Reference sound source calibration at various temperatures and site altitudes

Abstract

Adjustments are made for atmospheric temperature and barometric pressure when calibrating a sound power reference sound source (RSS) and when applying an RSS in the substitution method to evaluate an unknown sound power. Experience with Hemianechoic calibration of a dozen RSS units outdoors at various temperatures near sea level and a special test series on one RSS unit at three higher sites up to 9927 feet and applying a pistonphone reference to verify microphone and sound-level meter stability are described. According to these varied temperature results and the theoretical work of others [Allan Powell, J. Acoust. Soc. Am. 32, 982–990 (1960), Eqs. (41)–(42), after Lighthill], it appears that the mechanical fan-type RSS is a multipole—chiefly quadrupole and octupole—type sound source. It was apparent, according to these varied altitude results, that emitted sound pressure and sound power are both linear functions of barometric pressure. Experimental results will be presented. Practical applications to determine actual sound power, to report normalized sound power, and to predict actual sound pressure and sound power expected at a remote site under nonstandard conditions are described.