The Doppler Effect and Shock Waves
Motion of the source or the observer changes the observed frequency, with the two cases giving different formulas that agree at low speed. When the source outruns its own waves the fronts pile into a cone.
What a learner can do afterwards
- Applies the correct Doppler expression for a moving source and for a moving observer
- Explains why the two cases differ and when the difference matters
- Relates the angle of a shock cone to the speed of the source
1 · Read
A siren rolling toward you bunches its crests ahead, so you meet a shorter wavelength and a higher pitch. Rolling away stretches the crests and drops the pitch. A moving source rewrites the wavelength in the air, while a moving listener only changes how fast crests are met.
The two cases wear different formulas. At walking pace they nearly agree, and the gap only matters once speeds grow comparable to the wave speed.
A siren drives past you standing still. The note sounds high on approach and low after passing, even though the siren itself never changes.
Outrun your own waves and the crests pile into a trailing cone, called a shock wave. The cone half angle follows sine of theta equals wave speed over source speed, so twice as fast means sine of one half. The boom trails continuously and greets each ground point as it sweeps past.
Motion squeezes or stretches what you hear, and supersonic motion piles crests into a cone.
2 · Watch
Take it off screen
Where it sits
Learn first
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.