Breaking through the acoustic shock barrier

A sonic boom produced by a jet aircraft can shatter windows and flatten structures. Such a shock wave, whether generated in the open air or inside a closed, gas-filled tube, represents a significant concentration of acoustic energy.

 Nonetheless, the formation of a shock front also marks a limit on the amount of energy that can be pumped into a sound wave. Energy added to a shock wave would dissipate without increasing the wave's peak pressure.

 It is possible, however, to evade that limit by generating sound waves inside specially shaped cavities that prevent the formation of shock fronts, says Timothy S. Lucas of MacroSonix Corp. in Richmond, Va.

 Lucas and his coworkers have developed cavity resonators within which standing sound waves of extremely high energy can be produced. The gas pressure inside such a resonator can reach hundreds of pounds per square inch, making the technology useful for compressing gases and other industrial processes.

 For a long time, it was widely considered impossible to achieve such high-energy densities and acoustic pressures, Lucas says. "Our technology unlocks the power of sound."

 The researchers described their work this week at an Acoustical Society of America meeting held in San Diego.

"The key is the shape of the resonator," says Gregory W. Swift of the Los Alamos (N.M.) National Laboratory. At high energies, shock waves form within cylindrical tubes but not inside tapered, streamlined cavities of just the right geometry.

 Simply driving the new resonator back and forth at a frequency that depends on the size of the cavity and the type of gas it contains will produce a high-energy sound wave. As the oscillating pressure inside the cavity increases, gas molecules speed up, eventually traveling at about one-half the speed of sound.

 The extreme pressure fluctuations in the resonator are analogous to surface waves on an imaginary lake, 1 kilometer deep, that shoot to a height of several kilometers and dip down to within a few hundred meters of the lake bottom, Swift says. Conversational sound, in contrast, corresponds to water waves about 1 millimeter high, and sound painful to the human ear is analogous to a wave height of 10 centimeters to 1 meter.

 The new technology, known as resonant macrosonic synthesis, would offer a number of advantages if used in acoustic compressors for household refrigerators, in small turbines for generating electricity, and in chambers for separating, agglomerating, levitating, mixing, or pulverizing materials, Lucas says.

 An acoustic compressor, for instance, eliminates the need for moving parts, such as pistons, connecting rods, crankshafts, and bearings, and thus for lubricating oil, he notes. An appliance manufacturer is already working with the technology to develop reliable, durable, energy-efficient refrigerators and air conditioners.

"You can use any refrigerant you want," Lucas says. "Because we have a simple, empty cavity, you don't have to worry about chemical incompatibility between the lubricating oil and the refrigerant."

 Several scientific questions warrant further investigation. Lucas and his coworkers want to understand more fully the turbulence that accompanies the waves inside a resonator. "A large part of the dissipated energy goes into turbulence," Lucas says. "As we learn how to reduce turbulence, the energy efficiency of the machine would increase, perhaps double."

 The researchers would also like to develop more accurate models of the acoustic effects that occur inside a resonator.

"This has turned out to be very interesting scientifically," Swift comments. "It's great stuff."