Theoretical and Experimental Investigation of the Transmission of Sound Over Reflecting Surfaces

Abstract

The paper presents an experimental and theoretical discussion of acoustic signaling between a sending station and a receiving station near a plane reflecting surface, and is applicable to fog horn and submarine signaling. The experiments were made with a small scale apparatus employing supersonic waves of a frequency of 67.5 kilocyles per second. It is shown that at grazing incidence the direct and reflected waves cancel, or partially cancel, as predicted by theory. The cancelation for propagation in air near a water surface, or other surface of high sound velocity and high density, is found to be restricted to angles very near to grazing incidence. On the other hand, theory predicts cancelation for much larger departures from grazing incidence if the reflecting surface has high density but low velocity; experimental curves for transmission over beach sand and certain other substances resemble such theoretical results very closely. Acoustic mirages caused by temperature gradients were studied experimentally, and their effects are shown to be of striking importance in modifying the above results.