ECHOES OF BATS AND MEN 



sensitive microphone to convert the energy of sound 

 waves into electric voltages. Suppose further that we 

 have connected this microphone to an instrument such 

 as a cathode-ray oscilloscope, which draws a visible 

 graph of the sound waves almost instantaneously. A 

 cathode-ray oscilloscope is the forerunner of your tele- 

 vision set. Inside the picture tube a spot of Ught is 

 created, and one electric circuit moves this spot horizon- 

 tally and again and again at a uniform rate from left to 

 right while another moves it up and down. In this appli- 

 cation the up-and-down movement is produced by the 

 amplified voltage from the microphone. On the picture 

 screen the combined horizontal and vertical motion lit- 

 erally draws a graph of sound pressure agamst time. 



With such a machine we can watch the behavior of 

 the sound waves while we utter them. If we suddenly 

 stop talking, the movement of the spot of Hght on the 

 picture tube may seem to stop at the same instant. But 

 if one looks closely, and if the instrument is set up in a 

 large hall, one can easily see that the oscilloscope con- 

 tinues briefly to draw a diminishing graph of the sound 

 waves that are still traveling back and forth past the 

 microphone from wall to wall. Because sound waves 

 travel about 344 meters per second, and because less 

 than 100 per cent of the sound energy is reflected back 

 from each contact with the walls or floor, and finally be- 

 cause sound waves are reduced slowly by their frictional 

 effects on the molecules of the air, the continuing echoes 

 are appreciable for only a fraction of a second. But they 

 are there, and our eyes can see them on the oscilloscope 

 screen even when our ears do seem not to hear them. 



With instruments we can improve upon the ability of 

 our eyes to judge how fast the sound level decUnes and 

 how long it is detectable at all. One of the simplest 

 methods is to photograph the moving spot on the oscillo- 



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