When sound waves approach us, they can appear to be compressed or "bunched up" due to a phenomenon called Doppler effect. The Doppler effect describes the perceived change in frequency of a wave when there is relative motion between the source of the wave and the observer.
If a sound source is moving towards an observer, the sound waves emitted by the source get compressed, leading to an increase in the perceived frequency of the sound. This is known as a "blue shift" because it is analogous to the shift towards higher frequencies in the visible light spectrum. As a result, the sound appears to have a higher pitch.
The compression of sound waves occurs because the motion of the source towards the observer causes the distance between successive wave crests to decrease. As a result, the waves appear closer together, creating the perception of compression or "bunching up."
Conversely, if a sound source is moving away from an observer, the sound waves get stretched out, leading to a decrease in the perceived frequency of the sound. This is known as a "red shift" because it is analogous to the shift towards lower frequencies in the visible light spectrum. The stretching of the waves occurs because the motion of the source away from the observer increases the distance between successive wave crests, making the waves appear further apart.
The Doppler effect is a fundamental principle in understanding the perceived change in frequency of waves due to relative motion. It is applicable not only to sound waves but also to other types of waves, such as light waves.