= Acoustic phonon
{title2=$\omega_{\boldsymbol q}\to0\text{ as }q\to0$}
An acoustic phonon is a quantum of vibration on an <acoustic phonon branch>. At long wavelengths the atoms in each primitive cell move together and translational invariance makes the frequency approach zero. A three-dimensional crystal has three acoustic branches, corresponding to longitudinal and transverse polarizations. A linear small-wavevector dispersion has slope equal to the appropriate <sound speed>.
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