Audiometrically normal, male students (9-11 persons) were exposed to impact noise from an electronic switch (4pulses/sec, 1pulse/sec and 1pulse/4sec) for 40 minutes, and the TTS growths were investigated.
Sound level of impact noise was 125dB (B duration Ca 5msec and Ca 25msec) and 120dB (B durations Ca 100msec). The following results were obtained:
1) TTS growth of exposure conditions, 1pulse/sec (Test frequency of 4kHz) was significantly greater than that of 1pulse/4sec in either different B durations (Ca 5msec and 25msec), however TTS growth at the exposure condition of 4pulse/sec was not significantly greater than that at 1pulse/sec in either different B durations (Ca 5msec and Ca 25msec).
2) Ninety-five % of the confidential limits of TTS growths at different exposure conditions were calculated using the data of experiments included in this paper and the critical level which corresponded approx. to the CHABA limit was detected: the TTS growth after the 2400 times' exposure to impact noise of 1pulse/sec and 5msec of B duration and the TTS growth after the 1200 times' exposure to impact noise of 1pulse/sec and 25msec of B duration.
3) Impact noise with B duration 5msec and 25msec in those experiments needs correction, when the maximum allowable noise level is determined. When such corrections are made utilizing data in this paper, the results concering the allowable level are in good agreement with those proposed by Coles.
