AIRBORNE ECHOES 



learned to do this with great skill and success. As you 

 walk toward a building from 15-25 meters away, the 

 echo of the cUcker is at first clearly separated from the 

 original cHck but gradually merges with it until there is 

 only one sound as best you can tell. At this point you 

 should turn in some other direction, where no large ob- 

 ject will return echoes, and operate the clicker several 

 times. The clicks will sound different, and if in doubt 

 you can alternately point toward the building and then 

 in some other direction. After this difference has been 

 recognized, you can move in closer to the building, re- 

 peatedly clicking both toward it and away in non-echoing 

 directions. It is surprising how close you can come and 

 still be clearly aware of a difference in the sound of the 

 clicker when it is pointed toward and away from the 

 wall. At very close range, less than 10 feet for example, 

 the difference will begin to be one of loudness; the 

 echoes are of sufi&cient intensity that they add apprecia- 

 bly to the click with which they are fused. This is why 

 the horn is so important to shield you from the direct 

 sound; if the horn could be perfect, so that all the sound 

 energy of the clicker traveled away from you, then the 

 echoes would become unmistakable. 



It is helpful to digress at this point into a little 

 thought about the wave lengths of audible sounds and 

 the relationship of these wave lengths to the practicable 

 size for a horn to direct the click forward. It is a general 

 property of wave motion that specular (that is, mirror- 

 like) reflections can be obtained only from objects that 

 are larger than one wave length. Water waves on the 

 surface of a ripple tank or a bathtub can be reflected 

 from the edges of the tank or tub or from objects several 

 centimeters long. Such reflections obey the same rules 

 as those that hold for light waves; for instance, the angle 

 of reflection from a plane surface equals the angle of 



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