AIRBORNE ECHOES 



It can also be used to get distinct echoes from trees a 

 foot in diameter, and other objects can be located in the 

 same way. A good technique for a beginner is to sweep 

 slowly back and forth with the clicker while operating 

 it at a rate of one or two clicks per second. A few min- 

 utes of careful Ustening will show that much can be 

 learned about objects of this general size, provided that 

 they are at a sufficient distance to yield an echo which 

 is clearly separate from the emitted cHck itself. Experi- 

 ence will show that echoes are most easily recognized 

 when only one large echoing surface is within range. 

 Several trees in a courtyard surrounded by large build- 

 ings give multiple echoes that only careful scanning can 

 resolve. 



Before very long your hands become cramped from 

 the unnatural position in which they must be held in 

 order both to operate the cUcker and provide it with a 

 horn. It is not difficult to mount the clicker in a small 

 horn made of cardboard, Ught metal, or plastic. While 

 a paraboUc shape is perhaps ideal, a fairly deep cone 

 will serve fairly weU. The most important point is to 

 provide a means of bending the dented sheet of steel 

 back and forth without having any opening at the back 

 of the horn through which the cUck can reach the user's 

 ears directly at a high level of intensity. One device of 

 this sort is shown in Fig. 7. 



After you have learned to detect trees and houses by 

 hearing their echoes, you will find it worth while to ex- 

 periment with an easily recognized target such as a build- 

 ing. Keep moving closer as you click. If you find it diffi- 

 cult to be sure whether you are really hearing echoes, it 

 may be helpful to try using the chcker while blindfolded 

 or with your eyes closed. You will then be in much the 

 same situation as a blind man trying to find his way 

 about by means of echoes. Many blind people have 



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