ECHOES OF BATS AND MEN 



as one or two wave lengths, you will find them al- 

 most equally loud over a wide angular range. Of 

 course they are nowhere as loud as those from larger 

 structures such as buildings. This would be just as 

 true of light waves or water waves, and an appropriate 

 experiment in the ripple tank will show specular reflec- 

 tion of surface waves from long objects but would show 

 extensive scattering from something about one wave 

 length in size. 



This difference between specular reflection and scat- 

 tering of waves can be studied with a ripple tank or even 

 with the surface waves in a bathtub, although it is more 

 difficult to see them clearly in the tub. Just as echoes are 

 easier to hear if generated by sounds of short duration, 

 it is easier to study surface echoes by generating short 

 trains or pulses of water waves. This is probably why 

 water beetles interrupt their swimming motions at fre- 

 quent intervals— to provide intervals of "quiet" in which 

 they can better feel the echoes from objects at some dis- 

 tance across the water's surface. If one sets up a few sur- 

 face waves at a time by a quick light tap against the 

 water, reflections from the edge of the tank or tub are 

 of course easy to see. If an object of about one wave 

 length (for example, a short piece of broomstick or 

 wooden dowel) is placed in the water with its axis per- 

 pendicular to the surface, close observation will find 

 smaller waves scattering out in almost all directions from 

 this source of surface echoes. Of course all other waves 

 must be absent, but, once this phenomenon has been ob- 

 served, it is of some interest to vary the size of the cy- 

 lindrical object from the smallest that produces visible 

 scattered waves up to sizes well in excess of one wave 

 length. Such experiments convince one of the real dif- 

 ference between sharply directional specular reflection 

 and the diffuse scattering from small echo sources. These 



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