What Sound Is

The sound that people hear is due to vibrations in air or substances that are transmitted to the hearing organs. What are perceived as sounds are sonic pressure waves

How Sound Is Measured 153

that travel through the air (or different substances) and interact with the eardrum by entering the ear canal or by passing through the body. Thus the eardrum responds to "sound pressure." There is a very, very large change in sound pressure (perhaps as much as 10,000,000 times) as sounds vary from the "threshold of hearing" (the intensity of sound just barely detected by an average human ear) to the sound pressure created by a large jet engine operating nearby. People can "feel" sound when they touch a vibrating body and the vibrations pass through the body to the eardrum.

The sense of hearing also responds to sound frequency (or pitch), that is, the number of vibrations or cycles per second—Hertz (Hz).The range of sound frequencies that can be heard varies from about 20 to 20,000 Hz, depending on the individual. As the intensity of sound is increased to an appropriately high level for a given frequency, the hearing sensation becomes painful, and the "threshold of pain" is reached.

Human hearing also can distinguish between sounds that differ in quality, those combinations of frequencies and intensities that produce squealing, grating, grinding, or rasping sounds. When the frequencies and intensities are combined in suitable proportions, pleasant musical or vocal sounds result. Thus, the sounds that are heard can be quite complex, but their effects on humans are well established.

In this chapter, our concern is with that part of sound called noise and how it may affect workers and their work. Generally, excessive noise can lead to hearing impairment, fatigue, annoyance, and interference with performance. Noise also can serve as a warning of equipment malfunction or a signal of needed maintenance. We frequently rely on sounds (or no sound) to tell us that equipment is performing satisfactorily.

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