Acoustic Fiber Can Turn Fabrics Into Mics and Speakers

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.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.