Editing Speed of sound (section)

===Compression and shear waves===
[[File:Onde compression impulsion 1d 30 petit.gif|thumb|Pressure-pulse or compression-type wave ([[longitudinal wave]]) confined to a plane. This is the only type of sound wave that travels in fluids (gases and liquids). A pressure-type wave may also travel in solids, along with other types of waves ([[transverse wave]]s, see below).|alt=|305x305px]]
[[File:Onde  cisaillement impulsion 1d 30 petit.gif|thumb|[[Transverse wave]] affecting atoms initially confined to a plane. This additional type of sound wave (additional type of elastic wave) travels only in solids, for it requires a sideways shearing motion which is supported by the presence of elasticity in the solid. The sideways shearing motion may take place in '''any''' direction which is at right-angle to the direction of wave travel (only one shear direction is shown here, at right angles to the plane). Furthermore, the right-angle shear direction may change over time and distance, resulting in different types of ''[[Polarization (waves)|polarization]]'' of shear waves.|alt=|305x305px]]

In a gas or liquid, sound consists of compression waves. In solids, waves propagate as two different types. A [[longitudinal wave]] is associated with compression and decompression in the direction of travel, and is the same process in gases and liquids, with an analogous compression-type wave in solids. Only compression waves are supported in gases and liquids. An additional type of wave, the [[transverse wave]], also called a [[shear wave]], occurs only in solids because only solids support elastic deformations. It is due to elastic deformation of the medium perpendicular to the direction of wave travel; the direction of shear-deformation is called the "[[Polarization (waves)|polarization]]" of this type of wave. In general, transverse waves occur as a pair of [[orthogonal]] polarizations.

These different waves (compression waves and the different polarizations of shear waves) may have different speeds at the same frequency. Therefore, they arrive at an observer at different times, an extreme example being an [[earthquake]], where sharp compression waves arrive first and rocking transverse waves seconds later.

The speed of a compression wave in a fluid is determined by the medium's [[compressibility]] and [[density]]. In solids, the compression waves are analogous to those in fluids, depending on compressibility and density, but with the additional factor of [[shear modulus]] which affects compression waves due to off-axis elastic energies which are able to influence effective tension and relaxation in a compression. The speed of shear waves, which can occur only in solids, is determined simply by the solid material's shear modulus and density.