Hot spots and supermodes: The theory of ambient sound in the ocean‐surface bubble layer

Abstract

An elegant series of experiments was conducted recently by Farmer and Vagle [J. Acoust. Soc. Am. 86, 1897–1908 (1989)], in which the sound from individual wave‐breaking events was recorded by a hydrophone located a few meters beneath the ocean surface. Between 1 and 20 kHz, the observed spectra from La Perouse Bank contain relatively sharp, isolated peaks (hot spots) at intervals of approximately 3 kHz, whereas the data set from the FASINEX experiment shows energy spread over broad spectral bands (supermodes), perhaps 3 kHz wide, and separated by fairly well‐defined energy gaps (i.e., spectral minima). Both types of spectrum are a manifestation of waveguide propagation in the upward refracting, ocean‐surface bubble layer. A new, modal theory of sound in an upward refracting duct shows that, in general, the mechanism responsible for the observed hot spots and supermodes is intermode interference. The qualitative differences between the two data sets arise because the number of modes supported by the profile at La Perouse is significantly less than the number in FASINEX. In both cases, the detailed features displayed by the theoretical and experimental spectra show an almost one‐to‐one correspondence. [Research supported by ONR.]