Doppler Effect

Source races right: waves bunch up ahead (higher pitch), stretch out behind (lower pitch); the listener hears the pitch drop at the pass.

What it is

A moving sound source raises pitch as it approaches you and drops it as it leaves.

Key facts

• Speed of sound in air = 343 m/s at 20 degrees C (changes ~0.6 m/s per degree C)
• Observed frequency: f_observed = f_source x (v + v_listener) / (v - v_source); v = speed of sound, v_source = source speed toward you, v_listener = listener speed toward source
• Approaching source: waves compressed = WAVELENGTH SHORTER = pitch HIGHER. Receding: stretched = LOWER
• Pitch shift only happens while source OR listener moves; at rest = zero shift
• Car at 100 km/h (27.8 m/s) shifts a 1000 Hz horn to ~1088 Hz coming, ~926 Hz going = ~162 Hz drop (about a minor-third)
• 1 semitone = ~5.95% frequency change; an octave = 2x frequency = 12 semitones
• Wavelength: lambda = v / f, so a 1000 Hz wave = 0.343 m long in air
• Leslie rotary speaker: 'chorale' slow ~40 RPM (~0.67 Hz), 'tremolo' fast ~340-400 RPM (~5.7-6.7 Hz) horn rotation
• Doppler depends on RELATIVE motion; source and listener moving together at same speed = no shift
• Same principle scales to light: radar/redshift use c = 299,792,458 m/s

How it works

1. A source emits sound waves at a fixed rate (its frequency).
2. Moving toward you, each new wavefront starts closer than the last.
3. Waves bunch up in front: shorter wavelength, more per second = higher pitch.
4. Waves spread out behind: longer wavelength, fewer per second = lower pitch.
5. At the pass, pitch slides from high to low.
6. Bigger source speed = bigger shift; standing still = no shift.

Real examples

• Ambulance siren: high whine approaching, sudden droop to a lower tone as it passes.
• F1 car or motorbike flyby: the classic 'neeeeeyowwwm' as it screams past.
• Leslie rotary speaker on a Hammond organ: spinning horn smears pitch for that swirling wobble.
• Train horn at a level crossing dropping a musical third as it rushes by.
• A flyby whoosh in a film SFX, faked with a downward pitch sweep on a synth.

How it helps in live sound

• Use a Leslie/rotary sim plug-in (slow ~0.7 Hz or fast ~6 Hz) on organ, guitar or vocals for swirl.
• Patch a Doppler/'flyby' plug-in (Waves Doppler, GRM) for pass-by SFX in theatre and immersive shows.
• Automate a pitch-shifter sweep up-then-down to fake a vehicle flyby with no source moving.
• For surround/spatial: pair Doppler pitch with an L-R pan and a volume rise-fall for a convincing pass.
• Real room Doppler is tiny: a handheld mic walked across stage shifts pitch far less than 1 semitone, ignore it.
• Mic a spinning Leslie's horn and drum separately for true stereo movement.