Idea 4of 5Builds onBorn closer togetherThe medium sets the speed
Whose speed is it
A 600 Hz siren at 40 m/s. You hear?
Today you'll discover
- which speed goes where in the shift
- how much a siren's note rises on approach
- what happens as it speeds up
Think firstA 600 Hz siren, approaching at 40 m/s
A 600 Hz siren approaches at 40 m/s; sound travels at 340 m/s. What do you hear?
Predict first, then drag the speed. Watch the two readouts move apart.
A moving siren with crowded rings ahead and stretched rings behind, the spacing ahead bracketed. After a prediction, the doppler bench opens with a source-speed slider and readouts for the approaching and receding frequencies around the emitted 600 Hz. The last step doubles the siren's speed and asks for the note.
See the whole ideathe big idea and the key word, in one place
The big idea
f′ = f v ÷ (v ∓ vs): the medium's v.
Easy to get wrongThe shift is a ratio, not an addition. Doubling the source's speed more than doubles the rise, because the divisor shrinks.
About this modelEvery crest travels at the air's speed. Only where each one is BORN depends on the siren.
Every crest travels at the air's speed. Only where each one is BORN depends on the siren.
Ready to use it?
- Compute the frequency heard from an approaching or receding source.