Abstract:
The ``toughening'' effect in chinchillas exposed to interrupted impact noises was explored as the center frequency (CF) of a narrow-band (400 Hz), 115-dB peak SPL impact exposure was varied. CFs were: 0.5, 1.0, 2.0, 4.0, and 8.0 kHz; impacts were presented 1/s, 24 h/day, for 5 days or 6 h/day, for 20 days. Each pair of exposures had equal energy. Results: Initial threshold shifts (TS) for both the 5- and 20-day groups were similar; 40 to 50 dB at the most affected audiometric test frequency. ``Toughening'' in the interrupted exposures amounted to at most 30 dB. The 1-kHz impact was most effective in producing ``toughening'' at all audiometric test frequencies that showed a TS. The maximum effect was seen at 4 kHz; two octaves above the impact CF. For impact CFs >1.0 kHz the ``toughening'' effect, when it was measured, was small (<10 dB) and most audiometric test frequencies showed no ``toughening.'' When PTS and sensory cell losses were compared across the 5- and 20-day exposures, at all impact CFs, there were no statistically significant differences. The TS toughened cochlea is not necessarily a protected cochlea. [Research supported by NIOSH Grant No. R01 OH02317.]