I am aware that phonons are lattice vibrations - and that the amplitude of vibration would depend on the temperature. But say, at room temperature what would the order of magnitude of these lattice vibrations be ? In particular, in continuum limit these phonons can be treated as elastic waves. So if I have take an incident planar longitudinal elastic wave at a surface, what would I need to take the amplitude of this wave to be ? I do think that if I equate the energy stored in this wave integrated over all incident angles and frequencies to the heat capacity of the solid - I should be able to find the amplitude of this wave. Is this correct ? Thanks!