AIRBORNE ECHOES 



The Acoustics of Clicks and Echoes 



Because almost every object reflects sound to an ap- 

 preciable degree, it is very rare for any sound to reach 

 our ears without embellishment by echoes. Why then 

 are the echoes so rarely noticed? Seldom do they occur 

 separately; that is, they rarely arrive at a different time 

 from the sound that produced them. Usually they and 

 the original sound are mixed, and we ordinarily fail to 

 discriminate between the two classes of sound waves. 

 The simple experiments with a portable radio or tape 

 recorder suggested in the previous chapter demonstrated 

 that echoes were present indoors and that they could 

 make a tone or a noise sound different. The loudness is 

 increased by the addition of strong echoes from the walls 

 of a room and by standing waves that may be audible 

 when continuous pure tones are present indoors. But the 

 important point is that special experiments were neces- 

 sary to convince us that echoes really are so common a 

 part of the most famihar sounds. One major reason 

 echoes escape our notice so completely is the relatively 

 long duration of most sounds compared to the time 

 they require to bounce back in our customary places 

 of Uving and hstening. Even on the shore of a moun- 

 tain lake we are not Hkely to notice echoes of the songs 

 we may sing about a campfire, for they will usually be 

 masked by the notes that follow. Only when the song 

 comes to an abrupt ending will the echoes from the hills 

 intrude upon our consciousness. The masking of echoes 

 by the continuing sound explains much of our inability 

 to notice them in ordinary circumstances. 



But all sounds come to an end, eventually at least, 

 and there are always pauses or brief intervals of silence. 

 Why don't we hear the echoes then? Suppose we try to 

 investigate the physics of this question by setting up a 



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