ECHOES OF BATS AND MEN 



that sound, rather than anything which could not travel 

 along telephone wires, informed the blind man that the 

 screen was in front of him, the experimenters had to con- 

 sider the nature of these sounds. 



Footsteps were an obvious possibility, and when the 

 original experiments were repeated with the subjects 

 walking in their stocking feet on a soft carpet, their 

 ability to detect the screen was greatly reduced. The av- 

 erage distance of first detection fell from 6.9 feet when 

 the subjects were wearing shoes and walking on the bare 

 floor to 3.4 feet when the sounds of their footsteps were 

 dampened by the carpet. Some subjects snapped their 

 fingers or made clucking sounds, but others apparently 

 relied on whatever sounds were present in the hallway, 

 such as the sound of their own breathing or the rustle of 

 their clothing. This question had not been seriously con- 

 sidered in the design of the first experiments, but now 

 that the investigation had reached the point where the 

 microphone was to be mounted on a cart there would be 

 no sound from footsteps or breathing. Some other sound 

 had to be substituted, which provided the opportunity to 

 study the usefulness of various sounds in providing audi- 

 ble clues to the presence or absence of obstacles. Ob- 

 viously, too, the experiment involved echoes. If some 

 sound told a listener that the screen was present, it must 

 have been a sound which was different with the screen 

 than without it. 



In order to study the character of the echoes used by 

 blind people, the experimenters then equipped the cart 

 with a loudspeaker as well as a microphone. A variety of 

 sounds with known characteristics could now be gen- 

 erated by the loudspeaker for further tests. If a loud 

 hissing noise was used— that is, a noise containing a wide 

 range of audible frequencies— the screen could be de- 

 tected by the subjects hstening to the telephone in the 



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