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 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. [READ MORE]