sp:ct. 4] 



SOUND PRODUCTION BY MAKINE ANIMALS 



545 



compared with their inherent self-noise that they are virtually useless for 

 studying many animal sounds. However, several new hydrophone designs are 

 being introduced, and scientists interested in this field can hope for smaller, 

 more sensitive hydrophones having a broader frequency response. 



Amplifiers have long had more faithful response than hydrophones, ear- 

 phones, loudspeakers or recorders. In fact there is little excuse for using 

 amplifiers that limit the effectiveness of the ensemble. Dow's Suitcase amplifier 

 (Hersey, 1957) is representative of several early post-war amplifiers specially 

 designed for use with hydrophones for detecting very broad-band underwater 

 sounds. 



If possible, hydrophones and amplifiers should be selected so that their self- 

 noise is considerably less than the lowest anticipated values of ambient noise. 



+ 10 



■20 



100 



10,000 



100,000 



1000 



FREQUENCY (c/s) 

 Fig. 3. Self-noise of an AX-58 — Suitcase system compared with ambient noise at sea 

 state 0. (After Hersey, 1957, Fig. 10. By courtesy of the Journal.) 



For example, at low sea states the ambient noise of the 10 kc/s band of the AX-58 

 and similar hydrophones is of the order 0.5 dynes/cm^. The self-noise of the 

 amplifier should be considerably lower than the equivalent input voltage : say, 

 10~6 volts. Fig. 3 shows Dow's estimate of performance of the AX-58 and 

 Suitcase system. 



Radio telemetering buoys fitted wdth hydrophones have been used in various 

 studies of natural sounds of the sea (e.g. Snodgrass and Richards, 1956; and 

 Hashimoto, Nishimura and Maniwa, 1960). These offer attractions such as 

 reducing the expense and effort of shipborne observations and removing the 

 ship and the people from the immediate scene, thus reducing the possibility of 

 abnormal behavior of the animals. 



A. Recorders and Monitors 



We know so little about animal sounds of the sea that there is seldom any 

 reason for limiting the recording bandwidth, except to be as faithful as possible 

 over as wide a frequency band as possible. This is not easily done with com- 

 mercially available equipment. The magnetic tape recorder is now virtually the 



