S37 88th Meeting: Acoustical Society of America S37

process. The 500-Hz 100-msec signal was masked by wide- band Gaussian noise. Detectors followed by rectangular and exponential integrators were equally successful in accounting for 70% for the predictable variance in observers' responses. The time constants of the best correlating detector integrators averaged 300 msec for the rectangular and 150 msec for the exponential shaped functions, with single tuned filters of 20- or 40-Hz bandwidth. Energy variations in successive time inter- vals of 25-msec duration were correlated with responses of observers and energy detectors. A regression analysis which recovered the shape of the energy detector integration func- tions showed that the observers give significantly negative weight to energy prior to the signal when it is present, but positive weight to such energy when the signal is absent.

10:15

R6. Masking beats with beats. M.M. Taylor, H.A. Brenne- man, and S.M. Smith (D.C. I.E. M., Box 2000, Downsview, Ontario M3M 3B9, Canada)

The envelope of the output of any filter whose input is more complex than a sinusoid may be described in terms of the spectrum of beats among the frequencies in its input. Taylor and Smith [J. Acoust. Soc. Am. 53, 377(A) (1973)] suggested tlmt one beat might mask another. We now propose that mask- ing of beats by beats is analogous to masking of tones by tones, and that the auditory system has beat-sensitive filters whose inputs are the output envelopes of the primary (critical-band) filters. In the experiments, the frequencies of two masker tones were held constant. As a function of signal frequency, masking was greatest when the beat between the signal and the nearest masker had the same frequency as tlmt between the two maskers. For at least some listeners, the amount of masking depended primarily on the intensity of the softer, not the louder masker. For maskers at 470 and 490 Hz, the half- bandwidths of the masking maxima at 450 and 510 Hz were on the order of 5 Hz, which represents a rough estimate of the half-width of the "20- Hz beat at 500-Hz" filter.

10:30

R7. Audibility of infrasound. Daniel L. Johnson and H. von Gierke (Aerospace Medical Research Laboratory, Wright-- Patterson AFB, Dayton, Ohio 45433)

The distortion of pure tones (1--16 Hz) caused by the non- linearities of the middle ear was calculated. It is shown that

the slope of the audibility curves for infrasound of Yeowart and Evans could be predicted, thus implying that infrasound might not be heard in the normal sense, but only heard as distortion after being transduced through the middle ear. To verify this result, subjects were exposed simultaneously with the 1--10- Hz stimuli to a low-frequency masking noise (10--100 Hz). This noise was shown to mask pure tones of infrasound of 1--10 Hz even when the SPLs of these tones were 15--25 dB above

the masking noise overall sound-pressure level. Clearly, this result implies that the pure tones of infrasound below 10 Hz are not heard in the same manner as tones above 16 Hz. The

implications of these results to the importance of the infra- sound components of any broad-band noise and to the auditory effects of infrasound are discussed.

10:45

RS. Masking of tone by tone as a function of duration. L.A. Jeffress and Alan Sharpley (Department of Psychology and Applied Research Laboratories, The University of Texas at Austin, Austin, Texas 78712)

A 500-Hz tonal signal was masked by a continuous tone, both from the same oscillator. A two-interval-forced-choice pro- cedure was used to determine the level required for 80% cor- rect. The masker level was 50 dB SPL, and the signal was

J. Acoust. Soc. Am., Vol. 56, Supplement

always in phase with the masker. Signal durations from 20 msec to 2 sec were employed. The results showed a slope of approximately 3 dB per doubling over the entire range from 20 msec to 1 sec. Detection at 2 sec tended to be worse than at 1

sec. The slope of 3 dB per doubling is much steeper than has commonly been found with noise maskers, especially at the longer durations. Data from an electrical model are discussed.

11:00

R9. Masking of narrow-band noise by pure tone. I.M. Young and C. H. Wenner (Department of Otolaryngology, Thomas Jefferson University, Jefferson Medical College, Philadelphia, Pennsylvania 19107)

Threshold measurements of narrow-band noise with

Zwicker's critical bandwidth w. ere made in the presence of various pure tones in subjects with normal hearing. For a given masking level, the masking effect on the narrow-band noise by the center frequency decreased as the center frequen- cy increased. This finding agrees with the masking of pure tone by pure tone in that the slope of masking versus level of the masker was inversely related to the frequency of the masking tone. Spread of masking was greater by frequencies below the center frequency than above.

11:15

R10. Detection threshold for a two-tone complex. Man Mohan Sondhi and J.L. Hall (Bell Telephone Laboratories, Inc., Murray Hill, New Jersey 07974)

This paper reports on a measurement of the absolute thres- hold for a two-tone signal as a function of the frequency sepa- ration between the tones. When interpreted in terms of an energy detection model (peripheral filter followed by squarer, integrator, and threshold detector), the measurements give an estimate of the time constant of the integrator. This time con- stant has been estimated by many investigators, and published estimates range from 10 to 500 msec. Our measurements on two subjects give a value in the neighborhood of 150 msec.

*The order of names was decided by coin tossing.

11:30

Rll. Loudness contours and growth functions derived from difference limen data. Edith L.R. Corliss (National Bureau of Standards, Washington, D.C. 20234)

In 1933, R.R. Riesz pointed out that his data on difference limen for intensity (1928) paralleled several contours for equal loudness measured by Fletcher and Munson (1933). Another extensive set of difference limen data was measured by Zwicker and Kaiser in 1955. They agreed so closely with the Riesz data that both sets could be combined. In 1972, Schneider, Wright, Edelheit, Hock, and Humphrey published results of a loudness magnitude estimation study. These data make it possible to test Riesz's hypothesis: Over more than two orders of magnitude there is'a one-to-one correspondence between loudness and the difference limen for intensity. Thres- holds determined from the Riesz, and Zwicker and Kaiser data, considering the differential intensity sensitivity of the ear to stem from a "least count" in the hearing mechanism are in close agreement (Corliss, 1967) with the observed free- field threshold of hearing for frontal' incidence of sound. By scaling up from threshold, equal-loudness contours have been estimated for pure tones, working from difference limen data. As would be expected, they fit the Schneider et al. loudness data for high levels, and--as has been the experience in the past--agree with some but not all other contours.

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