The Not -So-Silent Sea

By Frank T. Dietz, Professor of Physics, University of Rhode Island

Although the writings of Aristotle, Pliny and others mention the fact that some kinds of fish make sounds, man has not been able to listen effectively underwater with the unaided ear. With improvements in hydrophones and amplifiers, the listener became aware of the diversity of sounds present in the sea. Because sea water is a good conductor of sounds in the human range of frequencies, sounds heard in a given location are often a combination of locally generated sounds and sounds which have travelled over large distances. The resultant background of noise is referred to as ambient noise.

The ambient noise at a hydrophone location may vary in intensity with the time of day, or the week, or the month, or the year. In character it may sound like murmurs, or like hissing, clicking, snapping, or throbbing. It may be punctuated by the whine of an outboard motor or the slower deep-throated sound of a larger engine. The listener may hear whistles, chirps, or weird, eery cries.

A fortunate eavesdropper may hear the infrequent sound produced by an underwater landslide or an earthquake. The sounds of and the sounds of a distant storm can be heard. Where ice covers the surface, the listener hears the ice tortured by expansion and contraction as the heat from the sun varies.

What sense can be made of the conglomeration of sounds, and why bother? In the past 25 years enough attention has been given to the problem to permit the classification of sound sources into three broad types. There are sounds of biological origin, sounds of man-made origin, and sounds of natural origin. In the last category are included the sounds of rain, waves, currents, seismic disturbances, and the like.

The biologists have studied the sounds of such animals as the snapping shrimp, the croaker, and the sea robin, and have identified the animal sources of scores of other sounds. Marine animal sounds have become of increased importance and have generated much interest. A knowledge of the sounds produced can aid the biologist in obtaining information about animals under open sea conditions where direct observation is presently nearly impossible. With the increased use of sonar and the capability of long-period submergence, the U. S. Navy became interested in the further identification of the noise-making species and in the significance, if any, of what is being "said." M. P. Fish of the Narragansett Marine Laboratory was the first to show that some marine animals make purposeful sounds which convey information.

Many of the sounds heard have not as yet been identified. In a recent study by J. S. Steinberg at the University of Miami it is stated that at least 25 categories of unidentified biological sounds were observed at Bimini, B.W.I., during a sixmonth period.

The man-made sounds present in the sea are of vital interest to the U. S. Navy. The detection and identification of ships and submarines within the perimeter of our defense areas present the sonar engineer with a series of complex problems involving such factors as the transmission of sound over long distances through an ocean capable of refracting and attenuating sound waves, and the detection of weak signals against an everpresent background noise.

What are some of the characterstics of the natural sounds which are the main contributors to this background noise? What are the predminant noise sources in a given location, and how is the intensity of sound related to them?

In the relatively shallow waters of the West Passage of Narragansett Bay one may hear biological sounds, bell buoys, small boats, and the sounds of aircraft. If we discount the sounds of biological and man-made origin and examine the residual noise, we have found that the sea noise is primarily influenced by two factors. The acoustic spectrum from 20 cycles per second to 10,000 cycles per second may be divided at 100 cps into low and high frequency portions. We have shown that the acoustic spectrum above 100 cps is mainly influenced by the wind, and that wind's blowing faster than five miles per hour cause the noise level to increase in a regular fashion. At the present time neither the interaction of the wind with the water surface nor the noise-making processes which occur in surface waves are completely understood. Figure 1 shows the relationship which exists between ambient noise and the wind speed for a portion of the acoustic spectrum.

Although the noise-making mechanism is still a mystery, graphs such as these may be used to predict the sound level from a knowledge of the wind speed.

In the frequency region below 100 cps, the effect of wind speed decreases and the observed variations in the ambient noise are found to be related to water currents. In a series of measurements J. Willis of the University of Rhode, Island was able to show that the acoustic signals were correlated with both the lunar and solar tides. Figure 2 presents some of these data.

When sound levels are averaged according to the hour of the day during which the observations were made, irrespective of the day of the month, one finds sound level peaks at approximately noon and midnight, showing the sun's influence. On the other hand, if the averaging is done by grouping the observations in terms of the hours after high tide that measurements were made, irrespective of the day of the month, a maximum is observed approximately three hours after high tide. This corresponds to the time when the ebb-flow in the West Passage is at its maximum. The smaller peak which occurs at approximately ten hours after high tide is connected with the time of maximum flood current. The difference between the two peaks is due to the fact that the flood current near the hydrophone is not as great as the ebb-flow in the same location. The sound producing mechanism is related to the current flow and involves sounds generated by turbulence. It is presently not clear, however, whether the sounds are generated in the body of the water or at the bottom of the bay.

Although these conclusions relating wind speed and current flow with ambient noise are peculiar to tidal estuaries, they form part of the broader picture and add to our knowledge of the cause and effect relationships which need further study. It is evident that present and future studies of sound in the sea require the combined talents of biologists, engineers, geologists, oceanographers, and physicists. A fuller understanding of the noisemaking mechanisms will need answers to questions such as, ''How does the air interact with the water to form waves?'', and, ''Exactly how does the disturbed sea surface generate sound waves?''

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