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 ac oustic trauma. We investigated the effects of these magnetic coil acou stic artifacts (MCAA) on the inner ear by exposing rabbits to computer simulated impulse noise designed to mimic the impulse noise of the co il in spectrum and acoustic energy. The simulated impulse noise stimul i (50 impulses) were varied in maximum peak sound pressure (160, 157, and 155 dB re: 20 muPascal), rise-time (100 mus and 1,000 mus) and dur ation. The frequency spectrum of the simulated impulse noises were kep t 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 c aused 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 rang e of 0.5 to 16 kHz, with the higher peak sound pressure stimuli ( 1 57 and 160 dB) causing greatest hearing loss. Increasing the rise-time o f the simulated brief impulse noise from 100 mus to 1,000 mus did not reduce the level of PTS significantly. The results suggest that for br ief acoustic signals of around 1 ms or less, the peak pressure and spe ctral content rather than the rise-time and duration were the importan t factors in the development of noise-induced hearing loss. A critical rise-time may exist, but is beyond the temporal range of this investi gation. We also compared the PTS resulting from the simulated impulse noises with that caused by a continuous noise exposure of 128 dB SPL f or 15 min. Although the continuous noise exposure had 100 times greate r acoustic energy, it caused considerably less PTS than the magnetic c oil and simulated impulse noises, further supporting the significance of peak pressure of impulse noise in inducing permanent threshold shif ts.