The Doppler Effect

Longitudinal Waves

A stationary sound source

Source moving with vsource < vsound Mach 0.7 - subsonic                                      

Source moving with vsource = vsound ( Mach 1 - breaking the sound barrier )                                      

The condition will last for only an instant, and once supersonic flow exists completely around the aircraft, sharp-angled sonic cones replace the normal shock waves.

Source moving with vsource > vsound (Mach 1.4 - supersonic)                                      

A bullet travelling at Mach 2. 45 A bullet travelling at Mach 2.45. The mach cone and shock wavefronts are very noticeable.

In which direction is the source of the wave moving?

If the source is moving towards you, you see a higher frequency, a higher pitched sound, or a color closer to the violet end of the spectrum. If the source is moving away from you, you see a lower frequency, a lower pitched sound, or a color closer to the red end of the spectrum.

How do we know stars are moving? By the Doppler effect.

Which way is this star rotating?

Is this star moving away from you or towards you?

Radar The Doppler effect is also used in some forms of radar to measure the velocity of detected objects. A radar beam is fired at a moving target—a car, for example, as radar is often used by police to detect speeding motorists

One of the most important applications of the Doppler Effect is Doppler Radar. In this technology, one uses a microwave transmitter (radar) and measures the frequency of the microwave radiation reflected off objects. From the frequency shift between the emitted and received electromagnetic waves, one can calculate the velocity of the object.

Medical imaging and blood flow measurement An echocardiogram can, within certain limits, produce accurate assessment of the direction of blood flow and the velocity of blood at any point using the Doppler effect. One of the limitations is that the ultrasound beam should be as parallel to the blood flow as possible. Although "Doppler" has become synonymous with "velocity measurement" in medical imaging, in many cases it is not the frequency shift (Doppler shift) of the received signal that is measured, but the phase shift (when the received signal arrives).

Underwater acoustics In military applications the Doppler shift of a target is used to ascertain the speed of a submarine