A source of frequency f lies between a stationary observer and a large stationary wall. The source moves towards the wall with speed u. The observer simultaneously hears the direct sound and the sound reflected from the wall. Which expression gives the beat frequency between the two received sounds?
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Question

A source of frequency f lies between a stationary observer and a large stationary wall. The source moves towards the wall with speed u. The observer simultaneously hears the direct sound and the sound reflected from the wall. Which expression gives the beat frequency between the two received sounds?

Options

G1 Physics · Accepted Answer

The source is moving away from the observer, so the directly received frequency is f₁ = f×v/(v+u). The source is moving towards the wall, so the wall receives f₁ = f×v/(v−u). The stationary wall reflects this same frequency, and the stationary observer receives fᵣ = f×v/(v−u). Therefore the beat frequency is fᵦ = fᵣ−f₁ = f×v[1/(v−u)−1/(v+u)]. Correct option: C.