Moving sourceThe thing making the sound, here a siren. It is moving to the right.
Direction of motionThe way the source is going. Waves bunch up on this side.
WavefrontOne ring of the sound, spreading from where the source was then.
Shorter wavelengthIn front the rings are close together, so the wavelength is shorter.
Longer wavelengthBehind the rings are far apart, so the wavelength is longer.
Observer in frontThis person hears a higher pitch than the siren gives out.
Observer behindThis person hears a lower pitch than the siren gives out.
Stationary sourceA source standing still. Its rings are evenly spaced, so everyone hears one pitch.
How this list was compiled: Every name and fact checked on 2026-10-01, a name kept only where two independent sources use it. (1) NASA Glenn Research Center, Doppler Effect, https://www.grc.nasa.gov/www/k-12/airplane/doppler.html (moving source; in the direction of the motion the waves bunch up and the wavelength decreases, behind they spread out and it increases; higher and lower pitch). (2) NASA Goddard Space Flight Center, Imagine the Universe: The Doppler Shift, https://imagine.gsfc.nasa.gov/features/yba/M31_velocity/spectrum/doppler_more.html (the circles are wave fronts; a siren at rest and in motion; the stationary observer in front hears a higher tone and the one behind a lower tone; the frequency heard on the truck has not changed; redshift and blueshift). (3) Wikipedia, Doppler effect, text only, https://en.wikipedia.org/wiki/Doppler_effect (source and observer; the center of each new wavefront is displaced; wavefronts bunch up in front and spread apart behind; Christian Doppler, 1842). (4) The Physics Classroom, The Doppler Effect, https://www.physicsclassroom.com/class/waves/Lesson-3/The-Doppler-Effect (moving source and observers; any type of wave). Licence: own work
The drawing: Drawn for this site on 2026-10-01 from plain geometry; nothing traced. Licence: own work
How it works
A siren moves to the right. Its sound spreads out in rings, and each ring starts from where the siren was at that moment. Tap a numbered ring to see what each part is called.
Read the full guide
Read the rings
Each ring is a wavefront. In the direction of motion the wavefronts bunch up, so the wavelength is shorter, the frequency is higher and the observer in front hears a higher pitch. Behind, the wavefronts spread out: longer wavelength, lower frequency, lower pitch. Around the stationary source the rings are evenly spaced.
Draw and label it yourself
Mark five dots in a row for where the source was, one step apart. Draw a circle round each dot, the largest round the oldest dot. Put the source just past the newest dot, add its arrow and an observer on each side, then label the parts.
Questions and answers
What is the Doppler effect?
The change in pitch you hear when a source of sound moves toward you or away from you. It is named after Christian Doppler, who described it in 1842, and it happens with any kind of wave.
Why does a siren drop in pitch as it passes?
While it comes toward you its waves are bunched up, so more reach your ear each second and the pitch is higher. Once it has passed, the waves reaching you are spread out and the pitch is lower.
Does the siren itself change its sound?
No. A person riding on the truck hears the same pitch the whole time. Only an observer the source is moving toward or away from hears a change.
Does the Doppler effect happen with light?
Yes. Light from a source moving away is shifted to longer wavelengths, called redshift. Light from a source moving toward the observer is shifted to shorter wavelengths, called blueshift.