AIRBORNE ECHOES 



point the clicker so that a distinct echo is heard. Since 

 the sound travels 60 meters from clicker to building and 

 back to your ears, this trip will require 60/344, or 

 about 0.17 second. If you operate the clicker twice per 

 second, you will hear an outgoing chck at a time you 

 may designate as zero, an echo at 0.17 second, a second 

 emitted cUck at 0.50 second, a second echo at 0.67 sec- 

 ond, etc. If we speed up our operation of the clicker, 

 the second click will eventually come at 0.17 second 

 and so will mask the echo. If we can operate the clicker 

 with sufficient regularity, this fusion of echo with second 

 click provides another way to measure distance— pro- 

 vided we know the velocity of sound. A mechanical de- 

 vice such as a metronome can control the rate of click- 

 ing more precisely, but with a little practice a good 

 approximation can be achieved. One practical difficulty 

 is that the click made by bending the strip of steel will 

 usually be slightly louder or different in quality from 

 that made when the strip is unbent. Thus successive 

 clicks alternate in level or quahty, and it is not always 

 easy to maintain an even rhythm. But it can be done 

 and, regardless of its practicability, it is worth while to 

 understand this simple method for estimating distance 

 by the rate of clicking necessary to cause each echo to 

 fuse with the following click. One effective way to es- 

 timate the critical rate is to have someone else count the 

 number of clicks in a 5- or 10-second period measured 

 with a stop watch or the second hand of an ordinary 

 watch. 



The same cUcker may also be used to demonstrate 

 convincingly the concentration of echoes into certain 

 directions when they have been reflected from surfaces 

 of various sizes relative to the wave lengths in the click. 

 Most toy clickers have a frequency range between 3 and 

 10 kilocycles, so that the most intense soimd waves have 



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