The nonlinearity and asymmetry present in this chain of compressed spheres can transform vibrations of one frequency, applied at one end of the chain, to vibrations with broadband frequency content leading to rectification. The amplitude of the vibrations are shown by the height of the peaks. Image: Chiara Daraio/Caltech
While many hotel rooms, recording studios, and even some
homes are built with materials to help absorb or reflect sound, mechanisms to
truly control the direction of sound waves are still in their infancy. However,
researchers at the California Institute of Technology (Caltech) have now
created the first tunable acoustic diode—a device that allows acoustic
information to travel only in one direction, at controllable frequencies.
The mechanism they developed is outlined in a paper published
in Nature Materials.
Borrowing a concept from electronics, the acoustic diode
is a component that allows a current—in this case a sound wave—to pass in one
direction, while blocking the current in the opposite direction. “We
exploited a physical mechanism that causes a sharp transition between
transmitting and nontransmitting states of the diode,” says Chiara Daraio,
professor of aeronautics and applied physics at Caltech and lead author on the
study. “Using experiments, simulations, and analytical predictions, we
demonstrated the one-way transmission of sound in an audible frequency range
for the first time.”
This new mechanism brings the idea of true soundproofing
closer to reality. Imagine two rooms labeled room A and room B. This new
technology, Daraio explains, would enable someone in room A to hear sound
coming from room B; however, it would block the same sound in room A from being
heard in room B.
“The concept of the one-way transmission of sound
could be quite important in architectural acoustics, or the science and
engineering of sound control within buildings,” says Georgios Theocharis,
a postdoctoral scholar in Daraio’s laboratory and a coauthor of the study.
The system is based on a simple assembly of elastic
spheres—granular crystals that transmit the sound vibrations—that could be
easily used in multiple settings, can be tuned easily, and can potentially be
scaled to operate within a wide range of frequencies, meaning its application
could reach far beyond soundproofing.
Similar systems have been demonstrated by other
scientists, but they all feature smooth transitions between transmitting and
nontransmitting states instead of the sharp transitions needed to be more
effective at controlling the flow of sound waves. To obtain the sharp
transition, the team created a periodic system with a small defect that
supports this kind of quick change from an “on” to an “off” transmission state.
Daraio explains, this means the system is very sensitive to small variations of
operational conditions, like pressure and movement, making it useful in the
development of ultrasensitive acoustic sensors to detect sound waves. The
system can also operate at different frequencies of sound and is capable of
downshifting, or reducing the frequency of the traveling signals, as needed.
“We propose to use these effects to improve
energy-harvesting technologies,” she says. “For example, we may be
able to scavenge sound energy from undesired structural vibrations in machinery
by controlling the flow of sound waves away from the machinery and into a
transducer. The transducer would then convert the sound waves into
electricity.” Daraio says the technology can also shift the undesired
frequencies to a range that enables a more efficient conversion to electricity.
The team plans to continue studying the fundamental
properties of these systems, focusing on their potential application to
energy-harvesting systems. They also believe that these systems may be
applicable to a range of technologies including biomedical ultrasound devices,
advanced noise control, and even thermal materials aimed at temperature
“Because the concepts
governing wave propagation are universal to many systems, we envision that the
use of this novel way to control energy might enable the design of many
advanced thermal and acoustic materials and devices,” says Daraio.