Taking the Shock out of Sound Waves

Spotted 150 years ago on the waters of a canal and today routinely generated in light-carrying fibers, the solitary, long-lasting waves called solitons have now been seen in yet another medium: sound. In the 15 November Physical Review Letters, researchers describe how they produced solitons in an air-filled tube in a way that could prove useful for taming unwanted shock waves from machinery.

Most waves quickly dissipate into a train of smaller waves, but in 1834 a British naval architect, John Scott Russell, was the first to spot an exception: a lasting solitary pulse of water racing away from the prow of a boat on the Edinburgh-Glasgow Canal. Such water solitons survive because the wave speed varies with frequency in a way that prevents the pulse from spreading apart. Optical fibers can also host solitons, but air--the usual medium for sound waves--behaves in just the wrong way to host a soliton. The speed of sound in air varies with intensity, not frequency, which causes blasts of sound to pile up into shock waves.

To try to change that, Nobumassa Sugimoto of the University of Osaka in Japan and his colleagues took a steel tube, 7.4 meters long and 8 centimeters in diameter, and grafted onto it about 100 so-called Helmholtz resonators--small tubes connected to spherical cavities that resonate at particular frequencies. They sent sound pulses through the large tube, using microphones to track how the sound propagated. The pulses kept their smooth profile, without forming shock waves. The Helmholtz resonators seemed to make the tube strongly dependent on frequency--more like a canal--creating the right conditions for solitons.

Solitons could never survive in the open air, notes acoustics researcher Junru Wu of the University of Vermont, Burlington. But a similar technique might squelch shock waves generated in exhaust pipes, says Wu. And like their optical counterparts, acoustic solitons might also transmit data in future acoustic communications systems, he adds.