Editing Dispersion relation (section)

===Phonons===
{{further|Phonon#Dispersion relation}}Phonons are to sound waves in a solid what photons are to light: they are the quanta that carry it. The dispersion relation of [[phonon]]s is also non-trivial and important, being directly related to the acoustic and thermal properties of a material. For most systems, the phonons can be categorized into two main types: those whose bands become zero at the center of the [[Brillouin zone]] are called [[acoustic phonon]]s, since they correspond to classical sound in the limit of long wavelengths. The others are [[optical phonon]]s, since they can be excited by electromagnetic radiation.