Subharmonic and Other Low-Frequency Emission from Gas Bubbles in Sound-Irradiated Liquids

Abstract

When a sound field of sufficient intensity is established in a liquid, the acoustic spectrum recorded on a wide-band microphone contains lines at frequencies below the fundamental (f0) as well as the expected components at harmonics of f0 with (sometimes) white noise. This paper discusses the various ways by which these low-frequency resonances can be generated by oscillations of gas bubbles in the liquid. There appear to be three mechanisms: (1) Integral subharmonics of f0 are generated by oscillations of bubbles whose radial resonant frequencies are submultiples of f0. (2) Bubbles of any size may be shock excited to emit at their radial resonant frequencies. It is shown that large bubbles of certain preferred sizes are normally present, and their resonances may account for other observed low-frequency lines that are not integral submultiples of f0. (3) Possibly, surface oscillations of bubbles contribute to the strong response at the first subharmonic (f0/2). Experimental results justifying these conclusions will be reviewed. [Work supported in part by a grant from National Institutes of Health, U. S. Department of Health, Education, and Welfare.]