Abstract
Much interest has centered in recent years in measurements of the noise of large machines as installed. Free‐field conditions above a reflecting plane are desired, and a means for evaluating conditions in situ are a necessary part of the measurement process. A current trend in drafting ISO and ANSI standards is to employ the method of enveloping surfaces. [G. Hubner, “Qualification procedures for free‐field conditions for sound power determination of sound sources,” in Internoise 73 Proceedings (Copenhagen, August 1973).] It has been found that spaces which fail to qualify by this method often pass a criterion like that of American National Standard ANSI S1.13‐1971, which requires only that the sound pressure level measured at one or more points in a direction away from the machine be at least 6 dB less than that at 1 m. It is argued that the latter criterion insures adequate accuracy for engineering measurements, and that the former one requires an unnecessary number of measurements.
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