ECHOES OF BATS AND MEN 



and again at three the weight will be halfway between 

 its highest and its lowest positions, and you might draw 

 a horizontal reference line along the wall below the 

 clock at this level. Now suppose that at other times in 

 the day you measured the height of the weight above 

 and below the reference line, calling this distance a, or 

 the ampUtude of motion of the weight. At noon the 

 amplitude would be + a and at six o'clock it would be 

 - a. Every six hours a would be zero, and around noon 

 and six the curve would slow its rate of rise or fall and 

 reverse direction. If you used the minute hand instead 

 of the hour hand, you would plot twelve excursions of 

 the weight during one revolution of the hour hand. 



This same graph would also represent the sine func- 

 tion studied in geometry and trigonometry. But this 

 convenient picture of a wave leaves out any possible 

 interactions with material objects. Hence, we shall find 

 it necessary to think about waves in somewhat differ- 

 ent ways or at least add to the simplified concept of a 

 continuous sine wave before we can use it to deal ef- 

 fectively with the message-carrying function of sounds. 

 These special modifications add new interest to the sub- 

 ject of wave motion as it is considered in physics courses. 



What are sound waves and how do they differ from 

 other kinds of wave motion? When sound is traveUng 

 through some medium such as air, the pressure of the 

 medium is changing rhythmically, increasing and de- 

 creasing at any particular point at a rate which we call 

 the frequency of sound (Fig. 3). To be sure, these 

 changes may not be regular, but even when they are too 

 irregular to be called a single frequency it is still true 

 that alternating pressure changes occur and that the at- 

 mospheric pressure fluctuates above and below its aver- 

 age value, designated as one atmosphere, which one 

 would measure with a barometer (at sea level the at- 



38 



