So I was part of the team working on electromagnetic metamaterials and cloaking with John Pendry
and David Smith, and the natural question that came out of that work was can you do
the same kinds of tricks to control other kinds of waves.
And soundwaves was a natural second choice to try and look at.
So I started sitting down with pencil and paper and working through equations for about
six months with lots of dead ends, and finally came to the conclusion and found a way to
make that trick work, and showed that you can also control sound waves if you can create
the right material properties.
Now that we have the capability of controlling sound with acoustic metamaterials, there are
a lot of possible applications I think that we can explore.
So this is an example of a sound-controlling metamaterial that we've built.
And if you look at the internal structure here, you can get some idea of how it actually
works.
At the outer edge here, there's not very much material, and the soundwave is essentially
free to travel like it would through air.
But if you move toward the middle of this structure you see that we have pieces that
force the soundwaves into longer and longer spiral paths, and that has the effect of slowing
down the soundwave as it travels through that structure, which ends up having exactly the
same impact as if the soundwave is traveling through a more dense, heavier fluid.
We're hoping to scale this down to make it work at ultrasound frequencies, kind of like
the backup bumper sensors on modern cars.
This is a retroreflector, so what it does is no matter what direction the soundwave
comes in, it reflects back in the same direction.
So something like this attached to an object would make it much, much easier to detect
using ultrasound sensors.
One area where we're working on is soundproofing or sound absorption.
The materials that are used right now are pretty big and not easy to deploy and absorb
essentially all frequencies of sound, all pitches.
With metamaterials we have the ability to design much more compact, thinner structures
that absorb only the frequencies or pitches that we want to absorb, leaving the rest of
the sound untouched or unaltered.
Another example where we can use acoustic metamaterials in the future is in medical
ultrasound imaging.
Those modern systems are really capable, but they require a lot of electric power and a
lot of computer power in order to create the real-time images that are used for diagnostic
purposes.
If we can integrate acoustic metamaterials into a system like that, creating things like
ultrasound lenses, then we can form the image without all that backing computer power and
create a medical ultrasound system that is a lot smaller, cheaper and much lower power.
Our first 3D proof of concept with acoustic metamaterials was a three-dimensional sound
cloaking device.
This is a material that could be put over an object on a sound-reflecting surface, and
when that reflecting object is covered by this cloaking shell, you can't detect the
object by incident sound waves and reflection like through sonar.
Most people are familiar with holograms for light - the kinds of small holograms you see
on credit cards or the more complicated museum-style holograms where there are surfaces that reflect
light, and the light that hits your eye makes it look like there's a three-dimensional object
there even though it's really just a surface.
That's a general trick that one can do with all kinds of waves, the key is just how do
you use a surface, a flat surface, to create a complicated three-dimensional wave field
- either light or sound.
What we did is create an acoustic metamaterial structure, like this, that we have a speaker
creating a simple sound tone that travels through this structure on one side, and what
emerges is a much more complicated sound field that, a certain distance away from the metamaterial,
takes the shape of the letter "A" in sound.
And so this is an example of how we could use metamaterial structure to create an arbitrarily
complex sound field.
And so we might be able to do something like mimic the complicated sound field produced
by a live orchestra out of a single speaker.