Impact noise: An evaluation of the equal energy hypothesis

Abstract

Four groups of chinchillas were exposed to impact noise. All groups received the same total energy of exposure; however, the intensity and total duration of the exposure was varied. The impact was 200 ms in duration and presented at the rate of 4 per sec at intensities of 107, 113, 119, and 125 dB peak sound pressure level. The exposure durations varied from 5 days to 1.9 h. Evoked response thresholds were obtained before and after exposure at test frequencies between 500 and 16 000 Hz. Thirty days after exposure final audiograms were obtained and the animals were sacrificed for surface preparation histology. There was a systematic increase in the maximum amount of TTS as a function of impact noise level particularly at the lowest test frequency. The three lowest levels produced relatively little PTS while the highest level produced as much a 40 dB PTS at 2 kHz. Average inner and outer hair losses were calculated over octave intervals along the cochlea for each group of animals. In general, there was an increase in the amount of hair cell loss as a function of level, particularly in the 4-kHz octave band region of the cochlea. On the basis of the limited range of parameters studied, both the threshold shift and hair cell loss fail to support the equal energy hypothesis. [Work supported by NIOSH 1-RO1-OH-00364.]