THE LANGUAGE OF ECHOES 



the two passages through the air-water interface. Or, to 

 put the matter in another way, the detection of insects 

 at 2 meters through the air means that bats are capable 

 of hearing echoes roughly as faint as those that might, 

 under ideal conditions, return from a minnow to a fish- 

 ing bat. The book Listening in the Dark goes into this 

 particular problem in more detail if you wish to pursue 

 it further. But it is significant that a hypothesis which 

 seemed so completely ridiculous when one first learned 

 of the milUonfold loss of energy during the round trip 

 from air to water should turn out, on closer examina- 

 tion, to be a serious possibility after all. Common sense 

 and first impressions may be misleading when we are 

 dealing with matters quite outside the range of ordinary 

 human experience upon which people have built what 

 we call common sense. 



Resistance to Jamming 



Up to this point we have been thinking about echoes 

 as more or less isolated sound waves that could be dealt 

 with one at a time. To be sure, we considered earHer the 

 likeUhood that a faint echo would be masked by the 

 louder outgoing sound. Experiments described in Chap- 

 ter 3 demonstrated that human hearing ignores echoes 

 arriving within a small fraction of a second after a loud 

 sharp chck. Bats obviously do better than we in dis- 

 criminating these echoes from the original sound. In the 

 experiments of Schevill and Lawrence a porpoise showed 

 that it could detect echoes from a small fish despite the 

 louder competing echoes from the bottom of the pond, 

 the surface, and the shore a few feet beyond this small- 

 sized target. But the expertness of bats goes even further 

 than anything we have yet considered. When they are 

 hunting insects, their ears receive a more complicated 



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