Raymond Carhart

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...

Masking Level Differences Encountered in Clinical Populations

Methods for measuring masking level differences (MLDs) at 500 Hz and for spondees were used with 290 subjects: 50 persons with normal hearing and 240 patients with various diseases. Of particular interest was whether techniques currently in clinical use could be used with ease, dispatch, and profit in determining MLD size. The methods selected, which were variations of Békésy audiometry and speech reception threshold procedures, proved clinically feasible. Results revealed differences in behavior from one group of subjects to another. Although MLDs were not affected by cortical lesions, they were very often abnormally small for patients with eight-nerve tumor, Meniéres disease, or multiple sclerosis. The high incidence of abnormally small MLDs in populations with normal sensitivity to pure tones and speech but with evidence of subcortical central lesions, such as patients with multiple sclerosis, suggests that the MLD tasks can be of diagnostic value in detecting retrocochlear lesions. However, in persons with hearing loss or significant interaural differences in threshold sensitivity, or both, the MLD tests are not always reliable in differentiating cochlear from retrocochlear disease.

Forward and Backward Masking: Interactions and Additivity

Monaural masked thresholds for clicks were established on three listeners. Masking functions across selected Δts (the time disparities between the masker and click) were determined for four masking conditions in which the 70 dB SPL masker was a 500‐msec burst of random noise. These four conditions were (1) simultaneous masking, (2) forward masking (FM) at Δts from 1 to 250 msec, (3) backward masking (BM) at Δts from −1 to −250 msec, and (4) forward‐backward masking at selected Δts between two 500‐msec noise bursts. In this last instance, the interval between bursts was varied to achieve seven interburst intervals (IBIs) ranging from 25 to 500 msec. Simultaneous masking was 3–5 dB less at the boundaries of the noise burst than it was in the middle of the noise burst. The functions for FM and for BM were not inversely symmetrical with one another, although both showed less threshold shift (TS) the further displaced in time the click was from the masker. The masking that appeared when the click was prese...

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.

Monaural and Binaural Discrimination against Competing Sentences

PB‐50 lists were presented in a sound field against a background of spoken sentences. Four signal‐to‐noise ratios plus one quiet condition were used. Test items came from a loudspeaker to one side of the subject and competing sentences from a second loudspeaker, on the opposite side. Sixteen normal hearers underwent both test and retest under one binaural and two monaural conditions. Discrimination functions for these conditions showed similar configurations, but were displaced from one another along the signal‐to‐noise axis. Head‐shadow effects caused poorer discrimination during monaural listening when test items come from the far side of the head. Monaural efficiency with the open ear facing the source of test items was almost as good as binaural efficiency. However, binaural performance here exhibited an advantage equal to approximately a 3‐dB shift in signal‐to‐noise ratio, despite the fact that the head‐shadow effect enhanced interference in the far ear by almost 13 dB. [Work supported by the Vetera...

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 ...

Influence of pulsed masking on the threshold for spondees.

The changes in masking for spondee words that result from varying both the level and the interruption rate of a white‐noise masker were studied with 14 normal hearing subjects and 14 subjects with sensorineural hearing loss diagnosed as resulting from cochlear otosclerosis. The masker was a white noise and was presented monaurally at either 90 dB sound‐pressure level (SPL) or 30 dB sensation level (SL). It was either continuous or was pulsed at rates of 1, 10, or 100/sec with 50% duty cycle. During the burst‐off half of each cycle, the noise was either dropped 14 dB in level or was fully interrupted. The masked speech reception threshold (SRT) was not improved re the masked SRT in continuous noise when the 30 dB SL masker was pulsed 100 times/sec. Reduction in masking was observed under all other circumstances of cycling the noise. This reduction was more pronounced when the noise was completely interrupted rather than only modulated by 14 dB, when the masker was at its higher level (90 dB SPL as opposed ...

Relations among temporal resolution, forward masking, and simultaneous masking

The data from this investigation suggest that both temporal resolution and forward masking are the products of a single underlying mechanism of auditory persistence. Persistence of auditory activity from a burst of noise not only limits the listener’s ability to perceive a trailing noise burst as temporally separate (temporal resolution), but also interferes with his detection of a click that follows the burst of noise (forward masking). After a relatively stable initial period of about 2.5 msec, the magnitude of this persisting residual activity decays linearly over log time, as evidenced by the progressive reduction in the level of both the trailing noise burst in temporal resolution and the click threshold in forward masking. The data also indicate that forward masking of the click bears the same relationship to masker level as does simultaneous masking of the click. The mechanisms of forward masking may be essentially the same as those for simultaneous masking, except that for the former the masking p...

Integration of acoustic power at threshold by normal hearers.

Threshold responses for seven durations of 250 , 1000 , and 4000‐cps and white noise were determined for 32 normal‐hearing persons. The time parameters and spectral characteristics of the stimuli were carefully specified. The following results were obtained. (1) There was no significant difference between the male or female groups employed here with regard to their threshold response to short‐duration acoustic stimuli. (2) The decrease in intensity required for threshold response as a function of stimulus length was a real difference when the stimulus was systematically doubled in length from 10 to 500 msec. (3) Changes in the intensity necessary for threshold response resulting from changes of signal length were highly similar for 250‐, 1000‐, and 4000‐cps and white‐noise stimuli of more than 50 msec in length, but there was an excessive increase in intensity needed for response when 250‐cps stimuli were made shorter than 50 msec in duration. (4) Test‐retest reliability of threshold response to short‐dur...

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...