Which effect describes the change in frequency or pitch when a sound source moves relative to the listener?

The correct answer is the Doppler Effect, which describes the change in frequency or pitch of a sound as it moves relative to an observer. This phenomenon occurs when a sound source, such as a passing ambulance or train, moves towards the listener, causing the sound waves to compress and produce a higher pitch. Conversely, as the sound source moves away, the waves are stretched out, resulting in a lower pitch.

This effect is commonly experienced in everyday life and is also utilized in various fields, such as radar and astronomy, to measure speeds of objects. Understanding the Doppler Effect is fundamental in the study of wave phenomena and sound mechanics, making it a crucial concept in fields related to physics and engineering.

The other options describe different acoustic phenomena that do not involve the relative motion of the sound source and observer in the same manner. For instance, interference refers to the interaction of different sound waves, harmonic effects relate to the overtones produced by vibrating bodies, and echoes are the reflected sound waves heard after the original sound. Thus, the Doppler Effect stands out clearly for its specific relation to moving sound sources.