What effect describes the apparent change in frequency when there is relative motion between a source of sound and an observer?

The apparent change in frequency due to relative motion between a sound source and an observer is known as the Doppler effect. This phenomenon occurs when the source of sound approaches or moves away from the observer. When the source moves closer, the sound waves are compressed, resulting in a higher frequency (or pitch) being heard by the observer. Conversely, when the source moves away, the sound waves are stretched, leading to a lower frequency.

This effect is commonly experienced in everyday situations, such as when an ambulance with a siren passes by; the pitch of the siren sounds higher as it approaches and lower as it moves away. The Doppler effect is crucial in various fields, including astrophysics for determining the movement of stars and galaxies, as well as in radar and sound technologies for measuring speed.

The other concepts mentioned, such as refraction, echo, and interference, do not pertain to the change in frequency caused by relative motion but rather deal with the behavior of sound waves in different contexts.