14. Doppler Effect for Sound

The Doppler effect (named after the Austrian physicist Christian Doppler, who first described it in 1842) refers to the apparent change in the frequency (and hence the perceived pitch) of a sound wave, as observed by a listener, when there is relative motion between the source of the sound and the listener (observer). It is important to note that the Doppler effect is a change only in the apparent frequency as perceived by the observer — the actual frequency at which the source is vibrating and emitting sound does not change at all; what changes is how many wave crests (or compressions) reach the observer's ear per second, due to the relative motion.

When a sound source is moving towards a stationary listener, successive sound waves emitted by the source are 'crowded together' (compressed) in the direction of motion, since each successive wave is emitted from a position slightly closer to the listener than the previous one; this effectively shortens the wavelength and increases the frequency as perceived by the listener, so the listener hears a higher pitch than the sound source's true, actual frequency. Conversely, when a sound source is moving away from a stationary listener, successive waves are 'stretched apart' (spread out) in the direction of motion, effectively lengthening the wavelength and decreasing the perceived frequency, so the listener hears a lower pitch than the source's true frequency. The Doppler effect also occurs (with the same qualitative outcome, though the underlying reason is slightly different) when the listener moves towards or away from a stationary source, or when both the source and listener are in relative motion.

The classic everyday illustration of the Doppler effect is the changing pitch of a passing ambulance, police car, or train's horn/siren: as the vehicle approaches a stationary observer standing by the road, its siren is heard at a noticeably higher pitch than its true pitch; the instant it passes and begins moving away, the perceived pitch drops abruptly to a noticeably lower pitch — this characteristic 'high pitch approaching, sudden drop to low pitch receding' pattern is a direct and very commonly experienced consequence of the Doppler effect. The Doppler effect has several important practical applications: Doppler radar/SONAR (used by traffic police to measure a vehicle's speed, and in weather radar to track the motion and intensity of storms and precipitation), Doppler echocardiography and Doppler ultrasound in medicine (used to measure the speed and direction of blood flow in blood vessels and within the heart, by measuring the frequency shift of reflected ultrasonic waves off moving red blood cells), and, notably, in astronomy — an analogous Doppler effect occurring with light waves (rather than sound waves) from distant stars and galaxies (observed as a 'redshift', a shift towards longer, redder wavelengths) provided crucial evidence that most galaxies are receding from us, supporting the theory that the universe is expanding.