FORGET ultrasonic probes: to work out the internal structure of an object,
all you need do is listen to its own vibrations. The new technique could be a
boon for everything from microchip manufacture to seismology.
In traditional ultrasound scans, a transducer sends sound waves into an
object. These reflect off irregularities inside the object, and the transducer
detects the returning waves. By analysing these reflections with a computer, it
is possible to reconstruct an image of the structure within the object鈥攁
fetus inside its mother鈥檚 womb, for example.
But sound waves aren鈥檛 necessary, say mechanics researchers Richard Weaver
and Oleg Lobkis of the University of Illinois at Urbana-Champaign in a paper to
be published in Physical Review Letters. You just have to listen to
tiny heat-carrying vibrations, called phonons, that course through every
object.
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Using ordinary transducers to record the vibrations, Weaver and Lobkis have
shown that the technique works with pieces of aluminium. The results have
astonished other researchers, says Dale Chimenti, a physicist at Iowa State
University in Ames. Chimenti likens the listener to a person standing on the rim
of a canyon: 鈥淚f he listens closely enough and has the proper signal processing,
he can hear the echo without having shouted.鈥�
The trick is to look for order in the apparently random noise, Weaver says.
Like sound waves, phonons reflect off irregularities and echo throughout the
object. So the noise patterns are likely to repeat themselves, Weaver says. 鈥淭he
transducer notices that the stuff that comes in at one time looks a lot like the
stuff coming in a certain time later,鈥� he says. To reveal the patterns, the
researchers used a computer to compare a recording of the noise with another
segment that started playing after a slight delay. As they varied the delay, any
matches, or 鈥渆choes鈥�, jumped out.
The new technique probably won鈥檛 replace conventional ultrasound imaging in
most medical applications, Weaver says, because sifting through lots of noise
takes longer than picking up a simple echo. It could, however, prove useful when
sound sources are scarce, he says, such as at the very high frequencies needed
to detect the tiny structures within microchips.
Slightly modified, the technique might also change the way seismologists
study the Earth, says Bart van Tiggelen, a physicist at the University of
Grenoble in France. At the moment, geophysicists only gain useful information
about the Earth鈥檚 interior when an earthquake or an explosive charge produces
detectable seismic waves. They might get the same data from the noise recorded
by their idling seismographs鈥� signals they currently throw away鈥攙an
Tiggelen says.
The new technique already reveals a connection between noise and structure
that could have been discovered decades ago but was simply overlooked, Chimenti
says. 鈥淲e鈥檙e all trained to think of noise as random,鈥� he says. 鈥淏ut it鈥檚 not
补濒飞补测蝉.鈥�