The application of a normal mode perturbation method to assess the effects of sound-speed fluctuations on the propagation of narrow- and broadband acoustic energy in the sea

Abstract

Fluctuations in the intensity of an acoustic signal are a common feature of propagation of sound in the sea. The fluctuations are a manifestation of the temporal variability of the index of refraction (sound velocity) of the medium which, in turn, is induced by a variety of random and deterministic ocean processes covering a wide spectrum of temporal and spatial scales. In this paper, a recently developed high-order perturbation method is described and applied to the problem of sound propagation in the sea. The basis functions used in the calculations are derived from sound-speed profiles obtained in an acoustic propagation experiment conducted in a shallow-water region of the Mediterranean. Each profile is treated as a perturbation from a mean profile, the latter obtained by depth-averaging the profiles. At particular source frequencies, calculations of modal functions and acoustic transmission loss were compared for the mean and several perturbed profiles. These initial discrete-frequency results were presented at the Scintillations Conference (University of Washington, Seattle, Washington, August 1992). Here, the method is extended to include the response of broadband, transient sound sources and sources at several water depths. The broadband (time-domain) approach enables one to readily determine the optimum frequencies of the shallow-water duct and, in addition, to assess the effect of sound-speed fluctuations on the optimum frequencies. These results, along with the earlier ones, confirm the significant effects on acoustic transmission of seemingly minor variations in sound speed and, moreover, demonstrate the efficacy of the new perturbation method in handling such problems.