---
title: "The Doppler Effect and Shock Waves"
description: "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 "
canonical: https://lightmysky.com/learn/science/the-doppler-effect-and-shock-waves-mt_UVfD_coqh6
source: https://lightmysky.com/learn/science/the-doppler-effect-and-shock-waves-mt_UVfD_coqh6.md
retrieved: 2026-09-12
---

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# 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.

Subject: Science · Area: Waves, Light & Sound · Ages 18 to 20
Page: https://lightmysky.com/learn/science/the-doppler-effect-and-shock-waves-mt_UVfD_coqh6

## Ready when they can

- 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

## Lesson: Why the siren changes pitch as it passes

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.

**Example.** A siren drives past you standing still. The note sounds high on approach and low after passing, even though the siren itself never changes.

**Tip.** 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.

**Recap.** Motion squeezes or stretches what you hear, and supersonic motion piles crests into a cone.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [The Wave Equation](https://lightmysky.com/learn/science/the-wave-equation-mt_wmKhTiXt1p)

## Opens up

- [Huygens's Principle and the Laws of Reflection and Refraction](https://lightmysky.com/learn/science/huygenss-principle-and-the-laws-of-reflection-and-refraction-mt_xxdniF2__w)