Explanation

(a) According to Doppler’s effect, whenever there is a relative motion between a source of sound and listener, the apparent frequency of sound heard by the listener is different from the actual frequency of sound emitted by the source. Let S be source of sound and L the listener of sound. Let v be the actual frequency of sound emitted by the source and $\lambda$be the actual wavelength of the sound emitted.
If v is velocity of sound in still air, then
$\lambda = \frac{v}{V}$
If velocity of listener is $v_{L}$and velocity of source is $v_{s}$, then apparent frequency of sound waves heard by the listener is
$v^{''} = \frac{v - v_{L}}{v - s_{s}} \times V$
Here, both source and listener are approaching each other.
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Then $v_{s}$ is positive and $v_{L}$ is negative.
$\therefore v^{''} = \frac{v - \left( - v_{L} \right)}{v - v_{s}}v = \left( \frac{v + v_{L}}{v - v_{s}} \right)v$
$i.e.,\ \ \ \ v^{''} > v$
Also,
$\lambda^{''} < \lambda$
So, listener listens more frequency and observes less wavelength.