What is Sound wave? — Physics

Definition 21.1 High School Physics · Chapter 21 — Sound and Acoustics

A sound wave is a mechanical pressure wave (Chapter 20): a vibrating surface creates traveling compressions (slight overpressure) and rarefactions (underpressure). Air parcels oscillate along the travel direction (sound is longitudinal), and a medium is needed: a bell under a vacuum jar falls silent (Chapter 8).

A loudspeaker drives a longitudinal wave: parcels bunch into compressions one wavelength = v/f apart. — A loudspeaker drives a longitudinal wave: parcels bunch into compressions one wavelength $\lambda = v/f$ apart.

Examples

Example 21.4 (The sizes of sounds)

The relation $\lambda = v/f$ sizes any sound: in air a $20\,\mathrm{Hz}$ rumble spans $340/20 = 17\,\mathrm{m}$ — a building — and a $20\,\mathrm{kHz}$ hiss fits in $1.7\,\mathrm{cm}$ ; the metre-sized ones bend easily around doors and corners.

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Examples

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