Final answer:
The change in frequency of sound waves as the source moves relative to a microphone is due to the Doppler Effect: it increases when the source moves towards the microphone and decreases when moving away.
Step-by-step explanation:
Explaining the Doppler Effect
As a source of sound moves from left to right, the frequency of the waves received by a microphone changes due to the Doppler Effect. If the source is moving towards the microphone, the wavelengths of the sound waves are compressed, leading to an increase in the observed frequency, and the sound is perceived as higher pitched. Conversely, if the source is moving away from the microphone, the wavelengths are stretched, resulting in a decrease in observed frequency, and the sound is perceived as lower pitched.
This phenomenon occurs because the motion of the source adds to or subtracts from the speed at which the sound waves reach the observer. So when the source moves towards the observer, each subsequent wave crest is emitted from a position closer to the observer than the previous crest, causing the wave crests to bunch up and the frequency to increase (higher pitch). When the source moves away, the opposite happens. The wave crests are emitted from points that are increasingly farther from the observer, leading to the wave crests spreading out and the frequency to decrease (lower pitch).
If both the source and the microphone are stationary, they observe the frequency exactly as it is emitted. However, any relative movement between them alters the frequency experienced by the microphone, with sound waves becoming either compressed or stretched depending on the direction of the source's motion.