Oto-traumatic effects of computer simulated magnetic coil impulse noise: analysis of mechanisms

The brief impulse noise artifacts of 1.0 ms or less generated by some magnetic coils used in extracranial magnetic stimulation may induce acoustic trauma. We investigated the effects of these magnetic coil acoustic artifacts (MCAA) on the inner ear by exposing rabbits to computer stimulated impulse noise designed to mimic the impulse noise of the coil in spectrum and acoustic energy. The simulated impulse noise stimuli (50 impulses) were varied in maximum peak sound pressure (160, 157, and 155 dB re: 20 muPascal), rise-time (100 microseconds and 1,000 microseconds) and duration. The frequency spectrum of the simulated impulse noises were kept constant at 0.5 kHz to 7 kHz with peak energy in the 2-5 kHz range. The results indicated that the simulated magnetic coil impulse noise caused extensive cochlear damage and permanent threshold shifts largely equal to those induced by the MCAA. The MCAA created slightly greater PTS than the simulated impulse of the same peak sound pressure. Each of the 3 experimental stimuli induced similar PTS in the auditory range of 0.5 to 16 kHz, with the higher peak sound pressure stimuli (157 and 160 dB) causing greatest hearing loss. Increasing the rise-time of the simulated brief impulse noise from 100 microseconds to 1,000 microseconds did not reduce the level of PTS significantly. The results suggest that for brief acoustic signals of around 1 ms or less, the peak pressure and spectral content rather than the rise-time and duration were the important factors in the development of noise-induced hearing loss.(ABSTRACT TRUNCATED AT 250 WORDS)