Hyperacoustic velocity of fluid hydrogen at high pressure

The hypersonic velocity has been measured in fluid hydrogen at room temperature in the pressure range 60 MPa-5 Gpa, by means of Brillouin light scattering in a large-volume optical gas cell and in a diamond anvil cell. These new data are compared with previous Brillouin results and with acoustic measurements, existing up to 2 GPa, to discuss dispersion of sound in high-pressure fluids. An accurate functional form for the fluid density as a function of pressure above 2 GPa has been derived from the present Brillouin results.