The effect is the change of pitch heard when the source of the sound approaches and then moves away from a stationary person.

The correct answer is Doppler effect.

Explanation:

When there is relative motion between the source and listener then there is an apparent change in the frequency of the sound wave. This phenomenon is called the Doppler effect.

Pitch depends on the frequency.

If the listener and the source are approaching each other then the frequency increases.

If the listener and the source are moving away from each other then the frequency decreases.

Therefore, the Doppler effect is the change of pitch heard when the source of the sound approaches and then moves away from a stationary person.

this is poiselle's equation from fluid mechanic in physics. mech eng

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