Doppler Effect or Doppler Shift

The change in pitch of a sound heard by a listener when the source and observer are in relative motion to each other. As the observer and sound source come together, the perceived pitch is higher than that of the source when stationary, and as they move apart it is lower.

Compare: Phasing, Sonic Boom, Tempophone.

The change in pitch is a function of the difference in relative velocity of observer and source. As the source and observer move closer, the sound waves may be thought of as being compressed together, and therefore, because the wavelength decreases, the apparent pitch rises. Similarly, as the source and observer move apart, the waves are expanded and the increased wavelength corresponds to a drop in pitch. The observed frequency f_o may be calculated from the relation:

f_o = f_s . (v - v_o) / (v - v_s)

where: v is the velocity of the sound in the medium (see speed of sound)

v_o is the velocity of the observer relative to the medium

v_s is the velocity of the source relative to the medium

f_s is the frequency of the source when stationary

For the pitch heard by a stationary listener to be a semitone higher or lower than the actual pitch, the source must be moving about 42 mph. However, since the pitch of the approaching source is a semitone higher than its actual pitch, and that of the receding source is a semitone lower, the drop in pitch as the source passes the listener will be a whole tone or major second. Compare: Glissando.

The effect is named after its discoverer, C.J. Doppler, a 19th century Austrian physicist. It is possible that the effect was first noticed with the advent of trains in Europe. In 1845, an experiment was carried out near the Dutch town of Maarsen where musically trained observers were stationed along the tracks of the Rhine Railroad and listened to trumpets being played in a passing railway car. By noting the pitch of the approaching and receding notes, the speed of the car was accurately estimated. (See J.J. Josephs, The Physics of Musical Sounds, Princeton, 1967, p. 20.)