Field measurements are measurements where little or no control over operating conditions is possible. Laboratory measurements are made in circumstances where some control is possible and conditions can at least be predicted. Usually less sophisticated equipment is used for field work; consequently, the experience and judgment of the analyst become important.
Background noise is normally present, and often it is not possible to maintain conditions while a machine under test is started and stopped. The effect of background noise should be minimized. Stationary noise sources, if small, can be approached to within a meter or so to raise the level of noise being measured relative to the background. (Questions of directivity, size of source, reflections and interferences, and other experimental problems always appear; such problems must be dealt with individually at the time they appear.) For large sources, such handling of experimental problems is seldom possible; where the effect of an entire building or a large cooling tower on a neighborhood, for example, is being studied, the ingenuity and experience of the analyst is taxed to the utmost.
Transient noises—auto horns, vehicle noises, passersby, and the like—must be avoided as far as possible. Measurements within buildings are easier in that traffic is controllable and atmospheric noises do not interfere, but the proximity of the walls introduces danger of reflections in addition to those from the analyst and his equipment.
Both the abilities and limitations of the measuring
Preamplifier ZE 0132
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