Single-wave sounds streak through air.

The oddity of a lone wave cruising tirelessly and undiminished across a body of water has fascinated scientists since the phenomenon was first observed in the 1830s. In recent years, similar solitary waves of light, or solitons, have become a hot topic in optics (SN: 4/6/91, p. 223). Now, Japanese researchers report creating the first solitary pulses of low-frequency sound in air.

"This is particularly satisfying," comments Hans Christian von Baeyer of the College of William and Mary in Williamsburg, Va. Solitary waves "were found in water, much later produced in light, and now are finally observed in sound," he says.

Nobumasa Sugimoto of the University of Osaka and his colleagues generated solitary acoustic waves by forcing compressed air bursts into a narrow, 7-meter-long steel tube studded on two sides with 148 hollow, knobby protrusions. Their findings, described in the Nov. 15 PHYSICAL REVIEW LETTERS, hint at practical payoffs; such as a new means to efficiently transport heat via pipes, Sugimoto says.

"It's a clever design, and they certainly exploit it," comments Andres Larraza of the Naval Postgraduate School in Monterey, Calif.

The Japanese group made its discovery while studying ways to suppress shock waves caused by fast trains entering narrow tunnels. While baffles reduce shocks that today's 300-kilometer-per-hour trains produce, the advent of even faster bullet trains is spurring more research.

In many materials, such as water and glass fibers, disturbances contain different frequencies that travel at different speeds, causing pulses to widen and smooth out. Because every sound frequency passes through air at virtually the same speed, air pulses don't tend to become smooth. Instead, intense pressure pulses in air develop into sharp-peaked shock waves.

Solitary waves arise in materials only when sharpening and smoothing effects balance, but air lacks a smoothing influence, Sugimoto explains.

The Japanese group has been exploring a model in which a series of regularly spaced hollows attaches to a tunnel via narrow channels. Excited by a wave in the tunnel, air jiggles back and forth inside these side structures at certain characteristic, or resonant, frequencies.

Sugimoto previously theorized that the resonating cavities could either dampen a tunnel shock wave or convert it into a solitary wave. Which effect takes place would depend on the dimensions--and therefore the natural frequencies--of the cavities. His team reports that the wave shapes and speeds measured in the recent experiments are in "good agreement" with predictions of a solitary wave.

Larraza says he finds the data "convincing." However, he notes that the pulse wavelengths are a sizable fraction of the length of the tube, so the team has yet to show that such waves can propagate very far.