ASA 128th Meeting - Austin, Texas - 1994 Nov 28 .. Dec 02

3pEA6. Determination of effective piston area by three methods.

S. L. Garrett

D. F. Gaitan

Phys. Dept. Code PH/Gx, Naval Postgraduate School, Monterey, CA 93943

The volumetric velocity of an acoustic source is sufficient to characterize its source strength if the acoustic impedance of the load presented to the source is known. Although a calibrated accelerometer can accurately determine the velocity of a piston, the volumetric velocity requires a determination of the effective area of the ``piston.'' For a thermoacoustic refrigerator operating at resonance, this load is purely resistive. Three methods for determining the effective area of an electrodynamic thermoacoustic driver will be presented. For in situ measurements, good agreement ((plus or minus)2%--4%) was obtained between direct measurement of the force, using the known Bl product and the resonant reciprocity method using the transfer impedance calculated by Rudnick [G. W. Swift, A. Migliori, S. L. Garrett, and J. C. Wheatley, Rev. Sci. Instrum. 53, 1906--1910 (1982)]. The result for effective area obtained by measurement of pressure under the reactive load presented by a small coupler [T. Hofler, J. Acoust. Soc. Am. 83, 777--786 (1988)] was smaller and more pressure dependent. A possible explanation for this discrepancy, based on bellows deformation under different loading, will be presented. [Work supported by ONR, the Naval Postgraduate School Direct Funded Research Program, and the Navy Science Assistance Program.]