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Sound power levels are established through sound pressure measurements; in a free field, sound is radiated spherically from a point source, thus

For precise work, barometric corrections are required. In practical situations, a directivity factor must often be introduced. For example, if a machine rests on a reflecting surface (instead of being suspended in free space), reflection confines the radiated sound to a hemisphere instead of a spherical pattern, with resulting SPL readings higher than for free-field conditions.

Actual sound power values of a source, in watts, can be computed from PWL values using Equation 6.1(8).

In all cases, the units should be stated when sound pressure or sound power values are listed (dynes/cm2, watts), and the reference levels should be made known when

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