SECT. 4] SOUND PRODUCTION BY MAKINE ANIMALS 543 



The practical design of hydrophones is different from that of microphones 

 because water is so much denser and so much less compressible than air. Also 

 the electrical parts must be waterproofed, and must be capable of withstanding 

 high hydrostatic pressures without structural damage or loss of sensitivity. 

 But the sensitive structures are the same. These are described elsewhere in 

 this book. 



All electronic systems for observing or recording underwater sounds from 

 animals of the sea, whether at sea or in aquaria, consist of a hydrophone, a pre- 

 amplifier and a means of reproducing the sound as sound in air or as a graph 

 of wave motion or pressure versus time. Earphones or loudspeakers have been 

 used for immediate sound reproduction, while various recorders, disc, wire, or 

 magnetic tape, have been used for "storing" the sound for later reproduction. 

 Equipment available for graphical representation of sounds includes the various 

 oscillographs, sound level recorders, and wave analyzers (see Chap. 12; also 

 Beranek, 1949, chap. 12, for extended discussion of methods and analysis 

 instruments). 



The hydrophone, pre-amplifier, and the recorder or reproduction instrument 

 each have a limited range of amplitude and frequency (or response time) over 

 which they perform faithfully. The central problem of instrument design is to 

 fit the range of faithful response to the needs of the investigation. Thus far we 

 have depended mostly on instruments designed for other purposes, particularly 

 those intended for sounds within the human audible range and those intended 

 either as naval underwater detection devices or as development aids to the 

 latter. 



After the war observations at sea continued to be made with surviving 

 military hydrophones. The Brush AX-58 hydrophone was popular because it 

 was designed for low self-noise in the human audible range ; its self-noise is 

 somewhat less than the ambient noise in open sea at sea state (Hersey, 

 1957). Commonly these observations consisted of long listening vigils from 

 small research ships or small craft that had been silenced as much as possible 

 by lying to and turning off" ship's power, the electronic equipment needed for 

 listening and recording being operated on batteries. Some disc recordings were 

 made, but were discontinued in favor of tape recording by 1950. Analysis of 

 the transient sounds thought to be of animal origin continued to depend on 

 subjective judgment of the quality of the sounds and what are best described 

 as gropings with oscillographic recording. These latter never proved out- 

 standingly useful because the presumed animal sounds were often not more 

 intense than the accompanying broad-band background. Furthermore, oscillo- 

 scope photography in that day was cumbersome and was simply not available 

 during the more significant early observations. All in all, the few years prior to 

 1950 brought many intriguing first observations, nearly all of which were 

 made with inadequate recording and analysis equipment. 



Since 1950 only a few individual scientists have maintained strong interest 

 in observations at sea. Much of their work has continued to be limited by 

 inadequate hydrophones. High quality hydrophones have not yet been designed 



