ECHOES OF BATS AND MEN 



opposite direction, and the net effect would be a reduc- 

 tion in the frequency of the echo. 



To return to our specific example, the final echo has 



150 

 a frequency at the sonar ship of 20,000 X (tt^)*? ot 



about 20,540 c.p.s. If this is translated into an audible 

 ping by combining it with a local frequency of 19,000 

 c.p.s., the echo beat note will be 1540 c.p.s., whereas 

 if both ships were stationary, the beat note would be 

 1000 c.p.s. This is a fairly extreme example of rapid 

 approach of the two ships, but in actual practice sonar 

 operators can tell when a submarine turns or even when 

 it speeds up or slows down. Though we understand far 

 less of what goes on in a bat or porpoise brain than we 

 know about the operation of this sonar system, it is rea- 

 sonable to infer that similar comparisons of outgoing 

 and echo frequencies may well be used to detect the 

 motion of flying insects or swimming fish. The horse- 

 shoe bats with their constant frequency pulses can per- 

 haps make better use of the Doppler effect than can the 

 FM bats, but even the latter seem to use less frequency 

 sweep when closing on insect prey than during cruising 

 flights when they are presumably seeking to make their 

 initial contact and detection. 



Prospecting by Echo 



Sound waves are not limited to air and water; they 

 can also travel through solid materials of any kind. Even 

 the echo sounder designed only to echolocate the bot- 

 tom may sometimes show a type of false bottom differ- 

 ent from the fish echoes or deep scattering layer de- 

 scribed earlier. Sometimes the records indicate a second 

 or third bottom below the real one rather than above it. 

 This means that after the bottom echo of the probing 



116 



