Conditioning the auditory system with continuous vs. interrupted noise of equal acoustic energy: is either exposure more protective?

The purpose of this study was to test the hypothesis that differences exist in the amount of protection provided by prior sound conditioning with continuous vs. interrupted, moderate-level noise. Differences were determined by monitoring the changes that occurred in cubic (2f1-f2) distortion product otoacoustic emission (DPOAE) amplitude growth functions subsequent to a traumatizing noise exposure (105 dB SPL, 1.0-2.0 kHz octave band noise presented 24 h per day for 3 days) in guinea pigs which had been conditioned with either continuous (89 dB SPL, 1.0-2.0 kHz octave band noise presented 24 h per day for 11 days) or interrupted noise (95 dB SPL, 1.0-2.0 kHz octave band noise presented on a 6-h 'on'/18-h 'off' schedule for 11 days) of equal acoustic energy. Results suggest that there are significant differences in the degree of protection provided by prior sound conditioning with the continuous and interrupted schedules of moderate-level noise used in this study. Specifically, the interrupted conditioning protocol afforded some degree of protection against the damaging effects of the traumatizing noise exposure, limited to frequencies above the noise exposure band. Conversely, there was a lack of any consistent and sizable protective effect found across the entire test frequency range for the continuous sound conditioning protocol.