Acoustic sound velocity of water in the supercooled regime shows a peculiar behaviour. Accordingly to the available data, sound velocity displays a complex behaviour below $-15$ $^\circ$C, here anomalous frequency dispersion phenomena seem to appear and need to be understood. In particular lower frequencies data at a few tens of KHz show leveling-off of the sound velocity value below -25 $^\circ$C, with a possible minimum around -30 $^\circ$C. Other data at higher frequencies, around 1 GHz, in comparison with the low frequency data, could instead suggest the existence of a "negative dispersion", yet other high frequency data show a normal dispersion behaviour. To get new insight into this topic, we made measurements of acoustic sound velocity in bulk water in the liquid and supercooled phase down to -29 $^\circ$C by means of a heterodyne detected transient grating (TG) experiment at an unexplored frequency value around 100 MHz.