Waveguide propagation of ambient sound in the ocean-surface bubble layer

Measurements of the ambient sound generated by breaking waves over the range 40–20 000 Hz reveal well‐defined spectral peaks, the frequency of which may remain generally consistent from one breaking event to the next, but which can change significantly over the course of a storm, or from one storm to another. A theory is proposed, based on the concept of trapping of a portion of the sound in the waveguide formed by the ocean‐surface bubble layer. Simultaneous measurements of the bubble population and size distribution as a function of depth and time were obtained with a multifrequency inverted echo sounder, allowing calculation of the resulting (dispersive) sound‐speed anomaly profile. Theoretical predictions of the spectral peaks, which are associated with modal cutoff frequencies, are in good agreement with the observations. It is suggested that this result might have application to the remote determination of ocean‐surface bubble fields relevant to the study of wave breaking, turbulence, and the air–se...