Difference frequency generation by forced-air bubbles

Abstract

An increase of 30 dB in the sound-pressure level at a 10-kHz difference frequency, measured at a range of 1.83 m, was obtained by introducing bubbles into the region of interaction of a parametric projector. The bubbles, produced by forcing air through a microporous filter, formed a thin screen that was situated perpendicular to the acoustic axis of the source transducer. The primary frequencies were 45 and 55 kHz. The difference frequency beam pattern was measured and appeared to be governed by the broadside directivity of the insonified region of the bubble screen. A theoretical model was derived that demonstrates the effects of bubble size and primary frequency on the difference frequency level. It was shown that the maximum difference frequency level is obtained from bubbles resonant at the primary frequencies. The results of this investigation are consistent with those of Clynch and Rolleigh [J. Acoust. Soc. Am. 55, S51(A) (1974)], who reported difference frequency level enhancement due to a small volume of bubbles produced by cavitation. [This work was supported by ARPA and the Naval Sea Systems Command.]