Reverberation in the Sea

Abstract

The apparatus used to measure ocean reverberation at 24 kc consists essentially of a sound projector which sends a signal of adjustable duration into the water; a hydrophone which translates the backward scattered sound into electrical voltage; an amplifier which increases this voltage; a cathode-ray oscillograph which converts voltage fluctuations into spot movements; and finally a camera which records the movements on a moving film. Simple reverberation theory indicates (i) that reverberation level increases 10 db with a tenfold increase of pulse length; (ii) that volume reverberation level decreases 20 db for a tenfold increase in range; and (iii) that surface reverberation level decreases 30 db for a tenfold increase in range. At certain times and under certain conditions, presumably when ocean conditions are those postulated by theory, observed reverberation levels agree with theoretical values. Such agreement, however, is relatively uncommon. Under most conditions, deep scattering layers cause volume reverberation levels to depart markedly from simple theory; also, a combination of refraction, wind, and other factors causes a decrease in surface reverberation level with range which is too rapid to be in agreement with simple theory. When a sound beam is projected horizontally in deep water, both surface and volume reverberation might be expected. Under a rough sea and for ranges less than 500 yards, surface reverberation predominates over volume reverberation. Beyond 1000 yards, even under a rough sea, volume reverberation usually overshadows surface reverberation. Also, for such long ranges, attenuation enters as an important factor and causes the reverberation level to fall off more rapidly than the rate predicted by simple scattering. In shallow water, bottom reverberation (which depends for its intensity on whether the bottom is rock, sand, mud and sand, or mud) is the dominant part of the observed reverberation.