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.