Physical damage from acoustic energy is not a new phenomenon. The first recorded event is in Biblical history when Joshua lead seven priests around Jericho seven times blowing trumpets and the people shouted and the walls fell. Ancient mortar and gun battery crews put cotton in their ears to lessen the sound, and this was a type of noise control. Since World War II much concern has been exhibited about hearing loss due to high noise levels in work environments, In order to limit the hearing loss of the average person, the Occupational Safety and Health Act of 1970 specifies the noise exposure of an employee for an eight-hour day shall not exceed 90 dBA. This has resulted in a massive noise control effort in industry.
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