In calculating the heat capacity of a solid due to the phonons in the low temperature limit, I am given the impression that the idea is to calculate the amount of standing wave modes available for the phonons in the solid. Is this the correct idea? But then in calculating the Debye temperature my book says: "for n primitive cells the number of acoustic phonon modes is n." What does it mean by this, what are acoustic phonon modes - are they different standing wave modes of the acoustical branch?