Tom W. Tillman

Perceptual Masking in Multiple Sound Backgrounds

Shifts in masked spondee thresholds during several conditions of listening (monaural, homophasic, antiphasic, and with interaural time disparity) in the presence of one to four competing maskers were measured. The maskers used were white noise, white noise modulated four times per second by 10 dB with a 50% duty cycle, the same noise with 75% duty cycle, connected speech by one male talker, and connected speech by a second male talker. Results from three experiments that employed various permutations of the aforementioned conditions are reported. The findings, after equating conditions to equivalent masker levels, were four. First, the modulated noise with 50% duty cycle produced about 3.5 dB less masking than that produced by unmodulated white noise. Second, the modulated noise with 75% duty cycle allowed only about 1 dB less shift than did the unmodulated noise. Third, mixing one speech train with noise (either modulated or unmodulated) induced about 3.2 dB excess masking. This excess is here termed per...

Release of masking for speech through interaural time delay.

Interference with binaural intelligibility of spondaic words produced by continuous white noise and of monosyllabic words produced by both continuous and modulated white noise as well as connected speech (single talker) was studied under a variety of interaural listening conditions. Performance during homophasic (N0S0) and antiphasic (NπS0) listening was compared with that achieved under conditions involving various interaural time differences of the noise and/or the speech. These time differences ranged from 0.1 to 0.8 msec. Several signal‐to‐masker ratios were employed, but for the conditions involving modulated noise, only two modulation rates (4/sec and 100/sec) and a single magnitude of modulation (14 dB) were used. Transition from homophasic to antiphasic listening produced masking‐level differences (MLDs) of about 7 and 4 dB for spondees and monosyllables, respectively. The MLDs produced by varying the interaural timing of either speech or noise increased systematically as the time differences were increased within the range studied here, but they never exceeded those for antiphasic listening and were usually appreciably smaller. As gauged by performance under 0.4‐ and 0.8‐msec interaural time delay, the MLD for monosyllables was the same regardless of whether the time difference was applied to the masker or the speech. Furthermore, the MLD produced by simultaneous but opposing interaural time differences (masker leading in one ear and speech in the other) did not exceed the antiphasic MLD, even when the aggregate timing discrepancy between the two signals reached 1.6 msec. Some implications of these findings are discussed.

Masking by Octave Bands of Noise in Normal and Impaired Ears

The masking of sinusoids by three octave lands of thermal noise was measured in normal and hearing‐impaired ears at each of two effective noise levels. For frequencies within the noise band results were essentially equivalent for all groups studied at either effective level. At frequencies both above and below the noise band no differences among groups were noted at the 10‐db effective level. At the 30‐db effective level, however, both presbycusics and sensori‐neural losses of presumably cochlear origin showed more masking than either normals, plugged normals, or conductives.

Release from Multiple Maskers: Effects of Interaural Time Disparities

Masked thresholds for spondees were measured during 37 binaural listening conditions covering homophasic, antiphasic, parallel time‐delayed, and opposed time‐delayed listening in the presence of one to four competing maskers at three nominal masker levels (65, 75, and 85 dB SPL). Two of the maskers used were white noise modulated 4 times per second by 10 dB with a 50% duty cycle; the modulation patterns were such that when the noises were combined they produced a continuous white noise. The other two were whole sentences spoken by different male talkers. Masking‐level differences (MLDs) re homophasic performance in the same masker complex emerged for every one of the 28 conditions of dichotic listening. MLDs were slightly smaller in the presence of a single competing talker (with or without noise also present) than with two talkers, and MLDs were always largest for the antiphasic conditions (range 4.4–6.8 dB). MLDs for parallel delayed listening (entire masking complex given 0.8‐msec lead to a single ...

Binaural Masking of Speech by Periodically Modulated Noise

The interference with intelligibility of monosyllabic words produced by continuous white noise, by modulated white noise, and by continuous speech (single talker) was studied during homophasic (N0S0) and antiphasic (NπS0) listening. Five signal‐to‐masker ratios, four modulation rates, and four magnitudes of modulation were used. Reception in the continuous noise was characterized by steeply sloping intelligibility functions and a 4.5‐dB masking‐level difference favoring antiphasic listening. Reception in modulated noise changed with the rate and depth of modulation. A 7‐dB modulation yielded intelligibility functions highly comparable to those for continuous noise having the same average level. By contrast, more extreme modulation (14, 21 dB, and complete interruption) produced better intelligibility under both homophasic and antiphasic conditions than did continuous noise. This effect was particularly great when noise was completely interrupted either 4 or 20 times/sec, under which circumstances intellig...

Effects of Interaural Time Delays on Masking by Two Competing Signals

Interference with thresholds for spondees and intelligibility of monosyllabic words (at 60 dB SPL) produced by combining modulated white noise (4 mod/sec, − 10 dB interburst ratio) and connected speech as simultaneous maskers was studied under a variety of interaural listening conditions at each of three masker levels (66, 72, and 78 dB sound‐pressure level). Monaural performance in the presence of both maskers (CmNmSm) was 7.8–10.5 dB poorer than with either masker alone (NmSm or CmSm). Thus, the two maskers produced added interference substantially in excess of the approximately 3‐dB increase to be expected on the basis of the summing of their powers alone. Very little release from masking was achieved in the transition from monaural (CmNmSm) to homophasic listening (C0N0S0). During antiphasic (CτNπS0) presentation, approximately 5 dB of release from masking occurred (re C0N0S0). A little less than half as much release from masking appeared when the maskers were given 0.8 msec interaural time delays, wh...

Postexposure Responsiveness in the Auditory System. I. Immediate Sensitization

Human ears were stimulated by 3‐min 65–90 dB sound‐pressure level continuous tones, and postexposure thresholds for tones of lesser frequency were examined. In most cases, such procedures allowed demonstration of auditory sensitization that was not preceded or succeeded by desensitization and that ran its course in the first postexposure minute. Such sensitization was noted at 200 Hz following certain 500‐Hz exposure tones, and at 2000 Hz following certain 3000‐Hz exposure tones. The magnitude and duration of the phenomenon seemed dependent on exposure tone intensity.

Reliability of Measures Obtained with the Zwislocki Acoustic Bride

An acoustic‐impedance bridge designed by Zwislocki recently became available. The bridge allows the specification of the absolute acoustic impedance of the human car by yielding independent measures of acoustic reactance and resistance. This measurement of absolute impedance requires an accurate measure of the volume of the external ear. The test‐retest reliability of volume measures, as well as those of resistance and reactance, was examined. Three examiners obtained volume measures by using two different techniques as well as measures of resistance and compliance (reactance) on 10 subjects in test and retest sessions. Results suggest that measurement of the volume of the ear canal is sufficiently repeatable to allow this procedure to serve as a first step in the measurement of absolute impedance with the Zwislocki bridge. Test‐retest reliability of the compliance measures was also high. Test‐retest measures of resistance yielded low correlations in spite of high interexaminer correlation in both sessions. It is postulated that unknown factors caused a real but nonsystematic shift in acoustic resistance over subjects, thus destroying test‐retest correlation while preserving high interexaminer correlation. Further investigation is required to establish the validity of this hypothesis, but it can be concluded that the bridge itself is a reliable measuring device.

LVIII Hearing Aids and Sensorineural Hearing Loss

It is still true, we think, that in some professional circles the role of a wearable hearing aid in the communicative rehabilitation of persons with sensorineural hearing loss is misunderstood due to a number of misconceptions. Four of these fallacies are particularly noteworthy, and it is our purpose to explore them with you in the hope of arriving at a more realistic approach to the problem of recommending hearing aids for patients with sensorineural loss.

Earphone versus Sound‐Field Threshold Sound‐Pressure Levels for Spondee Words

This experiment sought to establish the difference between earphone and sound‐field measures of spondee threshold SPL (sound‐pressure level). Recorded spondee words were delivered to both a conventional and an insert‐type earphone as well as to a single loudspeaker that generated the signal in the sound field. Monaural spondee threshold SPLs were established for both normal hearing and hypoacousic subjects under the three listening conditions. The intensity level of the spondee words under the three conditions was expressed as the SPL of an “equivalent” speech spectrum noise. For both groups of subjects, the mean sound‐field threshold SPL was about 7.5 dB lower than that measured under the conventional earphone and approximately 12.5 dB lower than that established under the insert‐type earphone. These results tend to confirm the reality of the “missing 6 dB.” Further, they indicate that the difference between minimum audible pressure (MAP) and minimum audible field (MAF) is a product of both diffraction ...