From the equation of state 



^ = RT (8.2) 



P 



where R is the gas constant and T is the absolute temperature, 

 substitute (8,2) in (8.1). The result is 



V = v^yRT . (8.3) 



Since v^ is constant, the velocity of sound in air is proportional 

 to the square root of its absolute temperature. 



Suppose a sound wave is propagated upwards through an atmo- 

 sphere of continually decreasing temperature. Then the velocity 

 will decrease proportionally and in a situation similar to that 

 shown in figure 9» the sound waves will be refracted. 



Although air temperatures decrease with height there is 

 nothing constant about the rate of decrease. On clear days, the 

 temperature lapse rate during the day is greater than on cloudy 

 days, but towards sunset the temperature becomes approximately 

 constant. 



At a given place a number of lapse rates have been observed 

 and statistically averaged. This average value for the lapse 

 rate is known as the "normal lapse rate" and is approximately 

 6''C per kilometer. 



The analogous model tested in the ripple tank was made in 

 the form of a sloping uniform surface, the waves bein^ propagated 

 in the deeper water to simulate the higher temperature at the 

 ground where the sound waves were produced (fig. 10), 



As the waves proceed into the shallow water they are notice- 

 ably refracted and at the portion of the model which is above water 



23 



