Cool sound

FEARS of ozone layer depletion have inspired research into refrigeration technologies that do not depend on ozone-destroying chemicals and which may even be more efficient than the conventional coolants.

The Los Alamos National Laboratory and the Naval Postgraduate School at California,. USA, have jointly developed thermoacoustic devices that chill nitrogen to its liquefaction temperature of -195.5' C. The sound energy is blasted into a kind of organ pipe to achieve cooling. A loudspeaker at the end of a tube blasts the very loud note to resonate inside a gasfilled tube to form what is called an acoustic "standing wave" in which individual molecules of gas are in rapid oscillating motion while the wave as a whole does not move at all.

These gas molecules move back and forth at the same frequency as that of the loudspeaker at the end of the pipe, and as they move they act as microscope workers carrying heat away from the tl@iinner regions of the standing wave towards its thickest bulge, its antinode. A metal plate within this antinode absorbs heat from the gas molecules.

Another technology, also developed by the Los Alamos laboratory, subjects liquids in place of the conventional gases used in refrigeration to cycles of compression and expansion to divert heat.