BUBBLE NOISE SPECTRA

When bubbles are formed by the enclosure of pockets of air, they must first undergo extreme distortions before settling down into a spherical shape. In two recent papers [M. S. Longuet-Higgins, J. Fluid Mech. 201, 525–541 (1989); 201, 543–565 (1989) ] it has been shown that the shape oscillations of bubbles will, at second order, contain radially symmetric “monopole” terms, having double the fundamental frequency. These terms can resonate with the radial, “breathing” mode to produce significant levels of sound in the ocean. In the present paper it is shown that any broad spectrum of bubble-generated sound may be expected to show peaks at the corresponding resonance frequencies. A first examination of the available oceanic data does indeed show evidence of such resonances, particularly at the even-numbered mode frequencies. The same mechanism may also help to explain certain resonances observed in laboratory experiments by Medwin and Beaky [J. Acoust. Soc. Am. 86, 1124–1130 (1989) ]. © 1990, Acoustical Society of America. All rights reserved.