SUPPOSE YOU WERE BLIND 



it. In much the same way, a bUnd man learns to antici- 

 pate colHsions with obstacles under certain conditions, 

 usually without realizing at all that these conditions are 

 the presence of audible echoes. Recognizing the prox- 

 imity of an obstacle and knowing from experience the 

 pain of bumping into it, he comes to believe that he felt 

 its nearness with his hands or face. All this adds up 

 to a warning not to interpret the workings of our sense 

 organs and our brains too hastily; they may be operating 

 in other ways than we are first incHned to think. But we 

 should not go to the other extreme and conclude that 

 measuring instruments will always improve upon our un- 

 aided senses. As we have learned, sense organs and 

 brains of men, porpoises, bats, and beetles accomplish 

 extremely difficult feats of detection and discrimination. 

 To return to the blind man's problems of orientation, 

 echolocation is certainly the technique by which skillful 

 blind people find their way in the "dark." But with the 

 general question thus answered, we are immediately im- 

 pelled to ask what type of sound will provide a blind 

 m.an with the most informative echoes. In the final ex- 

 periments by Supa, Cotzin, and Dallenbach where the 

 cart carried the loudspeaker and microphone up to the 

 test obstacle, it turned out that a hissing noise was more 

 effective than pure tones. But the average distance of 

 detection was only 3.75 feet instead of 6.9. Does this 

 mean that footsteps are more efficient sounds for this 

 purpose, or does it mean that the cart with the loud- 

 speaker and microphone was less easily controlled by the 

 remote listener? From the experiments described in 

 Chapters 2 and 3 it is clear that some sounds generate 

 more useful echoes than others, and that a very short 

 click has the advantage that it ends before the first echo 

 begins to return. But footsteps on the floor are not es- 

 pecially sharp clicks, even when the walker's shoes have 



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