Studies on TTS due to Exposure to Octave-Band Noise.

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  • Shoji Hikaru
    Department of Sanitary Engineering, Faculty of Engineering, Kyoto University
  • Yamamoto Takeo
    Department of Sanitary Engineering, Faculty of Engineering, Kyoto University
  • Takagi Koichi
    Department of Sanitary Engineering, Faculty of Engineering, Kyoto University

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Other Title
  • オクターブバンド騒音によるTTSの研究
  • オクターブバンド ソウオン ニ ヨル TTS ノ ケンキュウ

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Abstract

Temporary threshold shift (TTS) at 1, 2, 3 and 4 kc following exposure to four types of octave-bands (250-500cps, 500-1000cps, 1000-2000cps, 2000-4000cps) obtained by passing white noise through filters was measured with five subjects with normal hearing acuity. Table 1 gives the sound pressure levels of octave-band noises used in this experiment. Durations of exposure ranged from 5 to 155 minutes, and measurements of TTS were made within 3 minutes after cessation of exposure. (Fig. 3) Variations of pre-exposure thresholds of the five subjects are shown in Table. 2. All experiments were carried out in an anechoic soundproof room. (Fig. 1) Fig. 4 shows the TTS produced by four different octave-band noises at a level of 95dB. Among the four test frequencies (1, 2, 3 and 4kc), maximum effect was found at 3kc for 1000-2000cps octave-band, and at 4kc for 2000-4000cps octave-band. As regards the bands 250-500cps and 500-1000cps, TTS at 1, 2, 3 and 4kc decreased in that order. The relations of TTS to exposure time and sound pressure level are shown in Fig's 5 and 6. As Ward et al. had already pointed out, TTS increased linearly with the sound pressure level of exposure noise and the logarithm of exposure time. These relastion are expressed as TTS=a(S+b)log_<10>T+cS+d where S denotes the sound pressure level of each octave-band noise, T is exposure time in minutes, and a, b, c and d are constants that depend on octave-band and test frequency. These constants were calculated by the least square method. The empirical formulas obtained are shown in the text. (from (1) to (8)) Fig. 7 indicated the correspondence of the original data to a regression equation for TTS at 4kc following exposure to 2000-4000cps noise. Comparing the equations of Ward et al. 's with those of authors', TTS at 4kc due to 2000-4000cps noise was larger than that due to 2400-4800cps noise, (Ward et al. ) and TTS at 3kc due to 1000-2000cps noise were smaller than that due to 1200-2400cps noise (Ward et al. ). (Fig. 8) Fig. 9 shows the sound pressure level of octave-band noise which give, for 3 minutes exposure, TTS of 5dB at frequency half an octave above the center frequency of the octave band noise. Broken line represents the results obtained by Plomp et al. , and solidline is from the present equations. They are in good agreement.

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