Image: Tess Watson/Flickr
Up until now, acoustic waves traveling between two points in space always exhibited a basic symmetry summed up with the phrase, “if you can hear, you can also be heard.” Not anymore. A team led by Andrea Alù, associate professor at UT Austin's Cockrell School of Engineering, has created a “nonreciprocal acoustic circulator”—the first of its kind. It is a major step in engineering an acoustic analog of one-way glass.
“Our device allows to transmit sound without receiving it back from the same channel,” Alù told Motherboard. “It therefore can control sound propagation in unprecedented ways.”
The device is similar in principle to electronic circulators, though it may ultimately replace them. Typically made of three ports, traditional circulators transmit messages through microwave and radio frequencies sequentially from one port to another. But when one port is removed, the chain is broken, and the system becomes an “isolator” in which signals can flow in only one direction. The receiver cannot communicate with the transmitter (or, to wax poetic, the falcon cannot hear the falconer).
To get the same result with acoustic waves, the team built a sound circulator. According to their press release, the device is “a resonant ring cavity loaded with three small computer fans that circulate the airflow at a specific velocity. The ring is connected to three ports outfitted at each end with microphones that record sound.” The team's findings will be the cover story of tomorrow's issue of Science.
When the fans were turned off, the sound signals transmitted from “Port 1” predictably split the waves symmetrically to the receiving ports, 2 and 3. But when the fans were turned on, and the airflow disrupted the ring, the symmetry was thrown out of whack. Instead of a perfect split, Port 1's sound waves went straight to Port 2, causing Port 3 to left out, acoustics-wise. The same was true when Port 2 was the transmitter—in that case, Port 3 received the full acoustic load, while Port 1 “heard” nothing. And when Port 3 was the transmitter, it was Port 2 that was left in the auditory dark.
The study's figures of the nonreciprocal transmission. Image: Science/AAAS
“It is just the right spin of fluid (air) coupled with the strong resonance of our ring cavity, which makes our design powerful,” explained Alù. “These two combined mechanisms create strong nonreciprocity in a compact device. Sound waves are routed in one direction only—always contrary to the direction of the airflow.”
That's the concept at its most basic, but the implications are huge. One of its biggest applications will be in medical diagnostics, because the device has the capacity to make acoustic imaging devices like ultrasound much more effective. “If you can transmit an acoustic wave down a path and then reroute the reflected signal towards another path, where it can be efficiently collected, within a single device, you may improve significantly the way acoustic imaging works today,” Alù told me.
This setup would be fundamentally different from existing radio-based circulators: cheaper, lighter, smaller, and more precise than existing models.
The discovery also has broader theoretical implications. “Our paper proves a new physical mechanism to break time-reversal symmetry and subsequently induce nonreciprocal transmission of waves, opening important possibilities beyond applications in acoustics,” Alù said. “Using the same concept, it may actually be possible to construct simpler, smaller, and cheaper electronic circulators and other electronic components for wireless devices, as well as to create one-way communication channels for light.”
We couldn't just let that slip by, so I followed up with Alù about what grafting this concept into the electromagnetic spectrum would look like.
“For light the concept can also be groundbreaking,” he said. “There is a lot of interest today in realizing all-optical communication systems. A complete communication systems requires the use of nonreciprocal devices like the one we realized for sound. Since our concept is very general, we have been envisioning an equivalent setup that works with light, letting us send it down a channel, but rerouting the one coming back.”