The Physics of Sound

Sound is a mechanical wave consisting of regions of compression and rarefaction that travel through a medium such as air, water or a solid. Unlike light, sound cannot travel through a vacuum because it needs particles to transmit the vibration. As a longitudinal wave, the air molecules oscillate back and forth along the same direction in which the wave moves, alternately raising and lowering the local pressure.

The speed of sound depends on the medium and its temperature. In air at room temperature it is roughly 340 metres per second, and it travels considerably faster in liquids and faster still in many solids, because the particles there are more tightly coupled. The frequency of a sound wave determines its perceived pitch, while the amplitude relates to loudness. The human ear responds to a wide range of frequencies, and sounds above that range are called ultrasound.

The Doppler effect changes the pitch heard when a source and listener move relative to each other: an approaching siren sounds higher and a receding one lower. Resonance, standing waves in pipes and on strings, and the way sound fields spread and reflect all follow from these basic wave properties. Simulations let learners adjust frequency, source motion and medium to hear and see how each factor changes the resulting sound.

Frequently asked questions

Why can sound not travel through a vacuum?

Sound needs a material medium because it is transmitted by vibrating particles. A vacuum has no particles to carry the pressure variations, so no sound passes through it.

What makes the speed of sound change?

It depends mainly on the medium and its temperature. Sound generally travels faster in denser, stiffer media such as liquids and solids than in air, and faster in warm air than in cold.

What is the Doppler effect for sound?

It is the change in perceived pitch caused by relative motion between a sound source and a listener. The pitch rises as they approach and falls as they separate.