  We used optical laser pulses to create transient gratings (TGs) with sub-10
{\mu}m spatial periodicity in a Bismuth Germanate (310) (Bi4Ge3O12) single
crystal at room temperature. The TG launches phonon modes, whose dynamics were
revealed via forward diffraction of a third, time-delayed, heterodyne-detected
optical pulse. Acoustic oscillations have been clearly identified in a
time-frequency window not covered by previous spectroscopic studies and their
characteristic dynamic parameters have been measured as a function of
transferred momenta magnitude and direction.
