ECHOES OF BATS AND MEN 



constant sound source is carried in and out of doors. A 

 talkative companion might be one's first choice for a test 

 source of sound, but he will almost certainly change the 

 loudness of the conversation on moving from a closed 

 room to the open air. A portable radio is better, provided 

 that the building does not contain enough metal to act 

 as a shield for radio waves. First one might choose an 

 ordinary frame house for the experiment and set the 

 volume control of the radio to a level which produces 

 comfortably loud speech or music as the set rests on the 

 groimd. Carrying the same radio into a small room 

 makes it soimd much louder. Not only do the sound 

 waves reflected from the walls add to the total acoustic 

 energy reaching our ears, but also the announcer's voice 

 will seem to change its quaUty because the room has 

 selective effects on different frequencies of sound. 



Of course this experiment is a crude one, complicated 

 by many pitfalls. Perhaps there were distracting noises 

 on the street, or at one time you may have stood closer 

 to the loudspeaker. Perhaps the announcer happened to 

 talk louder during the time you had the set indoors. A 

 better experiment might involve some more constant 

 source— a whistle, typewriter, alarm clock, or other 

 noise-making machine, a baby's rattle or the louder kind 

 made for use on New Year's Eve. Best of all, in many 

 ways, is to use a tape recorder which can be carried 

 back and forth, indoors or outdoors. In this way you 

 can use the same sample of speech or music or perhaps 

 make up a tape recording in which the same sequence 

 is repeated often enough so that you can listen to it 

 repeatedly indoors and out. If you are still skeptical, 

 and you should be, you may wonder whether anything 

 changes in you, the Ustener, as you move back and forth. 

 Does your hearing become less sensitive when you are in 

 the open air? Many careful experiments have shown that 



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