  A time-resolved observation of coherent interlayer longitudinal acoustic
phonons in 2$H$-MoSe$_2$ is reported. A femtosecond pump-probe technique is
used to investigate the evolution of the energy loss of these vibrational modes
in a wide selection of MoSe$_2$ flakes with different thicknesses ranging from
bilayer up to the bulk limit. By directly analysing the temporal decay of the
modes, we can clearly distinguish an abrupt crossover related to the acoustic
mean free path of the phonons in a layered system, and the constraints imposed
to the acoustic decay channels when reducing the dimensionality. Loses
intrinsic to the low dimensionality of single or few layer materials impose
critical limitations for their use in optomechanical and optoelectronic
devices.
