Model Experiments Relating to Sound Propagation in Shallow Seas

Abstract

The object of model experiments on sound transmission in the sea is to supplement the somewhat empirical approach to the subject on a full scale. One serious difficulty in transmission research in the sea is that the conditions of the experiment are frequently not under our control—various perturbing factors are present which may mask the effect it is required to detect. The paper deals with the design of tanks, sound absorbers, transducers, etc., for small-scale work at frequencies of the order of 0.5 to l Mc/sec. This equipment has been used to record amplitude and phase of the received signal and to study the following factors influencing sound transmission: (1) nature of the bottom, (2) mean temperature and temperature gradients, (3) surface waves, and (4) depth of water. A study has also been made by visual observations of the sound distribution on the bottom and surface using (a) chemical methods, (b) abrasive methods, and (c) photography of “wedge” interference fringes on the water surface. These experiments reveal the complex nature of the interference phenomena occurring in the water. A new scanning technique has also been developed which gives a picture of a vertical section of the sound field (longitudinal or transverse) in the “model” tank. In this method the sound received is used to modulate the brightness of a cathode ray spot, and a “picture” record is made of the sound-intensity variations of the sound-field in the water. Comparisons have been made in this way between rocky and sand/mud bottoms.