A one-way road to sounds / Charles K. Choi

Engineers are about to overcome the challenge of transmitting sound waves in only one direction

One-way mirrors and one-way streets allow passage in only one direction, but it is difficult to create something that allows sound, or other waves, to move in only one direction and not in the opposite direction. This is because of a fundamental property called time-reversal symmetry. Usually, it doesn't matter which direction the sound is coming from, if you hear it, you can also be heard.

Electronics engineer Andre Alou and his colleagues from the University of Texas at Austin, published in January 2014 the results of their research on a device that might enable one-way voice transmission in a practical way. The device, known as an "acoustic circulator", is similar to isolators from the field of radio and radar communications, whose purpose is to limit the flow of radio and microwave waves to one direction. In such an insulator, the electromagnetic waves pass through a material on which a magnetic field is applied and this breaks the time reversal symmetry of the wave.

To mimic this effect in sound waves, Elo and his colleagues installed three tiny fans inside a resonant metal ring. The fans circulate air, which is the medium that transmits sound waves in the device, through the ring at a speed corresponding to the frequency of the sound. The ring is connected to three pipes that serve as openings at equal intervals, which can transmit the sounds to and from the ring. With the fans off, sound coming from one tube will reach the other two tubes with equal volume. But when the fans are turned on, the airflow breaks the time-reversal symmetry of the sound waves. As a result, the sound is transmitted almost entirely to one receiver tube and not to the others, and in the opposite direction to that of the air flow.

The Texas researchers' acoustic circulator, built from off-the-shelf products, reduces the amount of sound traveling in the unwanted direction ten thousand times. The findings were detailed in the January 31, 2014 issue of the journal Science. "They used a very sophisticated idea to do something that hadn't been done before," says electronics engineer Steve Kammer of Duke University, who was not involved in the research. Kammer adds that the device only works at very specific sound frequencies, and that future developments may focus on expanding the frequency ranges.

Elo and his colleagues are now also working on a device without moving parts for the one-way transmission of sound. The products could lead to new types of insulation and noise reduction products and sonar. In addition, Alou says, with the help of further research it will be possible to create new ways to manipulate light and radio waves.

The article was published with the permission of Scientific American Israel