Practical considerations for the design and construction of an acoustic dynamometer

Abstract

An instrumented, variable acoustic load of the type described by Fusco [Fusco et al., J. Acoust. Soc. Am. 91(4), 2229–2235 (1992)] and Gardner [D. L. Gardner and G. W. Swift, Cryogenics 37(2), 117–121 (1997)] has been implemented for the testing of high power (greater than 1.5 kW) electrodynamic loudspeakers for thermoacoustic refrigerator applications. The load serves to facilitate performance measurements for these loudspeakers under various operating conditions, a role analogous to a dynamometer in conventional rotating machinery. Attached to a primary acoustic resonator, the acoustic dynamometer consists of a 2-in angle globe valve and other flow constrictions connected to an acoustic compliance. The design of these flow restrictions employed the acoustic ‘‘minor loss’’ model developed by Swift et al. [Swift et al., J. Acoust. Soc. Am. 105, 711–723 (1999)]. Measurements of the dynamometer performance agreed with the calculated acoustic impedance to within 16%. At powers exceeding 1 kW, agreement between the acoustic load dissipation and other measurements of loudspeaker power agree to within 3%. To illustrate the utility of an acoustic dynamometer, a brief summary of measurements on two moving-magnet loudspeakers will be presented. [Work supported by ONR.]