  Phonon mean free path is generally inferred from the measurement of thermal
conductivity and we are still lacking precise information on this quantity.
Recent advances in the field of high frequency phonons transduction using
semiconductor superlattices give the opportunity to fill this gap. We present
experimental results on the attenuation of longitudinal acoustic phonons in
GaAs in the frequency and temperature ranges 0.2-1THz and 10-80K respectively.
Surprisingly, we observe a plateau, in the frequency dependence of the
attenuation, above 0.4THz, that we ascribe to a breakdown of Herring processes.
