Acoustic circulator stops sound reversal

When Baltimore residents think of a circulator, their minds probably jump to buses moving through the Inner Harbor. When Austin residents think of a circulator, they may think of something a bit more high tech. Scientists at the University of Texas at Austin recently developed an acoustic circulator that acts like one-way glass for sound.

Physical waves such as sound waves travel in two directions. This property, called time reversal symmetry, dictates that a wave moving in one direction can return in the opposite direction. Practically, this means that if you can hear or see someone, that person can also hear or see you with the correct technology and method of observation.

However, this property can be disrupted in the case of radio waves: Scientists have long discovered methods to split a radio signal and avoid having it bounce back. Using magnets, researchers create interference with a unidirectional filter that prevents outbound radio waves from returning to their source, dramatically increasing clarity of signal and making radio transmission of sound and data the ubiquitous technology that it is today.

The acoustic circulator operates on a similar principle. Because sound waves travel through air, manipulation of air pressure and current can make it difficult for sound to reverse and travel in the opposite direction. To test the idea, researchers Andrea Alù, an electrical engineer at the University of Texas at Austin, and Romain Fleury, a Ph.D. student working with Alù, developed a three-pronged, sealed box. Inside the box, a set of fans and CPUs allowed Alù and Fleury to manipulate any sound that entered the box. Each of the prongs had a microphone to pick up sound, as well as a small input port.

Initially, sound input at one port was equally received by the other two and bounced back to its initial port. When the device was activated, however, sound only traveled from port one, to two, to three, and back again, essentially requiring an input at a given port for anything to be heard at the following stage. This forced the sound waves to travel in one direction around the ring. By disabling one stage, it was possible to skip some of the ports, a promising development for sound insulation and transmission.

While practical applications for the device in spying techniques are obvious, the technology also has promising applications in conventional sound equipment such as amplifiers, microphones, acoustic-based computer sensing and noise-cancelling technologies. The latter development could be of great use for residents near highways or at airports.

In terms of future research, this acoustic circulator may spawn similar physical exploits to manipulate light to screen objects from sight and effectively render them invisible. Such light developments could also create novel light displays.

The developers of the original acoustic circulator are currently working on a new design that removes the need to include as many, or any, fans. They hope to regulate the sound transmission entirely by resonance and pressure. While commonplace use of acoustic circulators may be some years away, the potential for a quieter tomorrow is certainly exciting.