Illinois professor Nick Fang developed a two-dimensional acoustic cloak that makes objects in the center invisible to sonar and other ultrasound waves. Credit: L. Brian Stauffer
In one Univ.
of Illinois lab,
invisibility is a matter of now you hear it, now you don’t.
Led by mechanical science and engineering professor Nicholas
researchers have demonstrated an acoustic cloak, a technology that renders
underwater objects invisible to sonar and other ultrasound waves.
“We are not talking about science fiction. We are talking
about controlling sound waves by bending and twisting them in a designer
space,” said Fang, who also is affiliated with the Beckman Institute for
Advanced Science and Technology. “This is certainly not some trick Harry Potter
is playing with.”
While materials that can wrap sound around an object rather
than reflecting or absorbing it have been theoretically possible for a few
years, realization of the concept has been a challenge. In a paper accepted for
publication in the journal Physical
Review Letters, Fang’s team describe their working prototype, capable of
hiding an object from a broad range of sound waves.
The cloak is made of metamaterial. Fang’s team designed a
two-dimensional cylindrical cloak made of 16 concentric rings of acoustic
circuits structured to guide sound waves. Each ring has a different index of
refraction, meaning that sound waves vary their speed from the outer rings to
the inner ones.
“Basically what you are looking at is an array of cavities
that are connected by channels. The sound is going to propagate inside those
channels, and the cavities are designed to slow the waves down,” Fang said. “As
you go further inside the rings, sound waves gain faster and faster speed.”
Fang’s team designed a two-dimensional cylindrical cloak made of 16 concentric rings of acoustic circuits structured to guide sound waves. Each ring has a different index of refraction, meaning that sound waves vary their speed from the outer rings to the inner ones. Credit: L. Brian Stauffer
Since speeding up requires energy, the sound waves instead
propagate around the cloak’s outer rings, guided by the channels in the
circuits. The specially structured acoustic circuits actually bend the sound
waves to wrap them around the outer layers of the cloak.
The researchers tested their cloak’s ability to hide a steel
cylinder. They submerged the cylinder in a tank with an ultrasound source on
one side and a sensor array on the other, then placed the cylinder inside the
cloak and watched it disappear from their sonar.
Curious to see if the hidden object’s structure played a
role in the cloaking phenomenon, the researchers conducted trials with other
objects of various shapes and densities.
“The structure of what you’re trying to hide doesn’t
matter,” Fang said. “The effect is similar. After we placed the cloaked
structure around the object we wanted to hide, the scattering or shadow effect
was greatly reduced.”
An advantage of the acoustic cloak is its ability to cover a
broad range of sound wavelengths. The cloak offers acoustic invisibility to
ultrasound waves from 40 to 80 KHz, although with modification could
theoretically be tuned to cover tens of megahertz.
“This is not just a single wavelength effect. You don’t have
an invisible cloak that’s showing up just by switching the frequencies
slightly,” Fang said. “The geometry is not theoretically scaled with
wavelengths. The nice thing about the circuit element approach is that you can
scale the channels down while maintaining the same wave propagation
Next, the researchers plan to explore how the cloaking
technology could influence applications from military stealth to soundproofing
to health care. For example, ultrasound and other acoustic imaging techniques
are common in medical practice, but many things in the body can cause
interference and mar the image. A metamaterial bandage or shield could
effectively hide a troublesome area so the scanner could focus on the region of
The cloaking technology also may affect nonlinear acoustic
phenomena. One problem plaguing fast-moving underwater objects is cavitation,
or the formation and implosion of bubbles. Fang and his group believe that they
could harness their cloak’s abilities to balance energy in cavitation-causing
areas, such as the vortex around a propeller.