Long-term effects of simulated sonic booms on hearing in rhesus monkeys

Abstract

Two monkeys of the species Macaca mulatta were exposed at 1 min intervals to five simulated sonic booms lasting 200 ms at 200 Pa overpressure with a 10 ms rise time. Another group of five monkeys of the same species were exposed to 100 booms. Their hearing thresholds were tested 24 hours, two weeks, one month, two months, four months and six months later. In one animal exposed to five booms, changes of the hearing thresholds were observed 24 hours following the exposure, but not later. All five animals exposed to 100 sonic booms had threshold shifts in the high-frequency range 24 hours following the exposure. Of the three animals followed for the full period of six months, one recovered completely. In the two others, threshold shifts were still observed in the high frequency range. Histological examination revealed destruction of the organ of Corti in the basal turn of the cochlea. These data indicate that there is individual variability in the extent of the damage to the inner ear by the sonic boom (and, perhaps, by other types of impulsive noise). These data also indicate that there is a possibility of similar damage to human inner ears exposed either to sonic booms or to other types of impulsive noise, and that it may go undetected for a long time because the high-frequency hearing defect, over 8 kHz, may be overlooked when routine audiometric methods are used.