Why does a sound source approaching an observer seem higher in pitch?

When a sound source moves toward an observer, the sound seems higher in pitch due to a phenomenon known as the Doppler effect.

This occurs because the relative motion between the source and the observer causes the frequency at which the sound waves reach the observer to increase. In physics, the frequency of a wave—the number of wavelengths passing a point per second—is perceived physiologically as pitch. Because the source is approaching, the waves are compressed, resulting in a higher frequency and, consequently, a higher perceived pitch.123

This effect was first described by the Austrian physicist Christian Doppler in 1842 and is commonly observed with passing sirens or moving vehicles.23