ECHOES AS MESSENGERS 



this is not so in ordinary circumstances. Furthermore, 

 we can make objective measurements of sound intensity 

 in the two places with a microphone attached to a sensi- 

 tive voltmeter. Such measurements confirm our impres- 

 sion that the same source of a continuous sound such 

 as speech or music does produce a higher sound level 

 indoors. 



Let us pursue the matter a Uttle further and assume 

 that a tape recorder is available for experiments of this 

 type— perhaps you can borrow one from a friend or your 

 school. It will be more useful if you have a long exten- 

 sion cord, perhaps 50 feet in length, so that the instru- 

 ment can be operated well away from the building as 

 well as indoors. What sorts of sounds shall we compare 

 in the two situations in order to learn as much as we can 

 about the effects of echoes on how sounds sound with 

 and without echoes and reverberations? Speech and mu- 

 sic are excellent to get a better general understanding of 

 these effects. But no two passages will have the same 

 assemblage of sound waves, and it will be difficult to 

 compare the quaUty of the different notes, words, and 

 syllables in the same recording when heard indoors and 

 out. With the microphone of your tape recorder you 

 can record a sustained vocal note or one from any mu- 

 sical instrument. It is difficult to make a recording which 

 is really continuous and does not fluctuate in loudness. 

 But the best solution is to splice the tape into a con- 

 tinuous closed loop long enough to pass around both 

 reels so that the machine plays the same sample over 

 and over again. 



This experiment wiU immediately demonstrate one 

 important effect of echoes in a room. If a shrill, 

 high-frequency note is maintained at a constant level 

 in an ordinary room and if one Ustens to it carefully 

 while moving slowly across the room, its loudness will 



47 



