AIRBORNE ECHOES 



onstrates how these echoes behave. One should aim the 

 clicker at the wall 20° to 30° to one side of a perpen- 

 dicular from clicker to wall, while the second listens for 

 the echo. He will not hear it so clearly if he stands be- 

 side the clicker at point B as he will if he walks to one 

 side and a Uttle behind the clicker to a point such as 

 C. The position where the echo is loudest can be pre- 

 dicted on the same principle that governs the specular 

 reflection of light from mirrors; namely, that the angle of 

 reflection, r (angle EAC), equals the angle of incidence, 

 / (angle DAB). This experiment will give clearer results 

 if the listener stands a little behind the clicker, so that 

 he is shielded from the direct, outgoing click by the horn. 

 The same experiment can be performed more accurately 

 by mounting the clicker on a camera tripod and turning 

 it slowly to different angles relative to the wall. The lis- 

 tener may then move back and forth until he finds the 

 points where he hears the echo most clearly. Or the Us- 

 tener may stand still in various positions while the first 

 person turns the clicker slowly back and forth from right 

 to left according to his instructions. It is remarkable how 

 well the results of such experiments confirm the rule that 

 the angle of reflection equals the angle of incidence. 



There is an entirely different situation in which it is 

 easy to experience a simple type of echolocation. When 

 you ride in an automobile, sitting by an open window, 

 you hear a number of soimds from the engine, the tires, 

 and the rush of air past the window. As you drive past 

 a high stone wall, through an underpass, or close to any 

 large surface, these sounds will change in quality. A se- 

 ries of concrete guardrail posts, the masonry posts used 

 to support iron fences, or even a row of wooden fence 

 posts can be detected from a rapid series of swishing 

 sounds as the car moves by. Try Ustening with your eyes 

 closed as you ride along some familiar route and you 



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