ECHOES OF BATS AND MEN 



rise and fall at regular intervals of distance. It is of par- 

 ticular interest to observe this effect while moving slowly 

 near the middle of a room with the tape recorder at one 

 end of the room playing a note that is two or three 

 octaves above middle C. The comparison may be easier 

 if one ear is covered so that the tone is heard entirely 

 through the other. Listening in this way, one can usually 

 hear clearly the waxing and waning of the soimd level 

 and you can, with care, estimate the distance from one 

 loud spot to the next. A meter stick hanging horizontally 

 at about eye level may make it easier to judge the dis- 

 tance through which the ear must move in order to pass 

 from one point of maximum loudness to the next. 



Having observed these fluctuations in the level of our 

 recorded tone within a closed room, you repeat the same 

 experiment out of doors. Not only will the tone sound 

 fainter but the fluctuations will largely disappear; the 

 loudness will fall off gradually as one walks away from 

 the loudspeaker. Such a simple experiment as this will 

 demonstrate that reflected sound waves from the walls 

 of the room are interacting with those coming straight 

 from the tape recorder and that at some places there 

 is constructive interference producing maximum sound 

 levels, while elsewhere there is destructive interference 

 causing zones of relative quiet. Furthermore, it will be 

 foimd that the distance between the maxima is one half 

 wave length, provided that tones of nearly a single fre- 

 quency are used. C2, the second C above middle C, has 

 a fundamental frequency of 1024 c.p.s., or a wave length 

 of close to 30 centimeters, and it is therefore a con- 

 venient frequency to use for such experiments. At a 

 lower frequency, such as 100 c.p.s., the wave length will 



equal or exceed the dimensions of the room (X. = j^ = 



344 



-T— r = 3.4 meters), and at 10 kc or higher the successive 



48 



