The movement of air molecules brought about by a source vibration is called sound.

Here's how it works:

1. Vibration: A sound source, like a speaker or a musical instrument, vibrates.

2. Compression and Rarefaction: The vibration causes the air molecules near the source to compress (crowd together) and then expand (spread out) in a repeating pattern. This creates areas of high pressure (compressions) and low pressure (rarefactions) in the air.

3. Wave Propagation: These compressions and rarefactions travel outward from the source as a wave, much like ripples spreading across a pond.

4. Transmission: The wave carries energy through the air, causing the air molecules to vibrate as it passes.

5. Perception: When this wave reaches our ears, the vibrations of the air molecules cause our eardrums to vibrate, which our brains interpret as sound.

So, in essence, sound is the propagation of vibrations through a medium, like air, in the form of compressions and rarefactions.