Making waves with sound velocity in water.

The speed of sound in water is about 1,500 meters per second. When water freezes, this speed rises to about 4,000 meters per second. This behavior exemplifies the general rule that, for any given material, sound waves travel faster in the solid phase than in the liquid.

However, this rule doesn't necessarily hold for all wavelengths, reports Francesco Sette of the European Synchrotron Radiation Facility in Grenoble, France. His team has discovered that sound waves with wavelengths between 0.5 and 3 nanometers travel at the same velocity in both water and ice.

"These results show that, despite the fundamental structural and dynamical differences between water and ice, the dynamical response of the two phases is strikingly similar at very short wavelengths," the researchers conclude in the Feb. 8 Nature. These wavelengths are roughly equivalent to the spacing between molecules in the material.

The passage of a sound wave through a solid or liquid involves the cyclic displacement of atoms or molecules from their normal positions. As the wave travels through the material, particles get pushed closer together, then farther apart.

For wavelengths considerably longer than the interparticle spacing, the speed at which a sound wave moves depends, in part, on the orderliness of the material. A high degree of order, as in a crystalline solid, allows the wave to travel more quickly than in the same material's more disorganized liquid form.

At shorter wavelengths, this long-range order has less impact on the wave's speed. Instead, microscopic processes such as bonding between molecules and factors such as molecular structure would probably have an enhanced influence on wave propagation.

Sette and his coworkers used X rays to generate short-wavelength sound waves in water at 4#161#C and ice at -20#161#C. By precisely measuring changes in the wavelengths of reflected X rays, they could determine how much energy went into the materials and then deduce the characteristic velocity of any high-frequency, collective excitations resulting from the interaction. From these measurements, the researchers concluded that sound waves with wavelengths less than 3 nm travel through both water and ice at 3,200 meters per second. This value is more than twice the speed of long-wavelength sound in water but less than its speed in ice.

The findings suggest that, for these particular excitations, the interactions between particles in both water and ice have strikingly similar characteristics.

"This result, so far, is specific to water," the researchers caution, "and it will be of great interest to investigate the behavior of other liquids."