A source emitting moves at straight towards a large flat wall. A stationary listener stands behind the source, on the line from the wall through the source. The speed of sound is . How many beats per second does the listener hear between the direct sound and the sound reflected from the wall?

Answer and explanation

Correct answer: C. 59.0 Hz

  1. Step 1: Calculate the frequency of direct sound heard by the listener. Because the source moves away from the listener behind it, the observed direct frequency is .
  2. Step 2: Calculate the frequency received at the wall. The source approaches the stationary wall, so the wall receives frequency .
  3. Step 3: The fixed flat wall reflects this sound without changing frequency, and the stationary listener receives it as .
  4. Step 4: Compute the beat frequency heard between the two sound waves: .
Answer
59.0 Hz.
Why not the others: and consider only one Doppler shift instead of the frequency difference between both waves.

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