A team of researchers at the University of Texas At Austin's Cockrell School of Engineering has effectively disproved the adage that, “if you can hear you can be heard” by creating the world's first one-way acoustic circulator. The simple, compact device, which controls the direction of sound waves, allows the user to hear without being heard.

There is a fundamental underpinning of symmetry called "time reversal symmetry" in the transfer of acoustic waves through the medium of air between two points in space. If a sound wave goes in one direction it can also travel in the other – hence, if you can hear, you can always be heard. That is until now.

Andrea Alù, the engineer who led the project, stated that "using the proposed concept, we were able to create one-way communication for sound traveling through air." The device itself takes the form of a circulator.

The traditional definition of an electronic circulator is a three-port device through which microwaves/radio signals are sent consecutively from one port to another. When one of the ports of the circulator was left unused, it had the effect of working as an isolator, preventing the signals from heading back the way they came.

Professor Alù and his team exploited this design, applying it to the medium of sound and creating the first non reciprocal sound circulator. In the words of Romain Fleury, a PhD student who worked with professor Alù on the project, the device "can transmit acoustic waves in one direction but block them from another."

The sound circulator is comprised of a resonant ring cavity in which three computer fans were placed with the purpose of circulating the air at a predetermined velocity. The cavity is connected to three ports, each housing a microphone to record sound. Audio is then transmitted to one port (port one) with the fans switched off. The sound then splits symmetrically in both directions to port two and three.

When the fans are turned on and an airflow is created, the sound can only pass from port one to port two, leaving the third port inactive. As is the case with a standard electrical circulator, the third port then acts as a block, meaning that the sound can only move in one direction: from port one to two, from port two to three and from port three to port one, but not in the opposite direction. Thus the standard symmetrical model of sound moving between two points in space is broken.

The team believes that the relatively simplistic design could be highly adaptable, and could be scaled to different acoustic frequencies. There is a plethora of potential applications for the technology, the most obvious being in the field of surveillance. Further research and testing has the potential to create what is, in the words of Preston Wilson, associate professor in the Department of Acoustics, an "acoustical version of one-way glass."

It's safe to say that the research was undertaken with such applications in mind, being funded by the US government's Defense Threat Reduction Agency and the Air Force Office of Scientific Research. However, the technology has potential applications beyond the remit of surveillance, and could lead to advances in noise control, such as that employed in noise canceling headphones, and will likely have far-reaching applications in a wide variety of fields.

The team is now working to advance their original design by attempting to create a circulator that does not require moving parts whilst also attempting to apply the concept to other mediums, such as light.

The team's research is detailed in a paper published in the journal Science.