513.001 Molecular and Solid State Physics

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Counting phonons

A crystal has a volumne of 1 cm³ and a sound velocity of 6000 m/s. At T = 300 K, what is the number of phonons between the frequencies 4.0 × 106 Hz and 4.1 × 106 Hz?

Why is it not necessary to know the crystal structure in this case?