Physics
Doppler effect
Why a siren goes high then low as it passes — wavelength compresses, then stretches.
When the source moves relative to the observer, the wavelength changes — that's the Doppler effect.
Try this
20
f' = f · v / (v − v_s) Hz = 531
Doppler shift (source moving)
Use minus when the source approaches (pitch rises), plus when it recedes (pitch drops). v is the wave speed in the medium.
Your turn
An ambulance siren is really 600 Hz. As it drives toward you at 30 m/s (sound 343 m/s), what frequency do you hear?
Astronomers use the light version: galaxies moving away have their light stretched to longer (redder) wavelengths — 'redshift' — which is how we know the universe is expanding.
It's why a passing siren goes higher then lower in pitch. Astronomers use the same effect to measure how fast galaxies are moving away (redshift).