Frequency Smearing of Forward-Scattered Sound from the Ocean Surface

Abstract

An open sea experiment was performed in which forward-scattered CW acoustic signals of 100, 180, 250, 400, and 500 Hz were measured with surface grazing angles in the range 35°–60°. At the same time, the vertical acceleration and orthogonal slopes of a typical patch of ocean surface were measured by a float system which was sensitive to gravity wave components with frequencies of up to about 1 Hz. The acoustic and ocean wave data were digitized and fast-Fourier-transformed with frequency resolutions of 0.01 and 0.02 Hz, respectively. The results of the experiment show good agreement with existing theoretical and experimental results [e.g., W. I. Roderick and B. F. Cron, J. Acoust. Soc. Amer. 48, 759–766 (1970)]. Theory predicts that sound which is forward-scattered by the ocean surface is phase modulated and the resulting power spectrum contains the discrete carrier frequency component with sidebands symmetrically positioned about the carrier. The sidebands are displaced from the carrier by multiples of the ocean wave spectrum. Note, however, that the energy may be asymmetrically disposed. Typical measured surface spectra have prominent peaks at 0.10 and 0.15 Hz. The measured acoustic spectra have prominent first-order sidebands close to these frequencies, although a slight asymmetry is evident. Less prominent second-order sidebands are also identifiable at the theoretically predicted frequencies. A comparison of measurements made at the various acoustic frequencies indicates that the location of the sidebands for this frequency range is independent of the acoustic frequency and that more energy is concentrated in the sidebands (relative to the carrier) as the acoustic frequency is increased.