Clint Sprott earned his bachelor's degree from MIT in 1964 and his PhD in physics from the University of Wisconsin - Madison in 1969. His professional interests are in experimental plasma physics and nonlinear dynamics.

Safety Considerations with Sound

Most experiments involving sound are relatively safe except for the hazards not directly associated with the sound as discussed elsewhere. However, it is possible for very intense sounds to permanently damage the ear especially after prolonged exposure[1,2]. Consequently, in the demonstrations described here, the sound intensity should always be kept at a level well below the threshold of pain (see Table 3.1).

As a general rule, prolonged exposure to sound levels above 85 dB will cause slight hearing loss and above 90 dB will result in mild to moderate loss. Prolonged exposure to sound levels above 95 dB causes moderate to severe hearing loss. Above 100 dB, even short exposure can cause a permanent loss of hearing.

The ear has an acoustic reflex that protects the inner ear from loud sounds in the same way that the pupil of the eye protects the eye by contracting in the presence of bright lights. However, the reflex requires a few hundredths of a second to respond, and thus cannot be relied upon to protect the ear in the event of a short duration sound such as an explosion.

The damage is typically by way of tearing or ripping the microscopic hair cells of the cochlea. Such damage is usually only temporary unless the sound is frequent or sustained. Especially intense sounds are capable of rupturing the eardrum. The damage threshold is frequency-dependent, and the ear is most susceptible to damage by sounds of around 3000 Hertz in part because the auditory canal is a closed tube having a resonance in this region. Prolonged exposure to sounds of a particular range of frequencies can permanently reduce the sensitivity of the ear to those frequencies.

The monitoring of sound levels requires an A-scale sound level meter with a frequency response matched to the response of the ear or one capable of displaying the sound level for all of the various frequency ranges[3]. Protection can be provided by ear plugs and/or ear muffs. Cotton in the ears is not an effective protection unless the cotton is wax-impregnated.

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REFERENCES 1. K. D. Kryter, W. D. Wood, J. D. Miller and D. H. Eldredge, Journ. Acoustical Soc. Am. 39, 451 (1966).

2. E. A. Lacy, Handbook of Electronic Safety Procedures, Prentice- Hall: Englewood Cliffs, New Jersey (1977).

3. A. P. G. Peterson and E. E. Gross, Jr., Handbook of Noise Measurement, General Radio: Concord, Massachusetts (1963).

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