Vibration and Vibration Measurement

Noise is usually accompanied by vibration; noise is caused by vibration; noise causes vibration. The physiological effects of vibration have not been studied as intensively as those of sound, but usually discomfort provides warning so that hazard is easily anticipated. However, exposure to vibration often contributes to fatigue and, thus, to loss in efficiency and to accidents. Vibration study is an important part of noise control, especially in analyzing problems.

FIG. 6.4.3 Noise criteria curves. The abscissa shows both the older bandlimit frequencies (at top) and the band-center frequencies based on the preferred-number series (at bottom). Both are still in use.

Roughly speaking, vibration becomes perceptible when its amplitude reaches about 2 X 10-3 millimeters at 50 Hz and seems intolerable at about 8 X 10-2 millimeters at 50 Hz. These estimates assume exposure for rather long periods and close contact to the vibrating surface—as riding in a vehicle. Vibration of large surfaces at these amplitudes produces high sound pressures. Individuals vary in their tolerance of vibration, but probably few people are disturbed by vibrations with accelerations less than 0.001 times that of gravity, or 1 cm/sec2, and many can tolerate ten times that much.

Vibration can be described in terms of its frequency and either its acceleration, velocity, or amplitude as a = -2f = -(2^f)2x 6.4(4)

where a is acceleration, v velocity, x is amplitude of displacement, and f is frequency.

Vibration is measured by mechanical, optical, or electrical means (Liptak 1970) depending on conditions. The low-frequency, large-amplitude vibrations of an elastically suspended machine might be measured with a ruler, by eye. The high-frequency, small-amplitude vibrations of a high-speed motor are best measured with an electrical instrument which is sensitive to acceleration, velocity, or displacement.

General purpose vibration meters often use a measuring element sensitive to acceleration over a range of frequencies. The instrument also shows velocity by performing one integration or displacement by performing two integrations on the acceleration signal. The integration is done electronically, within the instrument.

Vibratory force applied to an elastic membrane produces vibratory motion, thus sound pressure waves cause a window or partition to vibrate. Vibratory force applied to an elastically supported mass produces vibratory motion, as in a machine with vibration-isolating supports. In both cases, the amplitude of vibration is affected by the ratio of the frequency of the applied force—the forcing fre-

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