The screen uses slower motion than real life so the wavefronts can be seen, but every frequency and wavelength shown uses the real speed of sound (343 m/s in air). Use the Learn panel for a guided tour and a quiz.
- What is the doppler effect formula?
- For a moving source and a still listener, the observed frequency is f′ = f·v / (v − vₛ·cosθ), where f is the source frequency, v the speed of sound (343 m/s in air), vₛ the source speed and θ the angle between the source’s velocity and the line from the source to the listener. Straight ahead (θ = 0) f′ = f·v/(v − vₛ); straight behind (θ = 180°) f′ = f·v/(v + vₛ).
- Why does pitch go up as a siren approaches?
- The source moves towards the listener while it emits each wave, so the wavefronts ahead of it are squeezed closer together. Closer wavefronts mean a shorter wavelength and a higher frequency. As the siren passes, the waves behind it spread out and the pitch drops.
- What is a Mach cone?
- When a source moves faster than sound (Mach number M > 1), its wavefronts overlap along a cone behind it. The half-angle of the cone is sin α = 1 / M. Crossing the cone produces a sonic boom.
- Does the doppler effect apply to light?
- Yes. Light from a moving source is shifted in frequency too, which is how astronomers measure how fast stars and galaxies move towards or away from us. The sound formula above is for sound in air; the light formula is different.
- What does the wavelength do in front and behind a moving source?
- In front of the source the wavelength is shorter, λ = v(1 − M)/f. Behind it the wavelength is longer, λ = v(1 + M)/f. The speed of sound does not change, only the wavelength the listener receives.