ECHOES OF BATS AND MEN 



and if he continues on his way, he invariably strikes them 

 without warning. Subjective impressions obviously can 

 be misleading-we do not always know just which of our 

 senses is informing us about our surroundmgs. This is 

 not to say that our senses are not keen, but rather that 

 our conscious thinking about them may lead us to the 

 wrong conclusion about how they operate. 



This is not a unique misapprehension concerning the 

 workings of our sense organs, although perhaps it is an 

 extreme one. Another example also involves the sense 

 of hearing. How do we know where a sound is coming 

 from? Sometimes we see the source and are thus in- 

 formed of its position, but everyone is able to locate the 

 origin of an unfamiliar sound heard in darkness, and 

 usually with great accuracy. Sometimes we locate a 

 sound source approximately by turning our heads until 

 the sound is louder in one ear than in the other, but more 

 often and with great precision we rely on the difference 

 in the same sound as it arrives at the two ears. Consider 

 for the present only one type of sound, a sharp click. 

 The most important property of the bundle of sound 

 waves constituting the click is the time of arrival of the 

 first sound waves at the two ears. If the click comes from 

 straight ahead, the two ears receive the first sound waves 

 at exactly the same time because they are equidistant 

 from the source. If, however, the click arises at some 

 point to the right of the direction you are facmg, it 

 reaches the right ear a small fraction of a second sooner 

 than the left. If the source is 90" to one side, the opposite 

 ear is about 20 centimeters farther away than the closer 

 one, and since sound waves in air travel about 30 centi- 

 meters per millisecond, this means that the maximum 

 possible difference in time of arrival at the two ears is 

 less than 1 millisecond. Yet such is the precision of the 

 auditory portions of our brains that we can easily dis- 



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