Dana Mackenzie

The loudest controlled sounds ever made by humans were produced earlier this month--not by a rock band, but by a physicist. At the Acoustical Society of America meeting in San Diego, Timothy Lucas of MacroSonix Corp. in Richmond, Virginia, demonstrated a new "acoustic compressor" that uses ultraintense sound waves to do the work of a mechanical pump. The technology may soon be used in everyday appliances such as refrigerators and air conditioners.

Sound concept. Cycles of low and high pressure driven by sound can draw a fluid into a compressor (left) and expel it at high pressure.

TIMOTHY LUCAS ET AL./MACROSONIX CORP.

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. But the water in the child's bathtub will splash out 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--essentially that of a bowling pin--Lucas was able to keep the shock waves from forming, even as the can vibrated at about 600 times a second.

How loud are the resulting sounds? The pain threshold is about 120 decibels, and a jet engine produces 150 decibels. If you stand next to a sound of 165 decibels, it will ignite your hair. The sound waves inside Lucas's compressor are about 3000 times more powerful, or about 200 decibels. But because the can's own vibrations are much smaller than the vibrations of the air, on the outside it sounds just like an ordinary compressor.

The intense sound waves oscillate between low and high pressure in certain regions; with the help of valves that open and close at the right moments, these pressure differences can suck gas into the compressor and shoot it out at high pressure. Lucas's compressor could be especially useful for refrigerators and air conditioners, which work by compressing a refrigerant--traditionally a chlorofluorocarbon. Steve Garrett, a physicist at Pennsylvania State University in University Park, explains that some of the ozone-sparing refrigerants now being used break down in the oil that lubricates a conventional compressor. But Lucas's compressor has no moving parts inside and therefore requires no lubrication. MacroSonix has already signed a licensing agreement with an appliance manufacturer.

Other specialists in acoustics call Lucas's compressor a breakthrough. "What Timothy Lucas has done is shift the debate from whether acoustic compression can be done to who can do it better," says Garrett. Lucas himself thinks his sound waves will ultimately find many other roles. "Electromagnetic waves have been commercialized for over 100 years," he says, "but the commercial application of sound waves has only scratched the surface."

Dana Mackenzie is a science and mathematics writer in Santa Cruz, California.

Volume 278, Number 5346 Issue of 19 December 1997, p 2060
�1997 by The American Association for the Advancement of Science.