Focus: The Sound of Solitary Waves

Published November 10, 1999  |  Phys. Rev. Focus 4, 24 (1999)  |  DOI: 10.1103/PhysRevFocus.4.24
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Faster than a speeding bullet? The maglev trains now under development in Japan have topped 550 km/h, and their high speeds may exacerbate the problem of tunnel noise. Research on the problem led to the demonstration of the first sound waves that travel long distances without distortion.

Communications engineers want complex light signals to travel through long fiberoptic cables without changing shape, so they have been developing optical solitary waves–waveforms that travel long distances without distortion. But solitary sound waves are thought to be very difficult to produce because the properties of air don’t seem to permit them. In the 15 November PRL a Japanese team demonstrates the first production of acoustic solitary waves in air, which they say could lead to the undistorted transmission of heat and other forms of energy, as well as the elimination of troublesome shock waves from train tunnels and air compressors.

Any large disturbance in air–like a lightening bolt or even a high-speed train entering a tunnel–normally generates a traveling sound wave that changes shape as it propagates. If it doesn’t dissipate first, the pulse eventually forms a shock wave–the thunder clap forms immediately in the case of lightening. Solitary waves in optical fibers maintain their shape because the material exhibits “dispersion,” a speed of light that depends on frequency in just the right way. But the speed of sound in air is relatively independent of frequency, so solitary sound waves have been a great challenge.

Over the last several years Nobumasa Sugimoto of the University of Osaka has developed a theory suggesting that adding extra branches to Japanese train tunnels would eliminate the formation of shocks, creating less severe waveforms instead. More recently he realized that, under more ideal conditions than those of a tunnel, the branching structure could theoretically support acoustic solitary waves.

In their latest work, Sugimoto and his colleagues demonstrated solitary waves in a steel tube 7.4 m long and 8 cm in diameter. A sudden pressure pulse at one end quickly steepened into a shock wave, but when the team added over 100 small tubes branching from the main pipe, each leading to a ping-pong-ball-sized air pocket, the pulse profile remained constant as it traveled through the pipe. Just as Sugimoto’s theory predicted, the air pockets caused the speed of sound to depend on frequency in a way that allowed the existence of solitary waves.

Sugimoto explains that several types of heavy machinery, such as compressors, generate acoustic shocks which engineers consider to be a nuisance, but suppression of them is difficult. He suggests that modifications of the branched pipe concept might help in these designs. Sugimoto also imagines “heat pipes” for transferring heat or other forms of energy waves over long distances without distortion, although he is hesitant to discuss the details of those unpublished ideas.

Jun-Ru Wu of the University of Vermont tried unsuccessfully to create acoustic solitary waves about 20 years ago, so he is impressed with the team’s accomplishment. Although he can’t name any specific applications for these waves, Wu is certain that engineers will come up with some.

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