SUPPOSE YOU WERE BLIND 



their subjects. They therefore wore earplugs of wax and 

 cotton, ear muffs, and padding over the sides of the head. 

 This compound series of barriers was necessary because 

 many sounds, particularly those of low frequency, pene- 

 trate ordinary earplugs or ear muffs. Everyone knows 

 from the ordinary experience of wearing ear muffs or 

 parka hoods in cold weather that by speaking slightly 

 louder than usual one can still converse with his com- 

 panions no matter how well the ears are protected from 

 the winter winds. 



So thorough was this muffling that the subjects could 

 not hear the sounds of their own footsteps, and instruc- 

 tions could only be given them by loud shouts. A loud 

 shout can easily have 10^ times the energy of a barely 

 audible whisper. Direct measurement of the intensity 

 necessary for them to detect a test sound showed that 

 their auditory sensitivity had been reduced by a factor of 

 about 4,000,000; that is, they could hear the test sound 

 only after its energy level had been increased four 

 millionfold above the level that was just audible without 

 the ear covering. 



When the same subjects were now asked to repeat the 

 experiments with their hearing thus impaired, the results 

 were spectacular. None retained any obstacle -detection 

 ability at all, and in each of one hundred trials every 

 subject bumped unexpectedly into the screen. One of the 

 blind men, who had stoutly maintained that sound 

 played no part at all in his "facial vision," complained 

 that he was now getting no sensation at all, and for the 

 first time he walked hesitantly and held out his hands 

 to guard against anticipated accidents. If sound does ac- 

 count for the obstacle-detection ability, one might ask 

 why there was any reduction in distance of first detection 

 when the subjects wore the felt veil and leather gloves. 



135 



