CHAPTER 3 

 Airborne Echoes of Audible Sounds 



The word "echo" suggests a quiet country scene where 

 a steep cliff or hillside looms up hundreds of feet away. 

 A shout or a gunshot suddenly breaks the silence, and 

 there follows a repetition of the sound, fainter than the 

 original. Knowing the velocity of sound, we could de- 

 termine our distance from the hillside if we accurately 

 measured the interval of time from the onset of the out- 

 going sound to the arrival of the first echo. This can be 

 done with a stop watch, provided that the hill is large 

 enough and distant enough so that a clearly audible echo 

 will return after some seconds. K the hill is too close, 

 the time interval will be too short for easy and accurate 

 measurement; if it is too far away, the echo may not be 

 audible at all. Often there are too many hills producing 

 multiple echoes, and if the first of these overlaps the 

 end of the outgoing sound or there are reverberations 

 from objects in our immediate vicinity, then the accurate 

 measurement becomes difficult. Nor is it always easy to 

 decide just which hill is sending back the echo; in fact, 

 the easiest procedure often is to time the echo and then 

 scrutinize a large-scale map in search of a steep hillside 



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