What phenomenon describes the change in pitch of a sound as it approaches and moves away from a source?

The phenomenon that describes the change in pitch of a sound as it approaches and then moves away from a source is known as the Doppler effect. This effect occurs due to the relative motion between a sound source and an observer. When the sound source moves towards the observer, the sound waves are compressed, leading to a higher pitch. Conversely, as the sound source moves away from the observer, the sound waves are elongated, resulting in a lower pitch. This principle can be observed in everyday situations, such as the changing pitch of a passing siren from an ambulance or a police car.

In contrast, reflection refers to the bouncing back of sound waves when they encounter a surface, which does not involve any change in pitch due to motion. Refraction is the bending of waves as they pass through different media, and while it can affect sound, it does not inherently involve changes in pitch caused by relative motion. Interference occurs when two or more sound waves overlap, leading to areas of constructive or destructive interference, but it does not encompass the concept of changing pitch due to movement. Thus, the Doppler effect is the correct phenomenon in this context.