VOICES OF EXPERIENCE 



and one half times faster in water than in air, so that 

 the time differences between echoes from objects at dif- 

 ferent distances are that much smaller and presumably 

 that much harder to detect. Furthermore, fish are very 

 similar to water in the way they affect sound waves, and 

 most of the energy of underwater sound that strikes a 

 fish continues through its watery body just as though no 

 fish were there. The same physical problem is present in 

 the body and hearing apparatus of the porpoise; it too 

 is nearly "transparent" to underwater sound, and it is 

 intrinsically difficult for sound waves to interact with 

 the various parts of its body. Indeed, it is likely that in 

 the experiments of Schevill and Lawrence the echoes 

 used by the porpoise came not so much from the 15- 

 centimeter fish as from its small air-filled swim bladder, 

 which reflected sound much as an air bubble would do. 

 It would take us too far afield to present all the physical 

 properties of underwater sound that are important for 

 this extremely skillful echolocation by porpoises, but the 

 interested reader may refer to Chapter 10 of Listening 

 in the Dark (see Further Reading). 



Two facts may help to explain the precision with 

 which porpoises detect echoes from fish: one is the wide 

 frequency range of their emitted sounds, and the other 

 the range of their hearing. Tests have shown that they 

 hear sounds as low in frequency as 150 c.p.s. and as 

 high as 150,000 c.p.s. Yet these may not be the true 

 limits of their hearing but only those set by the apparatus 

 used to test it. In water where the velocity of sound is 

 some four and one half times that in air, sound waves 

 of 150,000 c.p.s. have a shorter wave length than the 

 highest frequencies to which human ears have really 

 useful sensitivity. Except by young children, it is doubt- 

 ful whether frequencies above 15,000 c.p.s. are heard 

 we'll enough to be useful for detecting objects by their 



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