Frequency Spectra of Forward‐Scattered Sound from the Ocean Surface

This study consists of a theoretical analysis, a model tank experiment, and an ocean experiment to determine the frequency spread of surface‐reflected sound. The theoretical study is based on the concept that the phase and amplitude of the surface waves modulate the acoustic signal. The amount of spreading is a function of the incident acoustic frequency, the surface‐wave spectrum, and the geometry of the experiment. The ocean experiment was conducted in winter off the coast of Bermuda. Two frequencies (750 and 1500 Hz) were emitted simultaneously from a fixed projector. The tones were surface reflected and received on a narrow‐beam fixed bottom‐mounted vertical array. Oceanographic and meteorological data were obtained in the vicinity of the insonified area. The time variation of the surface elevation was obtained from a wave staff, and this information has been processed to obtain the one‐dimensional ocean spectrum. The experimental results are compared with theoretical predictions. It is shown that the comparison is excellent for both the model tank and the ocean experiments. Under all conditions for the ocean experiment, the frequency spread is less than 1.0 Hz, and under low sea‐state conditions, the spread is about 0.2 Hz.