Making rooms 'fully' soundproof

While many hotel rooms, recording studios, and even 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 was described 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 mechanism that causes a sharp transition between the transmitting and non-transmitting states,” says Chiara Daraio, professor of aeronautics and applied physics at Caltech, and lead author on the study. The system is based on a simple assembly of elastic spheres—granular crystals that transmit the sound vibrations—that may easily be used in multiple settings, and can potentially be scaled to operate within a wide range of frequencies.