Transmission of sound through a randomly rough air-sea interface

Abstract

The Helmholtz integral and the Kirchhoff approximation have been used to develop predictions for the transmission of sound through a rough air-sea interface. A model study was conducted with wind-driven surfaces generated in a large anechoic tank. Root mean square wave heights, o, ranged from .05 to .41 cm, windward correlation lengths, Lu, from 0.8 to 4.2 cm. The frequencies used (10-43 kHz) were scaled to be equivalent to low audio frequencies with moderate seas. Although the coherent component of the transmitted acoustic intensity showed an exponential decrease with increasing values of the roughness parameter, R [= k22o2 (c2/c1 cos o1 - cos o2)2, the decrease was greater than predicted. (Subscript 2 refers to propagation constant, speed and angle of transmission in water; subscript 1, in air.) This lack of agreement appeared to be caused by violation of the Kirchhoff require­ment, L/A2 >> l. An empirical correction factor o = l + 4.8e-Lu/1.75 was determined to give a corrected roughness, oR. The corrected theory appears to be valid for both coherent and incoherent components of intensity for all values of L/A2 provided that oR < 4.0 . For higher values of roughness there is a "leveling off'' of the transmission loss of the coherent component similar to that recently observed but unexplained in rough surface scattering.