An ambulance siren emits a frequency of 600 Hz while approaching a stationary observer at a speed of 30 m/s. What frequency does the observer perceive? (Speed of sound in air = 330 m/s)
Answer and explanation
Correct answer: B. 660 Hz
- Step 1: Doppler effect for approaching source: f' = f (v / (v - v_s)). Step 2. f' = 600 (330 / (330 - 30)) = 600 (330 / 300) = 600 1.10 = 660 Hz. Answer: 660 Hz. Why not the others: 550 Hz is for a receding source (v / (v + v_s) = 330/360); 720 Hz uses v = 300.
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