The physics of sound

Sound is a mechanical wave: a disturbance that travels through a material medium as a pattern of compressions and rarefactions. When an object vibrates, it pushes on the surrounding air (or water, or solid), creating regions of higher and lower pressure that move outward. These pressure waves are longitudinal, meaning the medium moves back and forth along the same direction the wave travels. Because sound needs a medium to carry it, it cannot travel through a vacuum.

The speed of sound depends on the medium and its condition rather than on the loudness of the sound. In air at ordinary temperatures it travels at roughly 340 metres per second, faster when the air is warmer, and considerably faster in water and faster still in most solids, because these transmit the disturbance more readily. The frequency of the wave determines pitch, while its amplitude determines loudness. Frequencies above the range of human hearing are called ultrasonics and are used in applications such as medical imaging and cleaning.

Several familiar effects follow from the wave nature of sound. An echo is sound reflected from a surface and heard again after a delay, which is the basis of sonar and echolocation. The Doppler effect is the change in perceived pitch when a source and listener move relative to one another, raising the pitch as they approach and lowering it as they separate. When a source moves faster than the speed of sound, the pressure waves pile up into a shock front that is heard as a sonic boom. The study of all these phenomena, including how sound behaves in rooms and materials, is the field of acoustics.

Frequently asked questions

Why can't sound travel through a vacuum?

Sound is a vibration carried by a material medium such as air, water or a solid. With no particles to compress and move, as in a vacuum, there is nothing to carry the wave.

What is the Doppler effect?

It is the change in the pitch of a sound caused by relative motion between the source and the listener: the pitch rises as they approach and falls as they move apart.

What causes a sonic boom?

When an object moves faster than the speed of sound, the pressure waves it creates pile up into a shock front, which is heard as a sudden loud boom.