ECHOES AS MESSENGERS 



the velocity of sound, on the one hand, and the distinc- 

 tion between reverberations and echoes on the other. In 

 air, where we do most of our Ustening, a sound lasting 

 one second extends 344 meters through the air, and only 

 if it is reflected from an object more than half that dis- 

 tance away (172 meters) will a listener close to the 

 source receive an echo which begins after the original 

 sound has ended. Several syllables can be uttered in 1 

 second— "one thousand one," for instance— and with a 

 little effort a short syllable such as de can be repeated as 

 rapidly as five times per second. If one spoke a single 

 short syllable lasting 0.2 second, an echo would be sep- 

 arated from the outgoing sound even though the reflect- 

 ing surface was only slightly more than 34 meters (172 

 X 0.2) away. It is not often that we hear echoes clearly 

 separated in time from the original sounds that created 

 them. This is partly because we seldom deal with single 

 sounds as short as 0.2 second, or reflecting surfaces as 

 distant as 34 meters, and also because our ears do not 

 distinguish two sounds as separate unless there is a frac- 

 tion of a second of quiet between them. Even when two 

 sounds are so close together in time that they seem to be 

 single, the combination usually sounds different from 

 either of its two parts if they are heard alone. Two clicks 

 that follow each other too closely to be heard as a double 

 click sound duller than either one all by itself. Or, if 

 closer still, the pair of chcks may simply soimd louder 

 than one alone. 



Echoes We Seldom Notice 



The echoes that usually follow every word we speak 

 add to its quality and impact even though we are not 

 aware of the reverberations as separate and distinct. This 

 can be illustrated by simple experiments in which some 



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