Fig. 3.

The bulk water isothermal compressibility χ_T = χ_T(Q, ω) evaluated according to Eq. 2 from the relaxing sound velocities. Shown are the compressibility values corresponding to the “fast” sound v_hf measured in the Brillouin, IXS, and INS (open squares) scattering techniques together with those corresponding to the US and acoustical levitation (AL) data (open circles), and the hydrodynamic values gathered from thermodynamic measurements (solid line) (25). Note that for the temperatures studied these low-frequency data coincide within experimental error, although a small rounding off in the AL data are seen at the lowest temperatures (T < 245 K). The T behavior of “normal” sound data measured in inelastic scattering experiments at the intermediate frequency regime (BS and IUVS), are reported as triangles (the BS data) and diamonds (the IUVS data).