ECHOES AS MESSENGERS 



someone else may walk into the room. Often this will 

 cause a shift in the positions of the maxima and minima 

 even though the room is fairly large. This is a more 

 compHcated type of interaction in which the exact loca- 

 tion of greatest interference is influenced by all sorts of 

 objects that add reflected sound waves to those arriving 

 directly from the tape recorder. Because of these changes 

 the standing-wave patterns could be used to tell us that 

 someone had entered the room. Usually we have much 

 easier ways of knowing this, but there are circumstances 

 where changes in standing-wave patterns have been put 

 to use to detect small changes in the position of objects 

 in a room. One type of burglar alarm operates on this 

 principle. Suppose you were walking about blindfolded 

 in the same room. The standing- wave pattern would at 

 least inform you that you were in a room and not out 

 of doors where nothing was reflecting enough sound to 

 set up standing waves. It could also tell you when some- 

 thing else moved into the room, provided you were 

 standing still and noticed the shift in the standing waves. 

 These may seem to be trivial examples, but blind men 

 do learn to pay attention to many aspects of the sound 

 fields in which they live and in this way learn much 

 about what goes on around them. Remember, too, that 

 these examples have been selected for their simplicity, 

 and from such crude beginnings we can go on to much 

 more difficult questions that can be answered by carrying 

 these experiments further. This is in essence to use 

 sound waves as tools or "sense extenders" for exploring 

 one's surroundings. Crude tools used with little skill 

 yield only crude information. But, as we already have 

 seen, even such small animals as bats have become ex- 

 pert at using sound waves as tools of this sort to learn 

 rather complicated facts about what goes on around 

 them. They have come to do this in the long course of 



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