Application of this method requires the measurements of a reference velocity to obtain values for the acoustic dispersion.
2.
The speed of propagation of an acoustic phonon, which is also the speed of sound in the lattice, is given by the slope of the acoustic dispersion relation, ( see group velocity . ) At low values of " k " ( i . e . long wavelengths ), the dispersion relation is almost linear, and the speed of sound is approximately " ?a ", independent of the phonon frequency.
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