Question

A police car siren moving toward the stationary observer will have a _____ pitch than when it is travelling away from the same observer.
A. Higher
B. Lower
C. Louder
D. Softer
E. Faster

Answer 1

In physics, the sensation of frequency of the sound waves is commonly known as pitch of the sound and it’s directly proportional to the frequency of sound waves. A higher frequency sound will have more pitch than the wave having low frequency.

Complete step by step answer:
Let us examine the apparent frequency for stationary observers and will determine the pitch.Suppose the original frequency of the siren of police car is denoted by $\nu$ and the apparent frequency for the observer when police car is moving away from the observer denoted as $\nu '$ which is given as:
$\nu ' = \nu (\dfrac{{{V_{sound}}}}{{{V_{sound}} + {V_{source}}}})$
Where, ${V_{sound}}$ Is the speed of sound in air and ${V_{source}}$ is the speed of the source which is the police car in the given question.

Now, calculating the apparent frequency when source moves towards the stationary observer:
$\nu '' = \nu (\dfrac{{{V_{sound}}}}{{{V_{sound}} - {V_{source}}}})$
Now, we can see that Denominator in second case is less than in first case, which shows
$\nu '' > \nu '$
So, the frequency of the siren approaching the stationary observer is greater than the frequency of moving away from the siren.

Hence, the correct option is A.

Note: It should be remembered that pitch is just a representation of how high or low the frequency of sound wave is and frequency of sound wave for a relative observer is also affected by speed of wind when this speed of wind is comparative to the speed of source and speed of sound waves.