General Radio Company

30 STATE STREET - CAMBRIDGE A, MASSACHUSETTS

BRANCH ENGINEERING OFFICES 90 WEST STREET, NEW YORK CITY 1000 NORTH SEWARD STREET, LOS ANGELES, CALIFORNIA

at very high frequencies when the amplitude of vibration is sufficient.

Sound, as generally encountered, is a vibration propagated through the air, but it is also transmitted through various other media and generally originates as an actual mechanical vibration in or of some solid structure. To trace noise to its source, therefore, through the many materials or mechanisms that may transmit it, some means of measuring vibration is desirable.

For some years sound and noise measurements have been increasing in importance to all branches of industry. A wide variety of mechanical devices, from clocks to automobiles, are checked with sound-level meters. The same type of meter is used for measurements on sound-absorbing and sound-insulating mate rials. Analyzers and various accessories have widened the scope of usefulness of this instrument, until today there are few engineering jobs which can be undertaken without some sort of sound-level rne a s ure ments.

To adapt the sound-level meter to vibration measurement, a vibration pickup, replacing the microphone, can be used. Essentially, a vibration pickup is similar to a microphone, excepting that the pickup is designed to respond to solid-borne rather than air-borne vibrations. Hence, if the pickup is held in a sound field, the output, if any, will be relatively small compared to that of a microphone, but if it is held against some vibrating mechanical part, the output will be relatively high. This is essentially a matter of mechanical impedance and coupling.

The sound-level meter as a vibration-

Figure I. View of the Type 761-A Vibration Meter with cover removed.

Figure I. View of the Type 761-A Vibration Meter with cover removed.

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