ECHOES OF BATS AND MEN 



such as trees or bushes. When played back in the reverse 

 direction, they will sound far closer to the original than 

 they did indoors. In short, this experiment shows the 

 extent to which our sense of hearing de-emphasizes 

 echoes. Sound waves which would be clearly audible if 

 they existed in isolation are almost totally ignored if 

 they happen to be part of an echo arriving a few tenths 

 of a second after another sound. This goes far to explain 

 why spoken words or other sounds do sound nearly the 

 same when heard indoors with strong echoes from the 

 walls or out of doors with few echoes or none. Of course 

 there is a difference if one listens carefully for it, and, in 

 addition to being louder, speech heard in a closed room 

 has a "thicker" quaUty. The echoing sound of footsteps 

 in a very large empty room is a common observation. 

 Almost everyone has also noted the forlorn sounds of 

 footsteps or conversation in a house emptied of its fur- 

 niture and draperies. All these effects are caused by 

 either the presence or absence of strong echoes. 



The mechanism by which we suppress echoes is one 

 of many subtle mysteries of the human ear and brain, 

 and no one understands how it is accompUshed. The 

 suppression lasts only a small fraction of a second; in- 

 deed, it has been shown to be greatest immediately after 

 the end of the direct sound and then to diminish grad- 

 ually until after half a second or so another sound can 

 be heard about as well as ever. An echo from a distant 

 hillside arriving four or five seconds after the end of the 

 outgoing sound is easy enough to hear if it is quiet where 

 one is listening. But the same strength of echo would be 

 inaudible if it arrived 1/lOth second after its original 

 was emitted. By playing a tape recording backward, we 

 remove the echoes from the time interval when our sup- 

 pressor mechanism is at work. 



In trying to learn what echoes sound hke, it is best to 



64 



