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Cool Sounds at 200 Decibels

The loudest controlled manmade sounds in history were produced this week--not by a rock band, but by a physicist. At the Acoustical Society of America meeting, Timothy Lucas of MacroSonix Corporation in Richmond, Virginia, demonstrated a new "acoustic compressor" that uses ultraintense sound waves to do the work of a mechanical pump--for example, to inflate a balloon. The new technology may soon be used in everyday appliances, such as refrigerators and air conditioners.

The idea of the compressor is simple: You shake a can back and forth to create vibrations in the air inside. Just as a child can produce huge waves in a bathtub by sloshing back and forth at just the right rate (a phenomenon called "resonance"), the air vibrations become especially intense if the can is agitated at a certain frequency. In the case of Lucas's compressor, it vibrates about 600 times a second. But the water in the child's bathtub will splash all over the place if the waves start to crest. For acoustical engineers, the analogous problem is shock waves, which dissipate the sound energy as heat. By making his compressor just the right shape, Lucas was able to accomplish something no one else thought could be done: keep the shock waves from forming.

How loud are the resulting sounds? The human pain threshold is about 120 decibels, and a jet engine produces 150 decibels at takeoff; if you stand next to it, a sound of 165 decibels will ignite your hair. The sound waves inside Lucas's compressor are about 3000 times more powerful than that, or around 200 decibels. But because the vibrations of the can itself are much smaller than the vibrations of the air, on the outside it sounds just like an ordinary compressor. That's exactly how Lucas expects it to be used.

The intense sound waves oscillate between low and high pressure in certain regions, and these pressure differences are used to suck gas into the compressor and shoot it out at high pressure. Because some of the chemicals used to replace chlorofluorocarbons in refrigerators break down in oil, there may be a big market for Lucas's compressor, which has no moving parts inside and therefore requires no lubrication.

Other specialists in acoustics call Lucas's compressor a remarkable breakthrough. Steve Garrett, a physicist at Pennsylvania State University, compares it to the flight of the Wright brothers: "What Timothy Lucas has done is shift the debate from whether acoustic compression can be done, to who can do it better. It is now possible to use sound waves to pump significant amounts of fluid--and not just in the lab."