AIRBORNE ECHOES 



tively. The toy clicker produces a number of frequencies 

 or wave lengths within each brief click, but it would re- 

 quire much more compUcated click generators to pro- 

 duce an ideal click containing only a single frequency 

 and a pulse short enough in duration to yield echoes 

 distinctly separate from the original. Indeed, this con- 

 sideration of separateness itself imposes limits on the 

 possible frequencies. Several waves are necessary to es- 

 tablish a clear frequency, and if our sound is to last only 

 1 millisecond it can contain only 10 waves of 10,000 

 c.p.s., or 5 waves of 5000 c.p.s. 



What I have been suggesting in these simple experi- 

 ments with a clicker is to act as though you were blind 

 and see what you can discover about the larger objects 

 in your surroundings solely by means of echoes. Later 

 on I shall discuss in more detail what blind people 

 actually do and the success they have achieved as well 

 as the limitations that seem to prevent echolocation 

 from warning them about aU the major obstacles that 

 threaten their safe progress. But before turning to this 

 direct apphcation to a pressing problem of a large 

 group of handicapped persons, we will find it helpful to 

 consider certain physical properties of echoes that de- 

 termine their strength and audibility. For this purpose 

 we can make good use of both real echoes from a clicker 

 and "echoes" in the ripple tank, which is so useful in 

 the physics laboratory for the analysis of wave motion. 



The Velocity of Sound Measured 

 by Means of Echoes 



As a beginning we may consider a simple method of 

 determining the approximate velocity of sound by an ex- 

 tension of the already-mentioned procedure of timing the 

 return of an echo from a distant hillside. If the distance 



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