Prolonged low-level noise exposure reduces rat distortion product otoacoustic emissions above a critical level

Prolonged noise exposures presented at low to moderate intensities are often used to investigate neuroplastic changes in the central auditory pathway. A common assumption in many studies is that central auditory changes occur independent of any hearing loss or cochlear dysfunction. Since hearing loss from a long term noise exposure can only occur if the level of the noise exceeds a critical level, prolonged noise exposures that incrementally increase in intensity can be used to determine the critical level for any given species and noise spectrum. Here we used distortion product otoacoustic emissions (DPOAEs) to determine the critical level in male, inbred Sprague-Dawley rats exposed to a 16-20 kHz noise that increased from 45 to 92 dB SPL in 8 dB increments. DPOAE amplitudes were largely unaffected by noise presented at 60 dB SPL and below. However, DPOAEs within and above the frequency band of the exposures declined rapidly at noise intensities presented at 68 dB SPL and above. The largest and most rapid decline in DPOAE amplitude occurred at 30 kHz, nearly an octave above the 16-20 kHz exposure band. The rate of decline in DPOAE amplitude was 0.54 for every 1 dB increase in noise intensity. Using a linear regression calculation, the estimated critical level for 16-20 kHz noise was remarkably low, approximately 60 dB SPL. These results indicate that long duration, 16-20 kHz noise exposures in the 65-70 dB SPL range likely affect the cochlea and central auditory system of male Sprague-Dawley rats.