What phenomenon occurs when the source of sound waves or the listener is moving?

The phenomenon that occurs when the source of sound waves or the listener is moving is known as the Doppler effect. This effect results in a change in frequency and wavelength of the waves in relation to an observer moving relative to the source of the waves. When the source of sound is approaching the observer, the sound waves are compressed, leading to a higher frequency, which is perceived as a higher pitch. Conversely, when the source is moving away from the observer, the sound waves are stretched, resulting in a lower frequency and a lower pitch.

In practical terms, this is why a car engine sounds different as it approaches you and then passes by; the pitch changes due to the relative motion between the car (the source of sound) and the observer (you). The Doppler effect is not limited to sound waves; it also applies to electromagnetic waves, including light.

Other phenomena listed, such as refraction, involves the bending of waves as they pass from one medium to another; resonance refers to the amplification that occurs when the frequency of a forced vibration matches the natural frequency of an object; and interference relates to the way waves can combine to produce regions of greater or lesser intensity when they overlap. None of these phenomena specifically address the changes in frequency