Sounds of the Sea

By William H. Mowbray

Countless are the number of persons who have held large sea shells to their ears and heard the familiar roaring sound associated with the ocean. In fact, it has only been within the past two decades that significant sound sources have been recognized other than the well known crash of surf upon the shores, together with the action of wind and waves, undersea earthquakes and volcanic eruptions, and ship disturbances, which contribute to the general background sounds of the sea. We now know that the sea is far from a silent tomb administered by Davy Jones and actually contains a deafening chorus produced by its marine animal inhabitants.

Consider for a moment the ancient tales from the era of wooden hulled sailing vessels which traveled the high seas in search of fame and fortune. Many of these pioneers believed their ships to be haunted and reported hearing chains clanking, moaning, tapping, and other unearthly sounds below decks. Similar reports from present day vessels in waters inhabited by noisy animals support the possibility that the haunted ship sounds did not originate in the supernatural, but were only vocal renditions from the sea's residents heard through the ship's hull acting as a sounding board; perhaps such phenomena observed on whaling ships were caused by the animals for which they searched. If the early sailors could hear recent recordings of fish and marine mammal sounds, they would swear that all the banshees of Hades had been set upon their ships!

Although we have long recognized that some fish made sounds in air upon capture, as evidenced by such names as croaker, grunt, and drum for common coastal species, there has been a long delay in realizing the possibility and importance of animal sounds in the sea. The similarity between sea and land animals, who use sounds for recognition, communication, frightening enemies, and attracting food and mates, has now been definitely. established and has opened a new field of study. Sensitive hydrophones (underwater microphones) with associated amplifiers and recorders have been the electronic tools for our sonic research to show that the above fishes, and many of their underwater neighbors, do indeed live up to their names by producing a great variety of sounds with little stimulation.

The impetus for research on underwater sounds came during World War II when unexplained interfering signals rendered passive sonar Systems useless at critical times. Normally the strong, characteristic sounds of a ship's propeller would signal-its position to the listening sonar' operators of our submarines, surface vessels, and shore installations at a distance of several miles. However, at times the noisiest battleship or large commercial ship could not be hoard directly over hydrophones, of harbor defense sonar units because of the even louder reception of what sounded like foghorns, pneumatic drills, and other weird noises. In restricted areas, mine fields were laid using acoustic mines designed to explode upon reception of loud propeller sounds of unsuspecting ships. 1n only a short time these devices had to be with drawn and a costly delay in the mine warfare program was caused when unexplained detonations of the mines occurred, caused neither by ships nor mechanical defects.

Subsequent investigations instigated by the U. S. Navy proved that both of the described sonar difficulties were the direct result of fish sounds. The "pneumatic drill" sounds, were produced by members of the wide-spread croaker family, of which our local squeteague, or weakfish, is a member. Perhaps if the enemy had known. that the croakers' maximum sound production occurred at dawn and dusk, the usual hours for steak submarine attacks, the harbor defense up-its would have had problems of much greater magnitude. The loud "foghorn" sounds, to which the mine detonations are attributed, were a product of toadfish during their mating season when nests are built in tin cans, bottles, and similar refuse resting in shallow water, and perhaps on the mines themselves. These clear blasts of sound, among the loudest ever recorded from smaller fishes, can be easily heard around Wickford harbor during the month of June and the early part of July, after which they are not heard again until the following year.

Following World War II, the Narragansett Marine Laboratory, under contract with the Office of Naval Research, originated a continuing research program on underwater sounds of animal origin. As a result of extensive listening in Atlantic coastal waters and controlled testing of several hundred animals, including marine mammals, fish and crustaceans, information is now available on over 150 different soundmaking species whose sounds are preserved on 35 miles of magnetic recording tape. In Narragansett Bay alone, over 30 species of fish have shown their ability to "talk" in laboratory auditions, including the more common striped bass, tautog, cod, sea robin, scup, sculpin, and whiting. Detailed studies have been made on the physical characteristics of the sounds, their significance in normal life, and the nature of the mechanisms used to produce them.

There is little analogy between the sound producing and hearing mechanisms of land and sea animals since the latter do not possess vocal cords or external ears. In fish, the sound producing mechanisms usually serve a dual purpose, such as the air bladder used to maintain hydrostatic equilibrium or the pharyngeal teeth in the throat used to grind food. The air bladders are often equipped with special muscles used for sound making which set the gas-filled organ in vibration, resulting in low pitched, booming sounds which are easily differentiated from the higher pitched, raspy sounds of grinding teeth. However, some fish are equipped with both types of mechanisms and the resulting sounds have a combination of both low and high pitched components. Other species exhibit close coupling between the air bladder and the Internal ear structure, substantiating the theory that fish can hear sounds of their own kind for use in rudimentary communications. Marine. mammals, such as whales and porpoises, have a different type of sound producing mechanism which is not fully understood but may be operated by the air which they breathe. These animals are capable of emitting very loud and varied sounds, including whistles, moans, creaks and clicks. The latter sounds are employed as an effective sonar system, together with their well developed ears, to spot food, other animals, obstacles, and to determine the water depth by picking up the reflected echos.

How loud are fish sounds? This is a question often asked for which there is no simple answer. The major factor which influences the sound amplitude reaching a listener or a hydrophone is its distance from the sound source. The received amplitude primarily varies as the inverse square of the distance; that is, if the distance is doubled, the amplitude will diminish to one-fourth of its previous value. Therefore, unless the sound is originally of tremendous magnitude, it will be reduced to a level below the threshold of hearing or equipment detection over a fairly short distance in spite of the excellent sound transmission properties of water. To give a rough comparison of a typical loud sound, such as the toadfish's foghorn blast, its measured intensity at a distance of about three feet might be likened to an automobile horn at about the same distance I in air. Since the human ear is a very inefficient device under water, a submerged person would probably detect such a fish sound, although he would not realize its loudness. In fact, skin divers and spear fisherman often report hearing large fish at close range after being speared or surprised in a resting place. However, very few of the undersea sounds ever escape into the air to be heard by man above the surface due to the great density difference in the two environments.

The information already obtained on underwater animal sounds has been a valuable training aid for naval electronic equipment operators by assisting them -to recognize the previously unknown interfering signals and predicting the waters in which they might be heard. Research is being continued to keep abreast of advances in new equipment and techniques for undersea warfare and for possible commercial applications. Some day it may be possible for the commercial fisherman, having spotted a school of fish on his echo sounder, to utilize another piece of electronic gear to question the fish in the same manner as radar IFF (Identification, Friend or Foe) and learn their identity on the basis of sounds received.
 
 

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WILLIAM H. MOWBRAY is an Electronics Engineer at the Narragansett Marine Laboratory, University of Rhode Island. Mr. Mowbray received his B.S. (1950) from the University of Rhode Island. His principal research Interest is in biological acoustics.

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