542 SCHEVILL, BACKUS, AND HERSEY [CHAP. 14 



measured and the techniques of the acoustician introduced to the marine 

 biologist. 



Good progress has been made during the years since the war in describing 

 identified sounds in physical terms, but the few most interesting studies con- 

 cern the functions of these sounds in the lives of the animals making them. 

 Although the descriptive catalog of sea-animal sounds is far from complete, it 

 is in the use of these sounds to the animal and in their use to the marine eco- 

 logist in pursuing other problems through them that the excitement of the 

 next few decades in this field lies. 



3. Instrumentation 



Instrumental aids to underwater listening are wholly developments of the 

 past century. So far as we can discover they have been used for studying 

 sounds of marine animals only since about 1940. Just as microphones have 

 been developed to convert sound in air to electrical signals, so hydrophones 

 have been developed for the same purpose in water. The question is often asked : 

 do sounds reproduced in air by earphones or loudspeakers sound as they would 

 under water were we to hear them directly? We are familiar with the same 

 problem respecting microphones, and we have pursued it relentlessly in our 

 current enthusiasm for "hi-fi" (high fidelity) sound systems. The hydrophone 

 requirements are essentially the same. Sound waves in air or water are oscil- 

 latory wave motions and variations of pressure of the medium. The hydrophone 

 must convert the motion or the pressure variation into an electrical signal 

 which is accurately proportional to whichever effect it is sensitive to. Since the 

 motions and pressure variations are proportional to each other it is immaterial 

 which effect is chosen. Once the electrical signal has been generated it may be 

 amplified and fed to earphones, a loudspeaker or a recorder. If the amplification 

 and conversion of the electrical signal is faithfully achieved, then the resulting 

 sound will sound as it would have under water. 



A second well-worn question is : how loud is a sound under water? Since our 

 natural environment is not under water, we cannot truthfully answer from 

 personal experience. Loudness is partly a function of the receiver and partly of 

 the sound. Sounds of marine animals can greatly exceed the local ambient 

 noise level, that is, they form a sharp contrast with their background. The 

 pistol prawn (snapping shrimp) is known to stun its prey with a sharp sound. 

 Animal sounds of the open ocean carry long distances under the right condi- 

 tions ; thus the sounds of the echoing fish (see Griffin, 1955, pp. 411-412) must 

 have travelled nearly three miles, and were still well above background. Where 

 soniferous animals congregate, sound intensities over a hundred times the 

 average background have been observed (in this they are just like people). On 

 the other hand, open ocean listening in calm weather often sounds quiet (even 

 with the gain of the amplifier turned high), and one can only conclude that 

 there is great variety in the loudness of underwater sound just as there is in 

 the sounds of our own daily life. 



