Acoustic Fiber Can Turn Fabrics Into Mics and Speakers

Enables two-way communications and monitoring for wearables and other applications

3 min read
An acoustic fiber [top left to middle right] can help transform a fabric into a microphone and speaker.

An acoustic fiber [top left to middle right] can help transform a fabric into a microphone and speaker.

Fink Lab MIT/Elizabeth Meiklejohn RISD/Greg Hren

Batteries, circuitry, displays, and other devices are increasingly made in fabric form, leading to more and more powerful electronic textiles. Now scientists have developed an acoustic sensor consisting of a single fiber that can turn fabrics into microphones and speakers, for potential applications in two-way communications, detecting the directions of gunshots, and monitoring fetal heartbeats during pregnancy, a new study finds.

The new sensor consists of a fiber that is piezoelectric—that is, capable of converting vibrations to electricity, and vice versa. This acoustic fiber is embedded within a fabric consisting of a relatively soft cotton yarn, as well as a yarn roughly as stiff as Kevlar.

Previous acoustic fibers suffered from low sensitivity in air. Moreover, fabrics normally muffle sound—for instance, carpets often help do so in homes. In contrast, this new research actually makes use of the fabric the acoustic fiber is woven into to help the sensor detect airborne sounds, a strategy inspired by the complex structure of the human ear.

The human ear possesses a thin sheet of tissue known as the eardrum or tympanic membrane. Sound waves make the eardrum vibrate, and the sensory apparatus within the ear converts these vibrations into nerve signals. In much the same way, the stiffer yarn in the fabric vibrates in response to even relatively weak sound waves—like human speech—and the acoustic fiber then converts these vibrations to electrical signals, says study senior author Yoel Fink, a materials scientist and electrical engineer at the Massachusetts Institute of Technology in Cambridge, Mass.

“The next computing environment is going to be fabric.” —Yoel Fink, MIT

The acoustic fiber is encased within a rubbery plastic cladding. This not only makes the fiber flexible, boosting wearability and protecting it to render it machine washable, but the cladding also concentrates vibrations onto the fiber to help make it more sensitive to sound, Fink says.

The scientists note that even a single acoustic fiber can convert dozens of square meters of fabric into a microphone—the fiber needs to make up less than 0.1 percent of the fabric by volume. The fabric can detect sounds in a wide range of loudness from a quiet library to heavy road traffic with performance on par with that of commercial microphones.

Acoustic fabrics on a weaving loom. Acoustic fabrics on a weaving loom.Fink Lab MIT/Elizabeth Meiklejohn RISD

In experiments, the fabric detected the angle of handclaps to within 1 degree at a distance of 3 meters away. This suggests this acoustic fiber could find use in helping detect the directions of gunshots, “or help people with hearing aids focus on particular directions they want to hear,” Fink says.

The acoustic fiber can also serve as an acoustic speaker when a voltage is applied. This suggests it could help lead to clothing that can both hear and produce sounds. ”You can also imagine the fibers finding use in noise-cancellation applications, such as a noise-canceling crib, so it stays relatively quiet within the crib while there is noise outside it,” Fink says.

Finally, when the researchers stitched the acoustic fiber to a shirt’s inner lining, they found it could accurately detect a healthy volunteer's heartbeat, as well as subtle variations in its sounds. This suggests it could find use in noninvasive monitoring of vital signs, including fetal heartbeats.

The acoustic fiber could find use well beyond human-computer interfaces. For example, the scientists noted it could be integrated with spacecraft skins to listen to falling space dust, embedded within buildings to hear cracks or straining, or woven into nets to track fish in the ocean.

“The next computing environment is going to be fabric,” Fink says. “Our broader mission is realizing a fabric computer.”

The scientists detailed their findings in the 17 March issue of the journal Nature.

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Europe Expands Virtual Borders To Thwart Migrants

Our investigation reveals that Europe is turning to remote sensing to detect seafaring migrants so African countries can pull them back

14 min read
A photo of a number of people sitting in a inflatable boat on the water with a patrol ship in the background.

Migrants in a dinghy accompanied by a Frontex vessel at the village of Skala Sikaminias, on the Greek island of Lesbos, after crossing the Aegean sea from Turkey, on 28 February 2020.

ASSOCIATED PRESS

It was after midnight in the Maltese search-and-rescue zone of the Mediterranean when a rubber boat originating from Libya carrying dozens of migrants encountered a hulking cargo ship from Madeira and a European military aircraft. The ship’s captain stopped the engines, and the aircraft flashed its lights at the rubber boat. But neither the ship nor the aircraft came to the rescue. Instead, Maltese authorities told the ship’s captain to wait for vessels from Malta to pick up the migrants. By the time those boats arrived, three migrants had drowned trying to swim to the idle ship.

The private, Malta-based vessels picked up the survivors, steamed about 237 kilometers south, and handed over the migrants to authorities in Libya, which was and is in the midst of a civil war, rather than return to Malta, 160 km away. Five more migrants died on the southward journey. By delivering the migrants there, the masters of the Maltese vessels, and perhaps the European rescue authorities involved, may have violated the international law of the sea, which requires ship masters to return people they rescue to a safe port. Instead, migrants returned to Libya over the last decade have reported enslavement, physical abuse, extortion, and murders while they try to cross the Mediterranean.

If it were legal to deliver rescued migrants to Libya, it would be as cheap as sending rescue boats a few extra kilometers south instead of east. But over the last few years, Europe’s maritime military patrols have conducted fewer and fewer sea rescue operations, while adding crewed and uncrewed aerial patrols and investing in remote-sensing technology to create expanded virtual borders to stop migrants before they get near a physical border.

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