The speed of sound may exhibit a minimum

Scientific Explanation

While the speed of sound in water shows a temperature maximum (anomaly 30), it can also exhibit a minimum under certain pressure conditions. At high pressures, the interplay between density and compressibility shifts so that the speed of sound first decreases, reaches a low point, and then rises again.

In a normal liquid, one would expect the speed of sound to increase monotonically with rising pressure, since the medium becomes denser. In water, however, two opposing effects overlap: pressure breaks up the open hydrogen-bond structure, which initially reduces local order and can increase compressibility. Only at still higher pressures does pure densification dominate, and the speed of sound rises again.

This minimum occurs primarily at low temperatures and intermediate pressures, where the structural conflict between the two forms of liquid water is most pronounced.

Everyday Relevance

In the deep ocean, where both pressure and temperature influence sound propagation, this minimum can contribute to the formation of sound channels. In such a channel, sound is trapped like light in a waveguide: waves that stray upward or downward are bent back by the increasing speed of sound. These channels make it possible to transmit underwater signals over enormous distances.