1What a note really is
Squished waves sound high, spread-out waves sound low
You only need two ideas. Watch each one:
How squished the waves are = the note
Sound is a string of waves. When the waves arrive at your ear squished close together, you hear a high note. When they arrive spread far apart, you hear a low note.
Sound has one fixed speed
Every wave travels through the air at the same speed — the siren can't make its sound go faster. It can only move itself while it keeps pumping out waves.
2Two ways a siren can sit
Parked rings vs racing rings
A siren makes the same note either way. But where it puts each wave depends on whether it's standing still or zooming:
Even rings, all around
Standing still, it drops each wave in a neat ring. Every direction is spaced the same.
Bunched in front, stretched behind
Zooming along, it chases its own waves, crowding them in front and leaving gaps behind.
3Your turn — first, a parked siren
Park the siren and pump out waves
Here the siren sits still in the middle and sends waves out in rings. Speed up how fast it pumps and watch: the rings stay even in every direction, so both ears hear the very same note.
4Now make it drive past
Send the siren racing past one ear 🚓
Same siren, same note — but now it drives along a road and one ear waits beside it. The new thing you control is its speed. Guess first, then drag it fast and watch.
Guess before you drive it
You make the siren drive past the ear fast. We know the note changes as it goes by — but which way? The very instant the siren passes the ear, does its note jump UP or drop DOWN?