Question

Both a point source of sound and an observer are at rest. If air starts blowing then:

• A. the velocity of sound will change
• B. the frequency detected by the observer will change
• C. the wavelength of sound will change
• D. the intensity of the sound will change.

Answer 1

Answer: (A), (C)

the velocity of sound will change
the wavelength of sound will change

Let the speed of sound be $v$, and the flow rate of air be $u$. The distance between source and observer is $d$, which is fixed. The source generates a sound with frequency $f$. The period is $T=\frac{1}{f}$.
The wavefronts leaving the source travel at speed $v$ relative to the air, and the air travels at speed $u$ relative to the source and the observer.
So the wavefront travels at speed $v+u$ between the source and observer.

That means that wavefront 1, leaving at $t=0$, arrives at time $t_1=\frac{d}{u+v}$ later.
If wavefront 2 leaves at time $t=T$, it arrives at time $t_2=T+\frac{d}{u+v}$ later. Since velocity changes, frequency does not change, wavelength of sound will change.
Now, $t_2-t_1=T+\frac{d}{u+v}-\frac{d}{u+v}=T$
Hence,the spacing at which the observer receives the wavefronts is still $T$ and is unaffected by the travel time.
The velocity of the emitted wave increases as the medium gains speed.