Banging into a noisy background.

The slams of a hammer heard against a noisy background may, over a long period of time, cause a greater hearing loss than the same steady banging heard in an otherwise quiet room, according to the results of recent experiments with animals. Donald Henderson and Roger P. Hamernik of the Callier Center at the University of Texas in Dallas show that in chinchillas, certain combinations of individually "safe" impulse noises and continuous noises produce a synergistic interaction, especiall when the frequencies of the two sounds overlap.

Because chinchillas provided a good model for human being, these experiments indicated a potential problem, particularly for workers in places like textile milles and iron foundries, say the researchers. The worst situations involve high-level impulse noise riding on background sounds close to the 90-decibel limit set by the Occupational Health and Safety Administration (SN:5/22/82, p. 347). "That's a deceptive situation," says Hamernik, "because the impulses are almost obscured by the background noise. They're glossed over."

In industrial and military settings, the combination of background noise and impulse noise is much more common than either by itself. However, no regulations govern this combination, partly because too little is known about its effect on human hearing. "The idea," says Hamernik, "is to generate some data so that you can identify whether there is a problem and under what conditions you have a problem."