Idea 3of 5Builds onThe medium owns the speedHow often
Born closer together
The ambulance passes, and the note falls. Why?
Today you'll discover
- what changes about the crests ahead of a moving siren
- what you hear when crests arrive bunched
- why the note drops as it passes
Think firstThe siren comes toward you. Look ahead of it
The siren moves toward you as it sounds. What are the crests ahead of it like?
Predict first, then press Next. Compare the ring spacing ahead with the spacing behind.
A moving siren with rings spreading from it, each born a little further to the right than the last, so the rings crowd together ahead and spread out behind, with a small listener at the far right. After a prediction, the spacing ahead is bracketed, then the spacing behind. The last step asks what the driver hears.
See the whole ideathe big idea and the key word, in one place
The big idea
Motion squeezes the crests ahead and stretches those behind.
Easy to get wrongThe pitch change is not the siren changing, and not the sound travelling faster. It is spacing: crests born closer together ahead, further apart behind.
Key word
- Doppler effect
- the change in the frequency received when a source and a listener move relative to each other; approaching raises it, receding lowers it
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?
- Say whether a listener hears a higher or lower pitch for a described motion, and why.