Advanced #32

Fast sound occurs at high frequencies and shows a discontinuity at higher pressure

At high frequencies, sound propagates faster in water, with a jump at high pressure.

Scientific Explanation

At low frequencies, sound travels through water at the familiar hydrodynamic speed of about 1500 meters per second. But at extremely high frequencies in the terahertz range, a second, much faster propagation mode appears: so-called fast sound, with speeds around 3300 meters per second.

This fast mode emerges because at terahertz frequencies, the sound waves have such short wavelengths that they directly excite individual molecules and their nearest neighbors. Propagation then no longer relies on the collective flow of the liquid but on the stiffer, short-lived hydrogen bonds — similar to what happens in a solid. The speed therefore approaches values found in ice.

Particularly remarkable is that this fast-sound mode shows a discontinuity at high pressure: the speed jumps abruptly, pointing to a structural transformation within the hydrogen-bond network. This observation supports the two-state hypothesis, which holds that water locally switches between a low-density and a high-density configuration.

Fast Sound in Water at High Frequencies Diagram showing the speed of sound in water versus frequency. At low frequencies the normal hydrodynamic speed applies. At terahertz frequencies a second, faster propagation mode appears at roughly 3300 meters per second, with a pressure-dependent discontinuity. Frequency Sound Speed (m/s) 1500 3300 GHz THz Normal Fast sound Pressure gap Fast Sound at High Frequencies in Water
Fast sound at high frequencies in water. The second mode at about 3300 meters per second indicates solid-like vibrations in the hydrogen-bond network.

Everyday Relevance

Fast sound is a purely scientific phenomenon that can only be detected with specialized techniques such as inelastic X-ray scattering or neutron scattering. It has no direct everyday relevance, but it provides deep insight into the nature of water: at the molecular level, the hydrogen-bond network behaves like a solid, even though water is macroscopically a liquid. This dual nature explains many further anomalies.