The report describes investigations of the accuracy and precision of various measurement methodologies for determining the estimated sound power output of 'large' machines in the free field over a reflecting plane. One purpose of this investigation is to place empirical error bounds on many of the free field measurement procedures currently proposed or in use; and in particular, compare the results of near-field and far-field measurements. The sources used for the investigation included 17 portable air compressors of various types (powered by internal combustion engines), a reference sound source, and a loudspeaker driven by a pure tone source. The data recorded include sound pressure level (A-weighted, linear, and 1/3-octave band) on an 84 point hemispherical array of seven metre radius, and near-field measurements, sampled every square metre, on a rectangular surface one metre from the machine surface. These data were reduced to provide information on the deviation of near-field sound power determinations from far-field power level (using subsets of the data as appropriate to various methodologies).

"COM-75-11399." "EPA 550-8-76-001." "Final report." "July 1975." Cover title. Prepared for Office of Noise Abatement and Control, U.S. Environmental Protection Agency, Washington, D.C. 20460. Includes bibliographical references.