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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’t just pie in the sky—although they’ve 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’s 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’s “vomit comet”. 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.

“The students asked me to be their adviser for the project,” 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. “I
thought that once they found out what was involved, it might be the end of their
bright idea.”

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’s 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’s 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—highly desirable
for precision manufacturing.

Such “acoustic levitation” 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 “tuned”—if its length fits the wavelength of the sound—the
inbound and outbound waves synchronise into a “standing wave”. This creates
areas where the sound is especially loud—and, crucially, special points
called nodes where there is silence.

Anything caught in a node can’t 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’t 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. “Particles could form a sheet across the entire
chamber,” Komerath says. He soon realised that you could use this method to form
solid objects, and thus the idea of “acoustic shaping” was born.

Komerath says it’s 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. “These are pretty simple
differential equations,” Komerath explains. “Many of the solutions are published
in undergraduate textbooks.”

Off the wall

The team’s 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’s corners at 1250 hertz. But they still didn’t know if their idea would
work in practice.

In the lab, things got off to a discouraging start. Komerath’s 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’t budge.

They kept trying, destroying several speakers in the process. The loud,
shrill whistle from the box—which was not soundproofed—proved 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’t 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’t
managed to create any shapes with the polystyrene balls. “People kept telling
them that this would never work,” Komerath says.

Not exactly glowing with confidence—and somewhat airsick—they
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. “When they saw it, they were so excited that
they forgot all about being sick,” 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. “It was something different from styrofoam,
allowing statistical measurement of particle properties,” Komerath says. “And if
we ran out while we were in Houston I could always run down to the store and get
more.” On a later flight, the group mixed together styrofoam, cake mix, tiny
plastic beads and other oddments.

They learned that different particles—beads and cake mix, for
example—will intermingle rather than stay with their own kind. That might
not sound significant, but it could be a crucial bonus for making composite
materials. “The particles jostle each other and fill in the gaps automatically,”
Komerath says. “To a large extent, it’s a self-assembling structure.”

Acoustic shaping now appears to be a plausible technology. Komerath believes
there’s hardly any shape they can’t make if they use the right shape of box and
suitable frequencies. So far, Komerath’s 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’s one drawback with this particular
experiment, though. “You can’t run the experiment for very long before the walls
of the box containing the experiment become too wet to see through.”

But knowing that liquids respond to acoustic shaping is crucial to the
group’s 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’s potential, demonstrating whether or not space-formed materials can
hold up under physical strain. “If that works, then we can think about using
epoxies and other more sophisticated materials,” Komerath says.

He sees it as the first step toward creating permanent “sound factories” 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’s 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. “If you know how to form one shape, you can tweak the
combination of frequency and chamber shape to see what results,” Komerath says.
“Our 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.” 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. “This is a significant
part of the cost and the lead time building new aircraft,” Komerath says.

Space-based acoustic manufacturing might be in a good position to compete
here, he believes. “Using acoustic shaping in space, we could make those parts
without a jig. We also could change them quite simply and quickly as needed.”
And delivering parts from space to Earth is a relatively simple matter, he
points out. “Governments have been bringing back camera film from spy satellites
since the 1950s,” he says. “You parachute the cargo down to the ground and
someone goes and gets it.”

With the raw materials in place—bags of resin or composite beads could
fit easily into any spare hold space aboard the shuttle—there should be
nothing to stop us creating a whole new manufacturing base out in the inky
blackness. “For 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,” Komerath believes. “This wouldn’t 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.”

That test is probably a while off yet. The Georgia Tech group doesn’t 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. “This technology will gather momentum as enough people start
to see the reasons for it,” he says.

And NASA is starting to show an interest. Although the agency isn’t 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—mined from the Moon, for example—an 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’s Johnson
Space Center near Houston, is certainly impressed by the potential. “There’s
nothing in the laws of physics that says that acoustic shaping can’t work,” he
says. It’s a sound idea, it seems. And it’s just beginning to take shape.