ECHOES OF BATS AND MEN 



generating not a single frequency but a sound containing 

 many different frequencies. Speech and music answer 

 this description, but the different frequencies change 

 rapidly with time, so the effects are compUcated. Still, it 

 is true that even though we do not ordinarily notice 

 standing waves of speech or music, in some very large 

 rooms there may be "dead spots" where interference 

 between the direct and reflected sound makes hstening 

 very difficult and unpleasant. Indeed, there is a whole 

 science of architectural acoustics devoted to minimizing 

 such "dead spots" and to controlling the echoes from 

 the walls of auditoria so that speech and music are car- 

 ried as faithfully as possible to all parts of the hall. 



A simple experiment with our tape recorder in an 

 ordinary room can demonstrate the effects of having 

 many frequencies present at the same time. A loud hiss 

 made vocally into the microphone will, when played 

 back, fill the room with a still louder hiss. But you will 

 probably have great difficulty in hearing standing waves. 

 The same experiment can sometimes be performed by 

 turning up the volume control of a radio or record 

 player until you hear a hissing sound that incidentally 

 stems from the random motions of molecules in some 

 part of the electronic circuit. It has a wide range or band 

 of frequencies, as does a vocal hiss, and all are about 

 equally loud. So many different wave lengths are present 

 that even though each one tends to set up standing waves 

 at its own wave length, all the others have equally strong 

 tendencies to estabHsh loud spots separated by their 

 wave lengths. The result is that the over-all level of the 

 sound is much the same from point to point within the 

 room. To obtain clear standing waves there must be 

 only one or a very few wave lengths prominent in the 

 sound that fills the room. 



Perhaps when you are listening for standing waves 



50 



