Sources of Sound at the Ocean Surface; Bubbles and Other Noise

Abstract

The source processes generating noise at the interface of two fluids with different speeds, densities, temperature and speed of sound have been the central theme of aero-acoustic studies since the noise of jet engines first became a problem. The theoretical foundation of that study is Lighthill’s classic (1951) work ‘Noise generated aerodynamically’, a theory so elegant and persuasive that it tended to bias experimenters to measure and report only those parts of the subject perceived to be in accordance with that theory. A frequency doubling of the sound over the velocity field of the source, the scaling of sound power on the eighth power of source speed and an amplifying effect of velocity shear and speed of sound variability were verified under rather special well defined circumstances. Real sound-generating flows are much richer in structure than that conforming with this crude characterisation; it has taken some thirty years to see clearly the acoustic consequences of density contrast for example. That work can guide now the modelling of the sea surface source region and it leads to some surprising new views on the subject.