bends sound waves
Technology Research News
While a lot of research has gone into figuring
out how to manipulate lightwaves for applications like optical communications,
there's been substantially less attention paid to sound waves, which work
on the same principle but are much longer.
The visible and infrared light wavelengths used in optical communications
are around 500 nanometers long, while ultrasonic waves measure about a
millimeter. Sound waves audible to the human ear range from about 22 millimeters
to about 22 meters.
Researchers from Spain and Mexico have built a device that bends ultrasound
waves as much as 90 degrees. The manipulation is an early step in a field
that may one day produce devices that manipulate sound in much the same
way mirrors and lenses manipulate light. The researchers' methods may
also translate to the realm of light, where they have potential to increase
the control of lightwaves in optics devices.
The researchers' ultrasonic twinned crystal device is made of a composite
material -- 2-centimeter-thick aluminum alloy patterned with cylindrical
holes filled with mercury.
This phononic material guides sound waves in the same way that electronic
materials guide electrons, and photonic materials guide photons. "The
composite materials used in these experiments are the phononic equivalent
of an insulator," said José-Luis Aragón, a researcher at the National
University of Mexico's Laboratory of Applied Physics.
To make the device bend sound waves, the researchers oriented three squares,
or domains of the material, at 45 degree angles to each other. At certain
frequencies, "waves traveling along one domain are capable of penetrating
the second domain," essentially bending at the angle the domains are oriented
to each other, said Aragón.
The principle is the same as lightwaves bending, or refracting when they
travel from air to water, producing the familiar bent-drinking-straw effect.
"It is an extreme refraction phenomenon," Aragón said.
The effect is caused by the interference of sound waves scattering off
the mercury rods. This interference happens at different frequencies depending
on the direction of the wave because the spacing between the rods is different
at different angles.
The key to the researchers' approach is it does not require a full bandgap.
A phononic bandgap prevents a certain frequency of sound waves from passing
through a material in any direction. Previous attempts to control the
direction of ultrasonic waves propagating through a material were generally
focused on using structural defects to form full bandgaps in the materials,
a difficult prospect.
The research is "an interesting example of how composite... phononic materials
can be constructed with unusual wave properties," said John Page, a professor
of physics at the University of Manitoba in Canada. In order to be practical,
however, the material will have to be improved, he added. Sound attenuates,
or fades quickly "even in the easy direction. They will need to improve
this to make a useful filter out of their wedge."
The research may eventually improve photonic materials as well, because
both light and sound exhibit similar wave phenomena, Page said. "The area
where the concept illustrated by the ultrasonic wedge would be most useful
is analogous optical -- photonic -- materials, where new ways of beam
steering could have applications in photonic devices for optical communications,"
The researchers are working to improve the attenuation tendency so the
material will allow sound through without the sound waves fading so quickly,
said Aragón. "The next step is to design an ultrasonic bandgap material
with low intrinsic losses [and with it make] the acoustic equivalent of
various electronic or optical elements, such as mirrors [and] lenses,"
he said. This will take around five years, he added.
In theory, phononic materials could eventually be used in circuits: "even
switches and transistors could be fabricated," Aragón said.
Aragón's research colleagues were Manuel Torres from the Spanish Council
for Scientific Research's Institute of Applied Physics and Francisco R.
Montero de Espinoza from the Spanish Council for Scientific Research's
Institute of Acoustics. They published the research in the May 7, 2001
issue of Physical Review Letters. The research was funded by the Spanish
Ministry of Education.
Timeline: 5 years
TRN Categories: Materials Science and Engineering
Story Type: News
Related Elements: Technical paper, "Ultrasonic Wedge for
Elastic Wave Bending and Splitting without Requiring a Full Band Gap,"
Physical Review Letters, May 7, 2001.
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