You have heard this a thousand times: a siren approaches at a high pitch, passes, and suddenly drops. The siren never changed. What changed is how often its wavefronts reach you.
Doppler Effect
See the wavefronts bunch up and hear the pitch changeWhat is really happening
A moving source emits each wave crest from a slightly different position. Ahead of it, the source has moved toward the previous crest before emitting the next one, so the crests are crowded together — a shorter wavelength and therefore a higher frequency. Behind it the opposite happens.
Approaching: f′ = f · c / (c − v)
Receding: f′ = f · c / (c + v)
where c is the speed of sound (about 343 m/s in air) and v is the speed of the source. Notice the asymmetry: the pitch rise when approaching is larger than the drop when receding, because of the way the source speed enters the denominator.
Try this
- Set the speed to 0. The wavefronts are perfect concentric circles and both readouts match the source frequency.
- Raise the speed and watch the circles crowd together on the right while they spread out on the left.
- Push it to 150 m/s. The approaching pitch is now well over 700 Hz against a 440 Hz source — a huge shift.
- Turn the sound on and compare the approaching and receding frequencies in the readout.
Beyond sound
The Doppler effect applies to any wave, including light. For light it is measured as a shift in colour rather than pitch — and it is one of astronomy’s most important tools. Light from galaxies moving away from us is shifted toward the red end of the spectrum, and measuring that redshift is how we know the universe is expanding.
The same principle underpins radar speed guns, weather radar that tracks storm rotation, and medical ultrasound that measures blood flow.