16 



SOUND WAVES 



Table 1.2. the relation between decibels and power and current or voltage ratios 



1.10. Doppler Effect. — The change in pitch of a sound due to the rela- 

 tive motion of the source and observer is termed the Doppler Effect. 

 When the source and observer are approaching each other the pitch ob- 

 served by the listener is higher than the actual frequency of the sound 

 source. If the source and observer are receding from each other the pitch 

 is lower. 



The frequency at the observation point is 



V- Vo 



1.58 



V — v. 



where v = velocity of sound in the medium, 

 Vo = velocity of the observer, 

 Vs = velocity of the source, and 

 fs = frequency of the source. 

 All the velocities must be in the same units. 



No account is taken of the effect of wind velocity or motion of the me- 

 dium in equation 1.58. In order to bring in the effect of the wind, the 

 velocity v in the medium must be replaced by v -\- w where w is the wind 



