AIRBORNE ECHOES 



scope screen with a camera in which the fihn moves at 

 a constant rate. While the spot moves up and down as 

 the sound waves strike the microphone, the motion of 

 the fihn draws a graph of sound pressure on the vertical 

 axis against time on the horizontal axis. The resulting 

 graphic picture of sound waves makes it easy to see the 

 echoes which continue to arrive at the microphone a 



Fig. 6. A graph of the sound pressure in a very short 

 word without any echo is shown in A, and the same 

 word with echoes is shown in B. Note the similarity 

 of the early waves in A and B, and a difference as the 

 echo returns before the original word has ended. 



good fraction of a second after the end of the sound that 

 came directly from a speaker's mouth. Such photographs 

 also show clearly the greater magnitude of the echoes 

 that follow the same word spoken indoors rather than 

 out. An example of this comparison is seen in Fig. 6, but 



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