  We have performed elasticity measurements in the bulk multiferroic BiFeO3
(BFO) using acoustical pump and probe spectroscopy. The sound velocities of the
(quasi)-longitudinal and of the two (quasi)-transverse acoustic waves along
three independent directions of the (110) surface have been measured. Moreover,
one surface wave and one longitudinal wave propagating perpendicular to the
surface have been detected. Based on initial input values of the six
independent Cij elastic constants determined by our density functional theory
calculations and our eleven experimental velocities, the numerical resolution
of the acoustic equations allows to determine all the Cij elastic constants of
BFO. The propagation direction dependence of volume and surface waves phase
velocities allows the unambiguously assignment of the waves, hence the
polarization of phonons.
