms may be involved in the encoding of low- and high-frequency information by the nervous system or (2) the difference in level dependence may be due to attenuation by the action of the middle ear muscles at high sound levels.

Acoustics↗

PubMed1983

The effect of random intensity fluctuation on monaural and binaural detection.

Two experiments were performed to determine the effects of random intensity fluctuation on NoSo and NoS pi performance. Noise was used as both signal and masker, and stimuli were bands of noise from either 0-2.0 or 2.0-4.0kHz. Signal and masker were either coherent (from the same source) or noncoherent (from independent sources). In the first experiment, noise fluctuation was achieved by modulating a wide band of noise. In the second experiment, fluctuation was achieved by narrowing the noise bandwidth. Results from both experiments indicated that NoSo performance was adversely affected by fluctuation and by noncoherent relation between signal and masker. NoS pi detection was not adversely affected by fluctuation at low frequency, and was affected less adversely than was NoSo detection at high frequency. This difference between NoSo and NoS pi performance is an important consideration when making inferences about monaural and binaural processing when the stimuli are fluctuating rather than temporally steady.

Adult↗

PubMed1983

Frequency discrimination and signal detection in band-reject noise.

An experiment was conducted in order to compare the importance of information from frequency regions remote from the nominal signal frequencies for frequency discrimination and signal detection. In both tasks, signals were presented within the "notch" of band-reject noise, and different notch widths were employed. The results indicate that information is integrated over a wider range in frequency discrimination than in signal detection. Further, experiments in which a noise floor was present as well as band-reject noise, indicate that disrupting the information from regions remote from the nominal signal frequencies impairs frequency discrimination even in the absence of any significant impairment of signal detection performance.

Auditory Perception↗

PubMed1984

Comparison of auditory filter shapes derived with three different maskers.

Auditory filter shapes were derived for three different masker types, by measuring threshold for a 1-kHz sinusoidal signal masked by: (a) a noise with a spectral notch of variable width; (b) two tones with variable frequency separation; and (c) a noise with a sinusoidally rippled spectrum with variable ripple density. In each case the masker spectrum was symmetric about the signal frequency, the signal level was fixed, and the masker level was varied to determine threshold using an adaptive, two-alternative, forced-choice procedure. Both simultaneous and forward masking were used. The auditory filter shapes derived from the data were broader in simultaneous masking than in forward masking for all three masker types. In simultaneous masking the derived filters were similar for the three masker types, although there was a tendency for the filters derived from the rippled-noise data to be broader than those for the other maskers. In forward masking the auditory filters derived from the data for three masker types differed considerably in bandwidth and the slope of the filter skirts, and in that a portion of the rippled-noise filter was negative valued. The results are consistent with the idea that suppression has the effect of enhancing frequency selectivity, and that this effect is revealed in forward but not in simultaneous masking. However, the degree and nature of the enhancement differs for different masker types.

Auditory Perception↗

PubMed1984

Evidence for a reappraisal of the psychophysical selective adaptation paradigm.

The human psychophysical adaptation literature infers the existence of channels in the auditory system sensitive to frequency modulation (FM) from selective increases in FM detection thresholds following adaptation with FM stimuli. Using this psychophysical paradigm to characterize the attributes of feature-sensitive channels requires knowledge of the phenomenon's stability over repeated testing. In this study FM detection thresholds were measured in human subjects with continued testing over numerous sessions. During adapting intervals within the sessions either FM upsweeps or silence was presented. Exposure to FM upsweeps initially resulted in an increase in FM detection thresholds by a factor of 2 to 3 relative to those measured following silence. These initial threshold elevations decreased markedly with repeated testing (more than five 30-min experimental sessions). Final threshold differences between adapted and nonadapted conditions approached zero. In one subject, such asymptotic threshold values were regained in a single session, after a 4-month hiatus. The findings suggest multiple determinants of the selective adaptation function, as well as a reevaluation of inferred mechanisms.

Adaptation, Physiological↗

PubMed1984

Learning to detect auditory pattern components.

Listeners' abilities to learn to hear all the details of an initially unfamiliar sequence of ten 45-ms tones were studied by tracking detection thresholds for each tonal component over a prolonged period of training. After repeated listening to this sequence, the presence or absence of individual tones could be recognized, even though they were attenuated by 40-50 dB relative to the remainder of the pattern. Threshold-tracking histories suggest that listeners tend to employ two different learning strategies, one of which is considerably more efficient. Special training by reducing stimulus uncertainty and extending the duration of the target component was effective in increasing the rate of threshold improvement. Strategies acquired with the first pattern studied generalized to new sequences of tones. The possible implications of these results for the perceptual learning of speech or other auditory codes are discussed.

Auditory Perception↗

PubMed1984

Acoustic injury and the physiology of hearing.

A critical bibliography of published articles concerning the effects of noise exposure on hearing has been compiled for the NIH. The review concentrated on articles published over the last 14 years; however, historical highlights of the past 50 years or so were included for continuity. This paper attempts to summarize in tutorial fashion the results of that review with regard to auditory physiology in general and explores some of the current issues with specific reference to acoustic injury. To date, most of the effort toward understanding acoustic injury has been focused on the auditory periphery; the emphasis is clear simply from the number of papers published in that area. On the other hand, the response of the central nervous system to noise exposure or to any type of damage in the periphery is essentially unknown. It would seem that it is now time to assume a more balanced approach and recognize the importance of the CNS with regard to understanding the overall consequences of acoustic injury.

Auditory Pathways↗

PubMed1984

Late-onset auditory deprivation: effects of monaural versus binaural hearing aids.

Performance on tests of pure-tone thresholds, speech-recognition thresholds, and speech-recognition scores for the two ears of each subject were evaluated in two groups of adults with bilateral hearing losses. One group was composed of individuals fitted with binaural hearing aids, and the other group included persons with monaural hearing aids. Performance prior to the use of hearing aids was compared to performance after 4-5 years of hearing aid use in order to determine whether the unaided ear would show effects of auditory deprivation. There were no differences over time for pure-tone thresholds or speech-recognition thresholds for both ears of both groups. Nevertheless, the results revealed that the speech-recognition difference scores of the binaurally fitted subjects remained stable over time whereas they increased for the monaurally fitted subjects. The findings reveal an auditory deprivation effect for the unfitted ears of the subjects with monaural hearing aids.

Audiometry, Pure-Tone↗

PubMed1985

Mathematical modeling of cochlear mechanics.

The recent discovery of oto-acoustic emissions [see Zurek, J. Acoust. Soc. Am. 78, 340-344 (1985)] and the newer measures of the micromechanics of the inner ear have generated renewed interest in quantitative descriptions of the biomechanics of the cochlea. Active elements (mechanical force generators) are thought to be essential for producing the high sensitivity and sharp tuning typically associated with normal cochlear function. A mechanical model with active elements is described which can simulate basilar membrane displacements with neural-like tuning and peak amplitudes of about 1 nm at the threshold of hearing. In addition, such models might help explain the source of oto-acoustic emissions. The paper describes the power of the recent attempts at providing quantitative descriptions and predictions of the mechanics of the cochlea.

Animals↗

PubMed1985

Release from masking caused by envelope fluctuations.

This paper examines how short-term energy fluctuations in a masker affect the thresholds for tones at frequencies above those of the masker. Two equally intense tones at 1060 and 1075 Hz produce up to 25 dB less masking than does a 1075-Hz tone set to the overall level of the two-tone complex. At wider frequency separations, two-tone complexes also produce less masking than the pure tone. These results indicate that envelope fluctuations in a masker, whose spectrum is confined to a single critical band, may result in release from masking. The release from masking probably is related to the comodulation masking release reported by Hall et al. [J. Acoust. Soc. Am. 76, 50-56 (1984b)] for modulated-noise maskers with bandwidths greater than one critical band. Further measurements with maskers, whose intensity level in the critical band around 1 kHz was 90 dB SPL, show similar masking by a pure tone and a 625- to 1075-Hz bandpass noise, but less masking by narrow-band noises. These results are inconsistent with a simple frequency selective energy-detector model and indicate that the auditory system can use periods of low masker energy as brief as a few ms to enhance detection of a tone. The results also imply that the upward spread of excitation is best represented by masking patterns for noises with bandwidths of several critical bands.

Acoustic Stimulation↗

PubMed1985

The effect of competing melodies on melody recognition by hearing-impaired and normal-hearing listeners.

For a group of 30 hearing-impaired subjects and a matched group of 15 normal-hearing subjects (age range 13-17) the following data were collected: the tone audiogram, the auditory bandwidth at 1000 Hz, and the recognition threshold of a short melody presented simultaneously with two other melodies, lower and higher in frequency, respectively. The threshold was defined as the frequency distance required to recognize the test melody. It was found that, whereas the mean recognition threshold for the normal-hearing subjects was five semitones, it was, on the average, 27 semitones for the hearing-impaired subjects. Although the interindividual spread for the latter group was large, it did not correlate with the subjects' auditory bandwidth, nor with their musical experience or education.

Acoustics↗

PubMed1985

The temporal course of simultaneous tone-on-tone masking.

Threshold for a 20-ms, 1-kHz signal was measured as a function of its temporal position within a longer duration gated masker; masker frequencies were below, at, and above 1 kHz. For a masker frequency above the signal frequency, there is a sizable temporal effect: As the onset of the signal is delayed, threshold decreases rapidly but then increases again as the signal approaches masker offset. Similar results can be observed for a masker frequency below the signal frequency, but that temporal effect is due to the detection of the cubic difference tone. The implication of this frequency-dependent temporal effect for measuring psychophysical tuning curves is discussed.

Acoustic Stimulation↗

PubMed1985

Behavioral audiograms of the bullfrog (Rana catesbeiana) and the green tree frog (Hyla cinerea).

Reflex modification was used in a psychophysical technique to measure absolute auditory sensitivity of two species of anurans. Behavioral audiograms for these animals reveal that the bullfrog can detect sounds from 100 Hz to 3.2 kHz and the green tree frog from 100 Hz to 5 kHz. The shape and the sensitivity of these behavioral audiograms are similar to those of neural evoked-response audiograms of these animals. Absolute auditory sensitivity of anurans is only partially related to the spectral composition of their species-specific vocalizations.

Acoustic Stimulation↗