When it comes to the sound-proofing of buildings, most people likely think of using materials that simply absorb the sound waves in a noisy room, so they can't proceed into a neighboring quiet room. Researchers at the California Institute of Technology (Caltech), however, are taking a different approach. They have created something known as an acoustic diode, that only allows sound traveling through it to go in one direction. If incorporated into building materials, such diodes would let sound travel from the quiet room to the noisy one, but would simply block noise transmission in the opposite direction.

The acoustic diode works much like a traditional electrical diode, which lets electrical currents pass in one direction, but keeps them from traveling back. In this case, sound waves are taking the place of electrical currents.

"We exploited a physical mechanism that causes a sharp transition between transmitting and nontransmitting states of the diode," said Chiara Daraio, professor of aeronautics and applied physics. "Using experiments, simulations, and analytical predictions, we demonstrated the one-way transmission of sound in an audible frequency range for the first time."

The diode contains an assembly of elastic spheres, made from granular crystals that transmit sound vibrations. The properties of these crystals can be tuned, allowing them to process a wide range of frequencies. They can also "downshift" the frequency of sounds, allowing them to pass through in the one direction, but at a lower frequency. It has even been proposed that this feature could be useful for energy harvesting, wherein sound waves would be converted to a frequency at which they could be converted into electricity.

Although other scientists have created similar systems in the past, those reportedly all lacked the distinct transition between transmitting and nontransmitting states - something that the Caltech team claim is necessary for optimum control of sound waves.

The research was recently published in the journal Nature Materials.