The Power of Sound Sound waves in "thermoacoustic" engines and
refrigerators can replace the pistons and cranks that are
typically built into such machinery An Acoustic Laser The
transparency of this device, literal and figurative, invites
analogies with the laser. Borrowing some vocabulary from optics,
one would say that a non-equilibrium condition (corresponding to
the population inversion of electron energy levels in a laser
material) is maintained across the heated stack. The test tube
amounts to an acoustic resonator, which, like a laser cavity,
allows a standing wave to build in amplitude as energy bounces
back and forth. The open side of the test tube serves the same
function as the partially silvered mirror at the output side of
a laser. Both allow some of the energy stored within the
resonant cavity to radiate into the surrounding environment.
Although Chen's "acoustic laser" produces only about a watt of
sound power, a similar device heated by the burning of natural
gas produces in excess of 10 kilo