What phenomenon describes how sound changes as the source moves toward or away from an observer?

The phenomenon that describes how sound changes as the source moves toward or away from an observer is known as the Doppler Effect. This effect explains the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source.

When a sound source approaches an observer, the sound waves are compressed, leading to a higher frequency or a "higher pitch." Conversely, as the sound source moves away from the observer, the sound waves are stretched, resulting in a lower frequency or "lower pitch." This effect is commonly experienced in everyday life, such as when a police siren passes by; the pitch changes as the vehicle approaches and then recedes.

In contrast, Bernoulli's Principle pertains to fluid dynamics and describes the behavior of fluid flow rather than sound waves. Refraction involves the bending of waves as they pass from one medium to another and also does not directly relate to the motion of the source of sound. Wave interference involves the interaction of multiple waves but does not specifically address the changes in sound frequency associated with relative motion between a source and an observer.