IT ALL began as a speculative idea from a bunch of undergraduates: can you
create solid, three-dimensional objects using nothing but the power of
sound?
It seems crazy, but the students at the Georgia Institute of Technology have
shown that their idea isn鈥檛 just pie in the sky鈥攁lthough they鈥檝e had to go
on a zero-gravity trip to prove it. In fact, this silly-sounding scheme has so
much potential that their sound factory will undergo trials aboard NASA鈥檚 space
shuttle next year. Its success so far has astounded everyone involved.
The initial inspiration for the students was simply that they fancied the
experience of flying on NASA鈥檚 鈥渧omit comet鈥�. Every year, NASA sets aside a
small amount of time for students to run promising experiments aboard its KC-135
plane, which flies 40 to 60 parabolic arcs in each two or three-hour flight.
Each parabola gives 30 seconds or so of microgravity, allowing scientists to run
experiments in this special environment. But to get on board, you have to come
up with a good idea for an experiment.
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鈥淭he students asked me to be their adviser for the project,鈥� says Narayan
Komerath, a professor of aerospace engineering at Georgia Tech. He told them to
find out what kinds of microgravity experiments NASA was interested in. 鈥淚
thought that once they found out what was involved, it might be the end of their
bright idea.鈥�
But two days later, the students came back with a book entitled
Opportunities for Academic Research in a Microgravity Environment. In it
were outlined the agency鈥檚 zero-g interests, and the one that caught
their eye was a project to use sound waves to suspend a small mass in a chamber.
The idea, under investigation by researchers at CalTech and NASA鈥檚 Jet
Propulsion Laboratory, was to melt a substance and then cool it while it is
floating. This would allow it to form a perfect sphere鈥攈ighly desirable
for precision manufacturing.
Such 鈥渁coustic levitation鈥� is possible because of the way sound behaves in a
closed chamber. The waves travel to the back wall, then rebound towards the
front. The returning waves overlap with incoming ones, and if the chamber is
properly 鈥渢uned鈥濃�攊f its length fits the wavelength of the sound鈥攖he
inbound and outbound waves synchronise into a 鈥渟tanding wave鈥�. This creates
areas where the sound is especially loud鈥攁nd, crucially, special points
called nodes where there is silence.
Anything caught in a node can鈥檛 move easily because the air pressure at all
surrounding points is higher. The Caltech and NASA researchers had taken great
pains to design their chambers so that, in microgravity, a particle would sit in
one particular place in the box.
But Komerath thought there might be advantages in taking a slightly different
approach. What would happen, he wondered, if you didn鈥檛 worry about isolating
just one point? His group did some calculations and worked out that if the
frequency of the sound was just right, it could set up a nodal plane, a flat,
vertical expanse of minimum air pressure stretching right across the width of
the chamber. In a concert hall, that would be a disaster: an entire row of
people would hear nothing. But in a zero-gravity box, it means that particles
would stack up into a wall. 鈥淧articles could form a sheet across the entire
chamber,鈥� Komerath says. He soon realised that you could use this method to form
solid objects, and thus the idea of 鈥渁coustic shaping鈥� was born.
Komerath says it鈥檚 surprisingly easy to do. Choosing the right sound
frequency to suit a given chamber is just a matter of using some simple maths:
the Helmholtz equation. It describes the spatial arrangement of sound waves in a
given space and allows researchers to figure out which frequencies to use in a
particular chamber to create a specific shape. 鈥淭hese are pretty simple
differential equations,鈥� Komerath explains. 鈥淢any of the solutions are published
in undergraduate textbooks.鈥�
Off the wall
The team鈥檚 calculations showed that nodal planes should form like walls
across the box at 800 and 1600 hertz, and quarter-hemispheres should appear in
the box鈥檚 corners at 1250 hertz. But they still didn鈥檛 know if their idea would
work in practice.
In the lab, things got off to a discouraging start. Komerath鈥檚 students
designed a simple acoustic-shaping box: a plastic container with a speaker from
a home stereo fixed on one wall. They poured in some polystyrene balls, each a
few millimetres across, and then pumped up the volume as loud as they could. The
balls didn鈥檛 budge.
They kept trying, destroying several speakers in the process. The loud,
shrill whistle from the box鈥攚hich was not soundproofed鈥攑roved so
unpopular in the aerospace engineering department that they were forced to do
their experiments after hours when everyone else had gone home. But still the
balls wouldn鈥檛 move.
By the time the students were ready for their first flight aboard the vomit
comet in April 1997, they had soundproofed the box. But they still hadn鈥檛
managed to create any shapes with the polystyrene balls. 鈥淧eople kept telling
them that this would never work,鈥� Komerath says.
Not exactly glowing with confidence鈥攁nd somewhat airsick鈥攖hey
cranked up the speaker just before entering the first parabola, piping a steady
800 hertz into the box at about 50 decibels. Then, a few seconds into free fall,
the polystyrene granules stood up and formed a wall halfway along the chamber
from the speaker. It was a triumph. 鈥淲hen they saw it, they were so excited that
they forgot all about being sick,鈥� Komerath recalls.
Their success has led to seven more trips aboard the airborne roller coaster,
as later undergraduates have inherited the project and taken it further. In one
flight, they tested Rice Krispies. 鈥淚t was something different from styrofoam,
allowing statistical measurement of particle properties,鈥� Komerath says. 鈥淎nd if
we ran out while we were in Houston I could always run down to the store and get
more.鈥� On a later flight, the group mixed together styrofoam, cake mix, tiny
plastic beads and other oddments.
They learned that different particles鈥攂eads and cake mix, for
example鈥攚ill intermingle rather than stay with their own kind. That might
not sound significant, but it could be a crucial bonus for making composite
materials. 鈥淭he particles jostle each other and fill in the gaps automatically,鈥�
Komerath says. 鈥淭o a large extent, it鈥檚 a self-assembling structure.鈥�
Acoustic shaping now appears to be a plausible technology. Komerath believes
there鈥檚 hardly any shape they can鈥檛 make if they use the right shape of box and
suitable frequencies. So far, Komerath鈥檚 group has refined the technique to the
point where they can form curved surfaces and cylinders. In tests back on solid
ground, the group has even succeeded in forming a 4-centimetre-high wall of
water across the box. He says there鈥檚 one drawback with this particular
experiment, though. 鈥淵ou can鈥檛 run the experiment for very long before the walls
of the box containing the experiment become too wet to see through.鈥�
But knowing that liquids respond to acoustic shaping is crucial to the
group鈥檚 next step. NASA has reserved a small corner of the space shuttle for
their experiment. In March next year, an automated test in space will hopefully
prove a big point: not only that sound can form 3D shapes, but that those shapes
can be made tough and long-lasting.
Inside a cylinder 4 centimetres in diameter and 30 centimetres long, sound
waves will shape a powdered resin into a 2.5-centimetre disc, before glue is
injected. The mixture of glue and plastic will have time to set and harden
before facing the rigours of re-entry. This will be the true test of the
technology鈥檚 potential, demonstrating whether or not space-formed materials can
hold up under physical strain. 鈥淚f that works, then we can think about using
epoxies and other more sophisticated materials,鈥� Komerath says.
He sees it as the first step toward creating permanent 鈥渟ound factories鈥� in
space. He envisions 3-metre cubic chambers, each of which could turn out a
2-metre-square panel of hardened composite in about two hours, allowing for
drying and setting.
But that鈥檚 just the start. Once we know how to whistle simple shapes out of
thin air, it may be possible to combine sound patterns in the same chamber to
form more complex objects. 鈥淚f you know how to form one shape, you can tweak the
combination of frequency and chamber shape to see what results,鈥� Komerath says.
鈥淥ur chamber today is an empty box with a speaker at one end. Eventually,
chambers may have any number of speakers and baffles placed at strategic
locations.鈥� It might eventually provide a relatively cheap way to deliver
complex, one-off or limited-edition items. The students at Georgia Tech are
speculating about making individually tailored shoes in space. More
realistically, Komerath believes aircraft panels might be the breakthrough
application.
Aerospace firms developing prototypes for jet fighters and other experimental
craft spend as much as a million dollars making a jig to turn out a precisely
shaped aerofoil or other body part. It can take up to a year, and if the design
is altered even slightly, the process must begin again. 鈥淭his is a significant
part of the cost and the lead time building new aircraft,鈥� Komerath says.
Space-based acoustic manufacturing might be in a good position to compete
here, he believes. 鈥淯sing acoustic shaping in space, we could make those parts
without a jig. We also could change them quite simply and quickly as needed.鈥�
And delivering parts from space to Earth is a relatively simple matter, he
points out. 鈥淕overnments have been bringing back camera film from spy satellites
since the 1950s,鈥� he says. 鈥淵ou parachute the cargo down to the ground and
someone goes and gets it.鈥�
With the raw materials in place鈥攂ags of resin or composite beads could
fit easily into any spare hold space aboard the shuttle鈥攖here should be
nothing to stop us creating a whole new manufacturing base out in the inky
blackness. 鈥淔or this kind of application, if you have raw materials already up
there, you can deliver complex, precision-made items on demand a lot faster than
you can build them on Earth,鈥� Komerath believes. 鈥淭his wouldn鈥檛 be cheap, but
the current way is so expensive and takes so long that we might be able to beat
it. This could be an initial practical test of the technology.鈥�
That test is probably a while off yet. The Georgia Tech group doesn鈥檛 have a
research fund at the moment, and the project is still too speculative to attract
money from aerospace companies or other potential partners. Komerath remains
hopeful, however. 鈥淭his technology will gather momentum as enough people start
to see the reasons for it,鈥� he says.
And NASA is starting to show an interest. Although the agency isn鈥檛 prepared
to bankroll more than a few additional experimental flights until the concept
has been fully proved, it has very good reasons to at least try out acoustic
shaping. In coming years more and more people are going to be living in space.
Space colonists will need building materials for their labs, offices, workshops
and living quarters. Ferrying enough building materials into orbit to fit out an
industrial village would take years of shuttle flights and a huge amount of
cash: the cost of launching things into orbit is currently more than
$10,000 a kilogram.
But instead of trying to reproduce an Earth-bound factory in space, you could
use acoustic shaping to build whatever you want. You just get some raw
materials鈥攎ined from the Moon, for example鈥攁n empty chamber not much
bigger than a bathroom, audio speakers and a computer. Humboldt Mandell, manager
of academic and community programmes in the exploration office of NASA鈥檚 Johnson
Space Center near Houston, is certainly impressed by the potential. 鈥淭here鈥檚
nothing in the laws of physics that says that acoustic shaping can鈥檛 work,鈥� he
says. It鈥檚 a sound idea, it seems. And it鈥檚 just beginning to take shape.
