短音に対する聴覚の周波数分解能について

抄録

The temporal process of frequency analysis in the ear has been investigated from some aspects, but further investigations are required because it involves complicated problems owing to the neural complexity. Actually the ear shows various response modes depending on the stimulus. In this paper, we describe the results of four experiments carried out using tone bursts of short duration, and frequency selectivity for short durations. The results obtained as follows:1) Experiment on simultaneous masking when the signal has the same duration as the masker. Fig. 1 shows a schematic diagram of the stimulus presentation. The masked thresholds obtained as functions of duration and signal frequency are shown in Figs. 2 and 4. In Fig. 2, the sharpness of the masking curve increases with the duration. The masking curve is much sharper for our experiments than the steady-state response curve of the basilar membrane calculated from the model of Flanagan even for a duration of 5 msec (Fig. 3). A comparison between Figs. 2 and 4 shows that the threshold shift at the center frequency for the narrow-band masking noise is smaller than that for broad-band noise when the duration of the stimuli are shorter. This result can be interpreted by assuming that the critical band in the ear is wider for short durations than normal durations. 2) Experiment on trill threshold. Two tones of different frequencies were alternated successively from several times to one hundred or more times per second (Fig. 5). When the difference in frequency was large, the alternation sounded like two unrelated, interrupted tones. The just noticeable frequency difference (ΔF) as so perceived was measured as functions of duration and interval (Figs. 6 and 7). ΔF increases with decreasing duration of the signal and the data are described reasonably well by a straight line with a slope of about-0. 5 on the logarithmic coordinates used here (Fig. 7). It is interesting that similar results were obtained by Liang and Chistovich (1961), Oetinger (1959), and Stevens (1952), namely, that the frequency difference limen is reduced in direct proportion to the square root of the increase in duration. 3) Experiment for comparison of loudness and pitch between sine and cosine pulses. Cosine phase sound is perceived as considerably louder and higher in pitch than sine phase sound for very short durations. The critical duration as so perceived was measured at signal frequencies of 250, 1000 and 4000 Hz (Figs. 8 to 10). The results shown in the figures can be explained assuming that there is a linear filter whose bandwidth is inversely proportional to the critical duration in the ear (Fig. 11). 4) Experiment on hearing threshold for complex sounds. The hearing threshold for multitone complex sounds composed of 1 to 24 sinusoidal tones evenly spaced 80 Hz apart were measured for signal durations of 5 msec and 200 msec (Fig. 12). Similar results are obtained for both durations (Fig. 13). These results can be interpreted if the critical band assumed to be independent of the duration. Thus, in experiment on the signal detection of noise (simultaneous masking) and the experiment on frequency discrimination (trill threshold), the critical band appears to widen with decreasing duration. However, in the experiment on the energy summation along the frequency axis (hearing threshold and loudness), the critical band appears to remain constant.