An Experimental Study of Human Auditory Effect of Noise

Author: Lao-teh Wang(Department of Health Education College of Education)

Abstract：

An experimental study on the noise-induced hearing loss or noise-induced temporary threshold shift (NITTS) of the normal hearing college students (30 male and 30 female) was performed with three kinds of narrow band noise, including 250Hz, 1 KHz and 4KHz octave bands, and six different sound pressure levels of broad band noise, which were used white noise at sound pressure levels of 65, 70, 75, 80, 85 and 90 dB as stimulating tones.
The mean age of this study subjects was 21.28 years in male group and 21.8 years in female group. First of all, all subject were measured with their hearing levels by using Rion AA39 audiometer, and then the persons with less 15 dB in average hearing level of speech range (500Hz, 1KHz, and 2KHz) were selected as the normal hearing subjects. The actual mean values were 9.55 dB in male group and 9.83 dB in female group.
Two minutes after exposure to each stimulating noise ceased, the subject was retested with the hearing levels on 4KHz and 6KHz, and then compared with that of preexposure to the stimulating noise. If the difference of value between pre-and post-exposure hearing level on 4KHz or 6KHz shows 5 dB or greater, it is defined as a positive response of NITTS.
The inciaence of positive TTS2 shows 65% among 60 subjects after 50 seconds of exposure to the broad band noise; and 3.3%, 16.7%, 30% after 50 seconds of exposure at 70 dB SPL to the 250Hz, 1KHz, 4KHz octave-band noises respectively.
According to Reed-Muench bioassay method, LD50 value shows 80.99 dB SPL for the white noise exposure, In other words, 50% of subjects in this study group reveals positive TTS2 after exposure for 50 seconds to white noise at 80.99 dB sound pressure level.
So the 81 dB SPL of the white noise may be referred to as the risk-criterion limit of "Noise Control Administration in Taiwan" for the broad band noise. If the environmental noise contains the 4KHz or higher pure-tone components and/or high octave bands, the risk-criterion limit must be decreased reasonably for the purpose of preventing noise-induced permanent threshold shift (NIPTS).

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