[The frequency spectrum in the interaction between continuous and impulse noise: an anatomic functional evaluation].

Abstract

In industrial setting impulse noise rarely occurs in isolation. Impulses are more often superimposed on a background of continuous noise. Since measurement of peak sound pressure level or energy considerations may not provide a sufficient index of danger to hearing, examining the frequency spectra of the noises appeared to be important. Seven groups of five chinchillas were used in this study. Four groups were exposed to octave bands of noise centered at 0.5, 2 or 4 kHz having intensities of 95, 90 and 86 dB SPL respectively or impulse noise of 113 dB peak SPL applied on time per 1 second. Three groups were exposed to the combination of impulse noise and one of the above continuous noises. Each exposure lasted five days. PTS was calculated on each animal using auditory evoked potentials. The cochleas were dissected and evaluated using conventional histological surface preparation and examined with scanning electron microscopy (SEM) employing a JEOL 35 microscope. The groups exposed to only the impulse or to continuous noise alone developed neither PTS nor hair cell loss. Considerable values of PTS and sensory cell loss were observed in animals exposed to impulse noise in combination with 2 and 4 kHz of continuous noise. This interaction effect increases as the spectral overlapping between the impulse and continuous noise increases. In these animals excellent information concerning the sensory cell losses or cell damage was obtained with SEM. The results of the present study demonstrate that interaction between impulse and continuous noise can indicate increased hazard, which is highly dependent not only upon the total energy of the complex exposure but also on the degree of spectral overlapping between impulse and continuous noise.