Model Experiments on Sound Propagation in Shallow Seas

Abstract

The experiments described deal mainly with the propagation of continuous waves of a particular frequency, and with the variation of signal strength with the depth of the receiver and the water using “point,” almost nondirectional, transducers. A simple method has been found for lining experimental model scale tanks to make them anechoic at the frequencies considered. It has been demonstrated that temperature changes and surface waves produce marked changes in sound propagation. More important influences, however, appear to be due to the nature of the bottom and the depth of the water. Point-by-point observations, described in Sec. II of the paper, indicate that multiple reflections between the water surface and a hard reflecting bottom, like steel or concrete, result in the propagation of many of the higher modes or orders of reflection and the records of sound intensity variation from surface to bottom is covered with rubber (equivalent to mud and sand), however, the higher modes are suppressed and the records of surface to bottom intensifies are much simpler, containing relatively few maxima and minima. All these observations are in reasonable accord with either the mode theory or the ray theory. It has been demonstrated also that the received signal (in all-round transmission and reception) is extremely sensitive to the depth of water. A change of depth of say 5 ft in water of depth 200 ft (full scale) may result in a change of the order of 30 db in received signal. The distribution of the sound field on the bottom and surface of a water layer has been demonstrated by using sounds of relatively high intensity (Sec. III). Interference patterns form not only on the bottom but also on the surface of the water. A simple “abrasive” method has been devised which provides a permanent record of the sound pattern on the bottom. A new scanning technique has been developed which gives a complete picture of a sound field of relatively low intensity, in a vertical plane either longitudinal or transverse in the water (Sec. IV) . In this method “point” transducers are used, one of which scans a vertical line several times a second whilst the other travels along or across the tank at a predetermined depth. The received signal modulates the cathode-ray oscilloscope spot which moves up and down in synchronism with the scanning transducer. The scan patterns reveal a striking difference between the two cases of a hard steel (rock) bottom and a rubber covered (mud and sand) bottom. The pattern for a steel bottom is very complex and contains many maxima and minima in each vertical scan. The rubber bottom, on the other hand, provides only a few maxima and minima at each scan. The patterns hitherto obtained are, however, not amenable to mathematical treatment. They confirm and amplify the results obtained by point-to-point methods described in Sec. II. This method of scanning should prove a useful tool for further studies of low-intensity sound fields in water.