Physics

1.14 - Sound Waves & Hearing

6 min

Sound in different materials

Changing medium

How we hearOutside your course

Range of hearingOutside your course

note 1 of 2

Sound is a longitudinal wave that needs a medium

A sound wave is produced by a vibrating object, which passes vibrations into the particles of the surrounding material.
Sound is a longitudinal wave: the particles vibrate back and forth along the same direction the wave travels.
As they vibrate, particles bunch into regions of high pressure called compressions, and spread apart into regions of low pressure called rarefactions.
The wave is this travelling pattern of compressions and rarefactions. The particles vibrate about fixed positions - it's the wave, not the material, that travels.

A row of particles with a crowded region boxed and labelled compressions and a spread-out region boxed and labelled rarefactions, above an arrow labelled direction of sound wave.

Sound needs a medium

Because it is carried by vibrating particles, sound needs a medium - a solid, a liquid, or a gas.
Sound cannot travel through a vacuum (such as space) because there are no particles to pass the vibrations on.

note 2 of 2

Speed of sound in different materials

Sound travels at different speeds in different materials: in general, it travels fastest in solids, slower in liquids, and slowest in gases.
The denser the medium, the more closely packed its particles are, so vibrations pass from particle to particle more quickly.
In air, sound travels at roughly 330 to 350 m/s, depending mainly on air temperature.

key terms

TermDefinition

CompressionA region in a longitudinal wave where the particles are squashed close together.

RarefactionA region in a longitudinal wave where the particles are spread further apart.

MediumA material that a wave travels through, which can be a solid, liquid or gas.

VacuumA completely empty space containing no particles.

end of notes

Key points to remember

Sound is a longitudinal wave that needs a medium

A sound wave is produced by a vibrating object, which passes vibrations into the particles of the surrounding material.

Sound is a longitudinal wave: the particles vibrate back and forth along the same direction the wave travels.

As they vibrate, particles bunch into regions of high pressure called compressions, and spread apart into regions of low pressure called rarefactions.

The wave is this travelling pattern of compressions and rarefactions. The particles vibrate about fixed positions - it's the wave, not the material, that travels.

Speed of sound in different materials

Sound travels at different speeds in different materials: in general, it travels fastest in solids, slower in liquids, and slowest in gases.

The denser the medium, the more closely packed its particles are, so vibrations pass from particle to particle more quickly.

In air, sound travels at roughly 330 to 350 m/s, depending mainly on air temperature.