Diamond has a Debye temperature of Dt = 2000 K and a density of 3500 kg/m3. The distance between nearest neighbors is 0.15 nm. Determine the velocity of sound using the Debye approximation. I have no idea where to even start with this question. Most books dont even mention the Debye approx. and all the others have very little or are so confusing!
Refer http://en.wikipedia.org/wiki/Debye_model There is a formula in there, that connects all the given parameters with the sound velocity. You can also follow the derivation. Of course to understand more, you might have to refer to some book such as Kittel.