13. SOUND SCATTERING BY MARINE ORGANISMS 



J. B. Hersey and R. H. Backus 



1. Introduction 



Many people are familiar with the concept of specular reflection, even 

 though their knowledge may be limited to the reflection of light from a mirror 

 or the echo from the flat wall of a large building or from a rocky cliff. The 

 bottom echo recorded in echo-sounding is the most familiar similar process in 

 underwater sound. Sound also reflects from the sea's surface, but since the 

 surface of the sea is seldom smooth one appreciates immediately that the 

 specular reflection "laws" of elementary physics do not necessarily apply to 

 reflection here. Obviously, similar remarks apply to reflection from the sea- 

 floor where it is rough on a scale comparable to the wavelength of the sound. 

 In both cases, the sound incident on the rough surface is deflected in all direc- 

 tions rather than following the paths that would be predicted for reflection 

 from a plane surface ; the sound is said to be scattered. The sum total of sound 

 arriving at a hydrophone after a sound wave has thus been scattered from the 

 sea's surface is known as surface reverberation ; that from the sea-floor (and 

 below it) is known as bottom reverberation. Sound is also scattered from objects 

 in the water such as animals and plants, solid debris, air bubbles, and even 

 from bodies of water having a composition or temperature different from the 

 surrounding water. The sound reaching a hydrophone because of these pro- 

 cesses is called volume reverberation. This is our subject. Our principal interest 

 in it arises from the predominance of scattering from marine animals and the 

 consequent promise it offers as a means of learning about their distribution and 

 behavior. 



A. Historical Notes 



Unlike the study of sound production by marine organisms, the present 

 subject has a short history, as it could only begin after the development of 

 efficient underwater sound projectors. This development was stimulated by 

 the Titanic disaster of 1912 which was shortly followed by the patent applica- 

 tion of a Briton, L. F. Richardson, for both an air and an underwater echo- 

 ranging scheme for the detection of icebergs. At the same time R. A. Fessenden 

 in the United States was working on a moving-coil transducer for underwater 

 echo -ranging and signalling. With his device an iceberg was detected at a 

 range of nearly two miles in 1914. In 1915, Constantin Chilowsky, a Russian 

 electrical engineer living in Switzerland, proposed to the French government 

 that this technique be used for locating submarines. During World War I a 

 considerable advance was made in Europe and America in developing practical 

 echo-ranging systems, although these came too late to be of tactical advantage 

 in locating submarines at that time.i 



It was not until after the war that these systems were used extensively for 



1 For the details of the foregoing history, see Hunt (1954). 

 [MS received November, 1960] 498 



