ECHOES OF BATS AND MEN 



familiar is this topic that we have no common words 

 analogous to transparent and opaque to express the fact 

 that sound waves penetrate a substance easily or not at 

 all. Nor have we any acoustical equivalents for shiny or 

 matte to describe surfaces which reflect sound waves 

 chiefly in one direction or about equally in all directions. 



Reflected sound waves are called echoes or reverbera- 

 tions and they have an important effect on what we hear. 

 We have seen that certain animals, such as bats and 

 porpoises, fimd their way by listening for echoes. Blind 

 men also make use of sound for orientation, and their 

 dependence on reflected sound waves will be taken up 

 more fully at the end of the book. But before we go on, 

 it will be helpful to specify the meaning of a few words 

 that are useful in describing the message-carrying ability 

 of sound waves. 



Echo generally suggests a distinct, separate reflection 

 of a sound from some surface at a considerable distance. 

 Reverberation implies the multiple reflections of a soimd 

 from surfaces at closer range, so that reflected sound 

 waves tend to overlap and become mixed with the origi- 

 nal ones. In a more general sense, however, an echo is 

 any sound wave that has had its direction materially 

 changed after striking an object. When reflected waves 

 travel through the same space as later waves from the 

 same sound source, they interact and either increase or 

 decrease the previous level of air pressure. If the pres- 

 sure at a given point at a given time is increased by the 

 presence of the echo waves, we say constructive inter- 

 ference or reinforcement has occurred; if the soimd 

 pressure is reduced from what it would have been with- 

 out the reflected waves, we speak of destructive inter- 

 ference or cancellation. These terms have just the same 

 meaning for sound waves as for Ught. 



It is important to appreciate the relationship between 



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