Doppler effect

A change in the frequency with which sound waves from a given source reach an observer when the source is in rapid motion with respect to the observer. Approaching sounds appear higher in pitch (frequency) than sounds that are moving away. This is because the wavelengths of the sound from an approaching source are progressively foreshortened, whereas the wavelengths from a receding source are stretched. The Doppler effect is used in ultrasound scanning techniques. An emitter sends out pulses of ultrasound (inaudible highfrequency sound) of a specific frequency. When these pulses bounce off a moving object (blood flowing through a blood vessel, for example), the frequency of the echoes is changed from that of the emitted sound. A sensor detects the frequency changes and converts the data into useful information (about how fast the blood flows, for example). Doppler ultrasound techniques are also used to monitor fetal heartbeat, to detect air bubbles in dialysis and heart–lung machines, and to measure blood pressure.