ECHOES AS MESSENGERS 

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maxima and minima (X = ^^ ^^^ = 0.034 meters or 3.4 



centimeters) will be too close together for easy de- 

 tection. Notes from musical instrmnents have so many 

 frequencies, or harmonics, each giving its own maxima 

 and minima at its own wave length, that it will be diffi- 

 cult to distinguish the loud and quiet spots for each 

 frequency. Hence, the purer the note the more obvious 

 the effect. You will find the flute more satisfactory be- 

 cause of its purer tone than a piano or violin. 



These maxima and minima are called standing waves. 

 A loud spot is the point where sound waves reflected 

 from the walls add to others arriving directly from the 

 loudspeaker. If several parts of the walls all send strong 

 reflections to the same spot, these various echoes are 

 likely to arrive at different times and fail to reinforce 

 each other as strongly as they would if arriving at the 

 same time. In some rooms of irregular shape the standing 

 waves may thus be inconspicuous, but most rooms are 

 regular enough and have sufficiently reflective walls so 

 that at least in the middle of the room the standing- 

 wave pattern is noticeable. If you have an opportunity 

 to experiment with a ripple tank in which surface waves 

 on water are generated to illustrate the various phenom- 

 ena of wave motion, you will fimd that the frequency of 

 the vibrating object producing the waves has to be ad- 

 justed rather carefully to obtain pronounced standing 

 waves. Otherwise the water's surface may show only a 

 shifting and confusing mess of wavelets chasing each 

 other back and forth without apparent order. If the tank 

 is not a simple shape, such as a rectangle, then the 

 standing-wave patterns are either very complicated or 

 are limited to a few areas where reflected waves do man- 

 age to reinforce those arriving directly from their source. 



Suppose we try to set up standing waves in a room by 



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