One-way mirrors and one-way streets both permit flow in just one direction, but it is difficult to create something that permits sound or other waves to ripple in one direction and not the opposite. This is because of a fundamental property known as time-reversal symmetry. It usually makes no difference from what direction a sound is created—if you hear, you can be heard.

In January electrical engineer Andrea Alù and his colleagues at the University of Texas at Austin published results on a device that could make one-way sound transmission practical. Called an acoustic circulator, the device is analogous to isolators used in telecommunications and radar, which restrict the flow of microwaves and radio waves to one direction. In an isolator, electromagnetic waves pass through a material that has a magnetic field applied to it. Traveling through this magnetically altered material breaks the time-reversal symmetry of the wave.

To mimic the effect with sound, Alù and his colleagues installed three tiny fans in a resonant metal ring; the fans blow air (the medium that transmits sound in the device) through the ring at a speed matched to the frequency of the sound. The ring is connected to three equally spaced ports that can carry sound waves in and out of the ring. When the fans are off, sound from one port will flow to both other ports with equal strength. But when the fans are turned on, the airflow interrupts the time-reversal symmetry of the sound waves passing through it. The result: nearly all sound ripples to one receiving port and not the others, in a direction counter to the airflow.

Using off-the-shelf components, the Texas investigators' acoustic oscillator suppressed the amount of sound traveling in the undesired direction by 10,000. The findings were detailed in the January 31 issue of Science. “They used a very clever idea to make something that had never been made before,” says electrical engineer Steven Cummer of Duke University, who did not take part in this research. Cummer notes that this device works only for very specific sound frequencies and that future work might focus on controlling wider ranges of frequencies.

Alù and his associates are now also pursuing a device for one-way sound transmission that has no moving parts. The finding could lead to new kinds of soundproofing, noise control and sonar.In addition, further studies could lead to new methods for manipulating light and radio waves, Alù says.