Does there exist an anomalous sound dispersion in supercooled water?

The acoustic sound velocity of water in the supercooled regime shows peculiar behaviour. According to the available data, the sound velocity displays complex behaviour below -15 degrees C, where anomalous frequency dispersion phenomena seem to appear and need to be understood. In particular, lower-frequency data at a few tens of KHz show levelling-off of the sound velocity value below -25 degrees C, with a possible minimum around -30 degrees 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 normal dispersion behaviour. To gain new insight into this topic, we made measurements of the acoustic sound velocity in bulk water in the liquid and supercooled phase down to -29 degrees C by means of a heterodyne detected transient grating (TG) experiment at an unexplored frequency value around 100 MHz.