Acoustic phonons provide one of the most well known examples of the elementary excitations that occur in solids.
We describe experimental techniques that allow us to generate and detect narrow bandwidth (nonequilibrium) populations of acoustic phonons and explain how we can use these techniques to perform phonon spectroscopy in manner which is analogous to time resolved laser spectroscopy.
We investigate various processes by which a nonequilibrium acoustic phonon population can relax and return to equilibrium including anharmonic relaxation, inelastic scattering by electronic states and the interaction with thermal phonons.
We compare our results to existing theories of phonon relaxation and
finally describe new techniques and and directions.