One-way sound transmission system allows for sound control, energy-harvesting

Researchers at the California Institute of Technology (Caltech) have created the first tunable acoustic diode — a device that allows acoustic information to travel only in one direction, at controllable frequencies.

The researchers used experiments, simulations, and analytical predictions to demonstrate 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, the researchers said. This enables 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.

To obtain a sharp transition between transmitting and non-transmitting states, 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. 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.

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. Its application could reach far beyond soundproofing, the researchers said.

Potential uses include architectural acoustics for sound control within buildings, biomedical ultrasound devices, advanced noise control, and thermal materials aimed at temperature control.

“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.