ECHOES OF BATS AND MEN 



the two ears can be studied with the aid of a simple 

 device illustrated in Fig. 15. This consists of a source 

 of clicks, which could be a loudspeaker or a mechanical 

 clicker, and a sound-tight box to house it. From the box 

 lead two tubes each ending like a physician's stethoscope, 

 but make sure the earplugs are soft to avoid accidental 

 injury to your ears. One tube is fixed in length, while the 

 other has a telescoping tube like that of a trombone so 

 that its length can be varied. When the two tubes have 

 different lengths, it will obviously require longer for the 

 first sound waves of the click to reach one ear than the 

 other. Since the velocity of sound is known, the differ- 

 ence in time of arrival of the cUcks can be calculated 

 easily from the difference in length of the two tubes. 

 When one listens to clicks through these tubes of un- 

 equal length, the effect is strikingly Uke that of a cHck 

 coming from one side. If the eyes are closed and one 

 makes even a small effort to imagine that the clicks are 

 coming through the open air rather than through the 

 tubes, there is a compeUing illusion that the source is at 

 the side of the ear receiving the shorter tube. Of course 

 it makes no difference where the box containing the 

 source of cHcks is really situated, nor does the actual 

 length of the two tubes matter. When precise measure- 

 ments are made with more refined apparatus of this same 

 type, the minimum time difference that leads to this illu- 

 sion of a source at one side or the other is less than 0.1 

 millisecond. 



We need experiments like these to tell us about one of 

 the principal ways in which we locate the source of a 

 sound. We never think, "That click reached my right ear 

 1/10,000 second before it got to my left ear; therefore, 

 it must have come from a little to one side of the median 

 plane of my head." We simply recognize that the click 

 came from one side without any idea how we located 



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