All sounds are vibrations traveling through the air as sound waves. Sound waves are
caused by the vibrations of objects and radiate outward from their source in all directions.
A vibrating object compresses the surrounding air molecules (squeezing them closer
together) and then rarefies them (pulling them farther apart). Although the fluctuations
in air pressure travel outward from the object, the air molecules themselves stay in the
same average position. As sound travels, it reflects off objects in its path, creating further
disturbances in the surrounding air. When these changes in air pressure vibrate your
eardrum, nerve signals are sent to your brain and are interpreted as sound.

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Audio Fundamentals