Offset Doppler

12PHYS - Wave Systems

Finn Le Sueur

2024

Mahi Tuatahi

A fire engine travelling at a speed of \(40ms^{-1}\) sounds a siren of frequency \(500Hz\). The speed of sound is \(330ms^{-1}\). Calculate the frequency of the observe when:

  1. The fire engine is moving directly towards the observer.
  2. The fire engine is moving directly away from the observer.
  3. Sketch your values onto a rough graph.

Answers

Doppler Skill 5: Offset Doppler Effect

  • When the moving object is not directly in line with the observe, we can call this the “offset” or “indirect” Doppler effect.
  • Note: The graph is not symmetrical around 440Hz.

Close To The Observer

  • When an object passes close by, the velocity of the object relative to the observer does not change until it is very near the observer.
  • This has the effect of the frequency changing very rapidly from high to low.

Far From The Observer

  • The further the moving object is from the observer, the more gradually the frequency changes from high to low as it passes by.
  • This is due to the effect of trigonometry on the velocity of the object.

Pātai

Textbook Activity 6A Q5 – focus on question c.