ECHOES OF BATS AND MEN 



may be surprised to find how many places you can rec- 

 ognize by ear. If you find a series of clearly "audible" 

 fence posts, compare their sound effects with those you 

 hear in passing through an underpass. Along the posts 

 it is primarily the high frequencies that return as echoes 

 from the relatively small surfaces; in the underpass al- 

 most the whole range of sounds of the car will be re- 

 flected from the large wall surface. K you make a care- 

 ful study of these sounds while your car is driven at 

 about the same speed, you will find that you can learn 

 to recognize many types of structures, such as parked 

 cars, from the echoes which they add to the roughly con- 

 stant sounds made by your own car. 



Echoes are used by bats and men to locate smaller 

 and more elusive objects than the walls of buildings, 

 and some interesting properties of reflected waves be- 

 come important once we begin to work with smaller ob- 

 jects. After you have acquired some experience with the 

 chcker, it is of interest to try it on trees, telephone poles, 

 or other objects that can easily be found out in the open 

 away from other echo-making objects. With care and 

 practice you can detect trees as small as 6 inches from 

 several feet away, and when this has been accomphshed, 

 you can again call upon another person to point the 

 clicker at the tree while you, the listening observer, move 

 about to different positions to find where the echo sounds 

 loudest. The result will usually be that the echo can be 

 heard over a much wider range of angles than hap- 

 pened with the louder echo from a building. This is 

 because the tree is only a Httle more than one wave 

 length in diameter and the echoes are spread over a 

 much wider range of directions, as indicated in Fig. 10. 

 Just as a horn less than one wave length in size fails to 

 concentrate sound, small objects scatter their echoes. 

 If you can hear echoes from trees or poles as small 



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