<p>In order to investigate whether the critical band concept can be applied to the problem of temporary threshold shift (TTS) , three experiments (I, II, III) were carried out using five subjects with normal hearing acuity. In experiment I, thirteen high pass and thirteen low pass noises obtained by filtering white noise were used. The cut-off frequency of these noises are shown in Fig. 1. They were at intervals of 1/6 octave. The over-all SPL of white noise was 95dB. Durations of exposure were 5, 15, 35 and 55 minutes, and post-exposure threshold measurements at 3, 4, 6 and 8kc were made whithin 3 minutes after cessation of exposure. Fig. 2 shows the results of experiment. TTS due to low pass and high pass noises increased to a certin value as the bandwidth became larger, but when it reached to this limiting value, it remained constant regardless of the bandwidth of exposure noise. It may be concluded from this fact that only those components of the noise which are included in limited frequency regions are effective and that the components beyond this regions are ineffective in TTS. This is in agreement with the basic notion of the critical band. In experiment III, twelve exposure noises having linear spectrum were used (Table 1). The spectra of these noises are given in Fig. 3. TTS at nine frequencies from 0. 5kc to 8kc were measured within about 6 minutes after cessation of 20 minutes' exposure. Fig. 4 shows the TTS due to exposure to these noises at a level of 100dB. As a whole, 0dB/oct noise was most effective and -6dB/oct noise least effective. But TTS at frequencies below 2kc were not noticeable in all cases. In experiment III, four 1/6 octave-band noises (2240-2500cps, 2800-3150cps, 4500-5000cps, 5600-6300cps) whose spectrum level are equal to that of 0dB/oct noise at 100dB were used. Test frequencies and exposure time were the same as in experiment II. Fig. 5 indicates the results of this experiment. The maximum effects were found at 3, 4, 6 and 8kc respectively for the exposure noise 2240-2500cps, 2800-3150cps, 4500-5000cps, and 5600-6300cps. Using the data obtained from experiment II and III, the center frequency and width of the critical band were estimated by the following method. 1) Estimation of the center frequency of the critical band. The basic assumption is that TTS at frequency F is expressed as TTS_F=aX+b. . . . . . . . . . (1) a, b: Constants that depend on exposure time, test frequency, and the time when TTS is measured. X: Critical band level and is expressed as X=S(F_c)+10log_<10>&lrtri;f. . . . . . . . . . (2) S(f_c): Spectrum level at the center frequency of the critical band f_c: Center frequency of the critical band &lrtri;f: Critical bandwidth When the spectrum of noise is a linear function of log_2f, S(fc)=αlog_2f_c+β. . . . . . . . . . (3) α: Spectrum slope (dB/oct) β: intercept (dB) From Equations (1), (2) and (3), TTS_F=a(αlog_2f_c+β-L). . . . . . . . . . (4) where L≡-(10log_<10>&lrtri;f+b/a) Equation (4) means that TTS becomes a linear function of the spectrum level at the center frequency of the critical band. Using the data of experiment II, the value of a, f_c, and L were calculated for 3, 4, 6 and 8kc by the following least squre method. &lrtri;=Σ{y_i-a(α_ilog_2f_c+β_i-L)}^2 ∂&lrtri;/(∂a)=0, ∂&lrtri;/(∂f_c)=0, ∂&lrtri;/(∂L)=0 where y_i is TTS produced by noise whose spectrum is α_ilog_2f+β_i. The results are shown in Fig. 6. From these figures, it is noticed that TTS is expressed as a linear function of spectrum level at the center frequency of the critical band. Center frequencies are about one-third to two-third octave below test frequencies. 2) Estimation of the critical bandwidth. Let the TTS at certin frequency produced by wide-band noise (I) in Fig. 7 be Y, and the TTS by narrow-band noise (II) whose cut-off frequencies are included in the critical band be y, then</p><p>(View PDF for the rest of the abstract.)</p>