sound wave

Every sound you hear, a voice, a drumbeat, a slammed door, reaches you as a sound wave: a travelling squeeze-and-stretch of the air. Clap your hands and you briefly shove the nearby air, and that push races outward until it eventually wiggles your eardrum. The question it answers is: what actually travels from a source to your ear?

A sound wave is a longitudinal mechanical wave, a pattern of compressions (higher pressure, bunched-up molecules) and rarefactions (lower pressure, spread-out molecules) that propagates through a medium. The molecules only jostle back and forth about their rest spots, and it is the pressure pattern that travels, at the speed of sound. Its frequency sets the pitch (fast wiggles mean high pitch) and its amplitude sets the loudness. Humans hear roughly 20 Hz to 20,000 Hz.

Because it is mechanical, sound needs a medium, air, water, or steel, and cannot cross a vacuum, which is why space is silent. It travels through liquids and solids too, usually faster than through air. An honest note: drawing a sound wave as a smooth sideways squiggle is a graph of pressure versus position, not a picture of the air moving up and down, the air actually moves back and forth along the direction the wave travels.