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Switch-up may lead to one-way sound

CALTECH (US) — A tunable diode that allows acoustic information to travel in one direction only may bring the concept of a true sound-proof room a step closer to reality.

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 California Institute of Technology (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.”

To understand the technology, Daraio says, imagine two rooms labeled room A and room B. The new technology would allow someone in room A to hear sound coming from room B; but 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 co-author of the study, published in the journal Nature Materials.

The system is based on a simple assembly of elastic spheres—granular crystals that transmit the sound vibrations—that could easily be used in multiple settings, tuned, and potentially be scaled to operate within a wide range of frequencies, meaning its application could reach far beyond soundproofing.

Similar systems have been demonstrated, but 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, meaning the system is 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,” Daraio 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.”

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 potential application to energy-harvesting systems and believes the systems may be applicable to a range of technologies including biomedical ultrasound devices, advanced noise control, and thermal materials aimed at temperature control.

“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.

The research was supported by the National Science Foundation, the Office of Naval Research, and the A. S. Onassis Benefit Foundation.

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