B Ahroon | PubMed Health

PubMed1995

[Impulsive noise: PTS and anatomic correlations].

The EEH (Equal Energy Hypothesis) postulates that permanent hearing loss (PTS) produced by exposure to noise is a function of sound energy of the exposure. The validity of EEH for continuous noise exposure was confirmed by large scale demographic studies and experiments in controlled laboratory setting. However, for impulse noise, EEH may not be as valid. This study was designed to test the applicability of EEH in impulse noise exposure in chinchillas. 9 groups of chinchillas were exposed to impulse noise. The intensity of the impulses (113, 125 and 137 dB SPL) and the rate (900, 3,600 and 14,400 impulses/hour) of the 9 exposure conditions were counterbalanced so that the 9 groups received the same total energy. PTS was obtained at 0.5, 2. and 8.0 kHz, by recording the evoked potentials from a chronic electrode implanted in the inferior colliculus. The cochleas were dissected and evaluated using conventional histological surface preparations and examined through scanning electron microscopy (SEM) employing a JEOL 35 microscopy. In our experimental conditions only the 113 dB SPL exposures were consistent with EEH (PTS less than 20 dB). The 125 dB SPL peak level is a critical value in that the validity of EEH also depends on exposure duration. As duration increases, so does PTS. At a 137 dB SPL peak level, EEH is not applicable. The amount of PTS is higher than 50 dB. It slightly increases when the impulse rate per hour becomes higher. In all cases examined PTS is less at 0.5 kHz than at 2.0 or 8 kHz. In these animals excellent informations concerning the sensory cell losses and cells damage was obtained with SEM.

Animals↗

PubMedDate not supplied

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

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.

Animals↗