THE EFFECT OF CONDITIONING EXPOSURES ON HEARING-LOSS FROM IMPULSE NOISE

Research has shown that prior noise exposures or 'conditioning' can mo derate the amount of permanent threshold shift (PTS) from subsequent h igh intensity noise exposures. The aim of this experiment was to study the effect of 'conditioning' on subsequent exposure to high intensity impulse noise. The subjects were seven experimental and 14 control mo naural chinchillas. Evoked potential (EVP) thresholds were measured be fore and after the noise exposures. Experimental animals received 10 d ays of exposure to an octave band noise (OBN) centered at 0.5 kHz OBN at 95 dB SPL (6 h on/18 h off) and allowed to recover for 5 days. The subjects were then exposed to an impulse noise at 150 dB SPL. The temp oral spacing of the impulses consisted of a series of 50 pairs of impu lses presented 50 ms apart with 1000 ms between the onset of each pair . The total duration of exposure was approximately 1 min. Control anim als received only the impulse noise exposure. PTS was measured after 4 weeks. The threshold shift (TS) patterns during the 'conditioning' ph ase were consistent with previous research, with the greatest amount o f TS occurring on the second day and decreasing with continued exposur es. Four weeks after recovery from the impulse noise, the experimental animals showed significantly less PTS than the control animals. In ad dition, histological examination revealed significantly less hair cell loss in the experimental than in the control subjects. The results ar e discussed in the context of previous studies on 'toughening' and on the effects of impulse noise.