ECHOES OF BATS AND MEN 



what we see in the sky, and since the particles are of less 

 than the wave lengths of visible light (4 to 7 X 10"^ 

 centimeter), short wave length light is more strongly 

 scattered than other colors. This is why the sky is blue. 

 By analogy we might say that the bat flying up to these 

 wires must hear "blue echoes." In any event, a case could 

 be made for evaluating bat sonar by means of an index 

 containing d* rather than d or d-, and the value of 

 R^/PWd^ is therefore listed in the last Une of the table. 

 The drastic results of changing the definition of our 

 efficiency index should now be clear. This may indeed 

 open serious questions as to whether such different sys- 

 tems for echolocation can be meaningfully compared on 

 a simple numerical basis. Furthermore, several other im- 

 portant factors have not yet been brought into the com- 

 parison. Bats operating with sound waves in air face 

 serious reductions in echo signal due to the absorption 

 of sound in air, especially at higher frequencies. During 

 the round trip from bat to target and return, sound of 

 50 kc loses power by a factor of 0.63 for every meter 

 of distance, in addition to the reduction due to the in- 

 verse fourth power reduction for echoes. At 100 kc the 

 reduction is by a factor of 0.44 over every meter. Radio 

 waves suffer no such severe losses in traveling through 

 the air. This fact puts the bat at a great disadvantage 

 over long distances. On the other hand, there is a con- 

 sideration which would favor most radar systems as 

 compared to bats. This is the duty cycle, or fraction of 

 the time during which energy is being emitted. In typical 

 cases, such as those included in our table, a bat would 

 be emittmg 10 to 20 pulses per second, each pulse lasting 

 2 to 5 milliseconds, so that the duty cycle would vary 

 between 0.02 and 0.1. The radar had a far lower duty 

 cycle, however, the interval between pulses having been 

 about 1500 times as long as the pulse itself, so that the 



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