Columbia Engineering researchers, working with colleagues at Disney
Research and MIT, have developed a new method to control sound waves, using a
computational approach to inversely design acoustic filters that can fit within
an arbitrary 3D shape while achieving target sound filtering properties. Led by
Computer Science Professor Changxi Zheng, the team designed acoustic voxels,
small, hollow, cube-shaped chambers through which sound enters and exits, as a
modular system. Like Legos, the voxels can be connected to form an infinitely
adjustable, complex structure. Because of their internal chambers, they can
modify the acoustic filtering property of the structure—changing
their number and size or how they connect alters the acoustic result.
"In the past, people have explored
computational design of specific products, like a certain type of muffler or a
particular shape of trumpet," says Zheng, whose team is presenting their
paper, "Acoustic Voxels: Computational Optimization of Modular Acoustic
Filters," at SIGGRAPH 2016 on July 27. "The general approach to
manipulating sound waves has been
to computationally design chamber shapes. Our algorithm enables new designs of
noise mufflers, hearing aids, wind instruments, and more - we can now make them
in any shape we want, even a 3D-printed toy hippopotamus that sounds like a
trumpet."
He adds, "We also have proposed a
very intriguing new way to use acoustic filters: we can use our acoustic voxels
as acoustic tags, unique to each piece we 3D print, and encode information in
them. This is similar to QR codes or RFIDs, and opens the door to encoding
product and copyright information in 3D printing."
Last year, Zheng's team used
computational methods to design and 3D-print a zoolophone, a xylophone-type
instrument with keys in the shape of zoo animals. The zoolophone represented
fundamental research into vibrational sound control, leveraging the complex
relationships between an object's geometry and the surface vibrational sounds
it produces when struck.
In this new study, Zheng's team came up
with a computational approach that would enable better design for manipulating
acoustic propagation of many products, such as automobile mufflers and
instruments.
"With 3D printers today, geometric
complexity is no longer a barrier. Even complex shapes can be fabricated with
very little effort," Zheng notes. "So the question is: can we use
complex shapes to improve acoustic properties of products?"
They proposed using acoustic voxels, single, modular acoustic filter shapes, whose acoustic filtering behavior can be precomputed using numerical simulation. They developed a new algorithm that allowed them to assemble the acoustic voxels—like Lego bricks— into complex structures to produce the targeted acoustic filtering properties.
The creation of acoustic voxels has
also led Zheng's team in a completely new direction: acoustic tagging to
uniquely identify a 3D-printed object and acoustic encoding to implant
information (like a copyright) into an object's very form. Acoustic filters
work by manipulating sound waves; acoustic voxels have given the team a way to
exactly control that manipulation. A unique voxel assembly produces a unique
acoustic signature. Two objects may have the exact same exterior appearance,
but if their hollow interiors contain different voxel assemblies, each object,
when filtering a sound wave, produces a sound unique to that object. The
researchers recorded the sound made by objects with different voxel assemblies
and used an iPhone app they created to accurately identify each object.
Acoustic tagging could be a valuable
complement to QR codes and RFID tags, both of which entail operations entirely
separate from manufacturing. If fabricators can build ID information directly
into the object, they will save the time, effort, and expense of individually
labeling parts, especially useful when building larger structures from many
separate pieces. Acoustic tagging could also encode copyrighted originals, such
as 3D-printed figures from individual artists like Jeff Koons or companies like
Disney or Marvel.
Zheng's current acoustic voxels project
is for fabricating sizable objects producing audible sounds, and his team has
been able to demonstrate how information and identification can be embedded
into the acoustics of an object, requiring no additional procedures or labor
after fabrication. They are looking ahead to how they might use acoustic voxels
to computationally control ultrasound waves. Says Zheng, "We are
investigating some of the intriguing possibilities of ultrasonic manipulation,
such as cloaking. Then sound propagation can be distorted to hide objects from
sound waves. This could lead to new designs of sonar systems or underwater
communication systems. It's an exciting area to explore."
The work was funded in part by the
National Science Foundation and Adobe.
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