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.