Washington DC

SOUND waves crammed with 1600 times more energy than ever before have been bottled up by physicists in Virginia. They say the achievement, announced last week at the Acoustical Society of America's meeting in San Diego, will lead to powerful machinery driven by sound.

Waves of sound behave in a similar way to waves on the beach. When a wave contains too much energy it becomes too high and breaks. With sound, the equivalent phenomenon is the formation of a shock wave, in which the energy spreads over a wide band of frequencies and is lost as heat. Some energy from a sound wave leaks into waves of higher frequencies, called overtones. A shock wave is thought to form when these waves pile up at the same place, creating a sudden change in pressure.

Tim Lucas and his colleagues at MacroSonix, a company in Richmond, Virginia, realised that they could prevent this happening by tuning a cavity to damp down the overtones. "You can control the shape of the wave with the shape of the chamber," says Lucas. Using a cavity shaped liked an elongated pear, he has achieved spectacular success. When this cavity is vibrated so that its walls move back and forth over a distance of about 100 micrometres, it resonates with a smooth, shockless wave of huge energy.

"It's the equivalent of shaking a dishpan full of water and getting waves 40 centimetres tall," says Gregory Swift of the Los Alamos National Laboratory in New Mexico. "I've never seen anything quite this high before, and I'm quite a fan of high-amplitude waves."

MacroSonix is now developing sound-driven machines, including an acoustic piston that acts as a compressor. "We have reached the power levels of machines of the industrial revolution," says Lucas. "Now we can replace those machines— and their moving parts—with sound waves."