A sound source of frequency f moves directly towards a stationary observer with speed uₛ through still air. The speed of sound is v, with uₛ < v. Which expression gives the observed frequency?
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Question

A sound source of frequency f moves directly towards a stationary observer with speed uₛ through still air. The speed of sound is v, with uₛ < v. Which expression gives the observed frequency?

G1 Physics · Accepted Answer

During one source period, T = 1/f, the wavefront moves a distance v/f, while the source moves towards the observer by uₛ/f. Therefore the wavelength in front of the moving source is λ₁ = (v−uₛ)/f. The stationary observer receives frequency f' = v/λ₁ = f×v/(v−uₛ). Correct option: B.