The Doppler effect

In short: When an ambulance with its siren on drives towards us, the sound is higher; as it drives away, it is lower. This change in pitch caused by movement is called the Doppler effect.

In more detail: When a sound source moves towards us, the sound waves in front of it are squeezed together: the wavelength gets shorter and the frequency higher. Behind the source the waves are stretched and the sound becomes lower. The effect is named after the Austrian physicist Christian Doppler.

In detail

• Everyday example: Sirens, passing trains and racing cars show the effect clearly. The jump in pitch happens at exactly the moment the source passes us.
• The listener counts too: The effect also occurs when the listener moves and the source stands still. What matters is the movement relative to each other and to the air.
• Strength: The faster the source is compared with the speed of sound, the more the pitch changes.
• Sonic boom: When an aircraft flies faster than sound, the waves can no longer get ahead of it. They pile up into a shock wave that is heard on the ground as a bang.
• Radar: Police speed guns use the effect: radio waves reflected from a moving car come back with a changed frequency, which gives its speed.
• Medicine: In Doppler ultrasound, scanners measure how fast and in which direction blood flows through the vessels.
• Weather: Doppler weather radars detect how raindrops move and so help to spot wind and rotation in thunderstorms.
• Light from stars: Light shows the effect too. If a galaxy moves away, its light is shifted towards red; if it approaches, towards blue.
• Connections: The effect is based on the properties of waves (schall-und-dezibel, echo-und-schallgeschwindigkeit, urknall-und-expansion-des-universums).