Sound waves are produced when vibrating objects displace the particles in a medium, transferring energy to them and causing them to oscillate. This creates regions of high and low pressure, relative to atmospheric pressure, that propagate away from the object. We refer to the regions of high pressure as compressions and the regions of low pressure as rarefactions. You hear the sound when your ears detect these variations in pressure.

The figure below shows how a sound wave is produced by a vibrating guitar string. This sound wave will have the same frequency as the oscillation of the string. When we plot pressure, relative to atmospheric pressure, against the position near the string, we see the familiar sinusoidal waveform.


Sound is a mechanical wave as it requires a medium to travel through. This means that if there are no particles present, such as in the vacuum of space, then it cannot propagate. This is unlike light, or electromagnetic waves, which can travel through a vacuum, and explains why we can see the Sun but we can’t hear it. Sound is also a longitudinal wave because the particles in the medium oscillate parallel to the direction of propagation.