ECHOES OF BATS AND MEN 



hard soles. Some blind men prefer shoes with metal heel 

 plates, perhaps because of the sharper footsteps that re- 

 sult. If you have carried out some experiments with 

 clickers, like the one illustrated in Fig. 7, it must be 

 obvious that if you, a rank beginner, can detect trees, 

 an experienced blind man can do at least as well. Many 

 blind men have used clickers of one sort or another— 

 aside from metal heel plates, the canes used by many 

 blind men to lengthen the reach of their hands are used 

 as cUckers by tapping the ground or pavement. The re- 

 sulting cUcks provide a standard noise which gives a 

 useful echo. 



But we can properly ask whether footsteps, cane taps, 

 or even toy cUckers mounted in horns are really the best 

 types of sound for a blind man's purposes. Do they gen- 

 erate the most informative possible echoes or are there 

 other types of sound that would be superior? The ques- 

 tion is simply asked, but the search for a convincing 

 answer has been difficult and frustrating. Various types 

 of cHckers and portable sound sources have been built 

 and tested. Some, particularly the directional clickers, 

 have been used extensively by a small number of blind 

 men, including their inventors. But the results have been 

 far from satisfactory, and many users find it too difficult 

 to hear consistent echoes or find the added facility at 

 orientation not worth the embarrassment caused by a 

 conspicuous audible sound that calls attention to their 

 handicap. Yet almost every object that a blind man needs 

 to detect does interact in some way with audible sound 

 waves. This being so, why can we not devise a probing 

 sound which will produce audible echoes that are recog- 

 nizably related to the objects a blind man needs to locate? 



One difficulty has already been called to our attention 

 in the experiment where tape recordings of clicks or 

 other impulsive sounds were played backward on a tape 



144 



