546 SCHEVILL, BACKUS, AND HERSEY [CHAP. 14 



sole recorder employed at sea for preserving sounds for playback. Neither direct 

 nor FM (frequency-modulated) recording provides distortion-free playback ; 

 the student is well advised to learn their limitations. FM recording is limited to 

 frequencies below roughly 5 to 10 kc/s ; it is especially useful for frequencies 

 below 100 c/s. Direct recording is generally (but not inevitably) of poor quality 

 below 100 to 150 c/s, but can provide good recordings to 100 kc/s. Wave forms 

 are relatively faithfully preserved in FM recordings, whereas phase distortion 

 is characteristic of direct recording, even though uniform amplitude response 

 is achieved for sinusoidal signals over a broad frequency band. Direct tape 

 recorders are available in bewildering variety of size and quality. Lightweight, 

 small portable tape recorders operated on dry batteries are commonly limited 

 to frequencies between 50 or 100 c/s and 6 or 8 kc/s. Still portable but bulky 

 professional models are available that will perform equally well from 50 or 

 150 c/s to 15, 30 or 100 kc/s. Wide bandwidths are achieved in part by very 

 fast tape transport speeds (60 inches per sec or more). The wide -band recorders 

 have been comparatively bulky and generally not suited for use in small boats. 

 However, they have been used successfully in small research ships. One cannot 

 but hope that the space -saving of solid-state circuitry will soon provide us 

 broad-band, small tape recorders. They are sorely needed for small boat work 

 or other techniques to permit the observer to close range on suspected soni- 

 ferous animals. 



Earphones must be selected carefully for the intended research. Some 

 designs emphasize low, others high, frequencies. For work to be done from 

 ships rather than small craft, a high-quality loudspeaker may be used in quiet 

 spaces. As with tape recorders, it is well worth the investigator's time to know 

 the limitations of this part of the gear ; the possible distortions caused by the 

 wrong earphones may prevent the observer from detecting subtle components 

 of great significance in the ambient sounds of the sea. The best recorder is of 

 no use if the observer has decided there is nothing of interest to record. 



B. Analysis of Sounds 

 Sounds may be analyzed by measuring their intensity, their variation in 

 time, or their frequency composition (also generally a function of time). The 

 objective of the research should determine the method, but since this field of 

 inquiry has not generated new methods until recently, its devotees have been 

 forced to choose the best available from related fields. The methods of industrial- 

 noise analysis have not proved useful excepting for the study of sounds 

 generated by large groups, as the snapping shrimp or croakers. Other animal 

 sounds of the open ocean are commonly heard from single individuals or from 

 small numbers. Further, the sounds divide roughly into three categories : short 

 transients spaced many times their own duration in time (clicks or ticking 

 sounds), sequences of short transients closely and regularly spaced (creaking or 

 grating sounds), and prolonged sounds containing discrete frequency com- 

 ponents (squeals, grunts). Thus the methods of transient and vibrational analysis 

 are indicated. 



