The acoustic fiber, woven into the fabric, converts sound vibrations into electrical signals for sound detection capabilities. (Image Credit: Greg Hren/MIT)
MIT engineers developed a flexible piezoelectric fabric microphone, which picks up and interprets sound surrounding the wearer's body. For example, it can sense varying decibel sound levels in a quiet room to heavy traffic. It also detects a wearer's heartbeat and identifies the direction of a sudden sound, like a handclap. The team says this could have spacesuit applications, listening for space dust that surrounds or lands on an astronaut.
The researchers found inspiration from the human auditory system to develop this soft, durable, comfortable device with sound detection capabilities. They also realized that the fabric requires stiff or "high modulus" fibers that convert sound waves into vibrations. Plus, the team needed to fabricate a fiber that bends with the fabric while generating an electrical output.
The team produced a preform containing a piezoelectric layer and contents to boost the material's response to sound waves. Afterward, the preform was heated and stretched into thin, 40-meter-long fibers. Next, they performed a sound sensitivity test, outfitting it on an individual mylar sheet. A laser then measured the fiber and sheet's vibration in response to sound sourced from a nearby speaker. This caused the fiber to vibrate and produce a current matching the sound decibel level.
"This shows that the performance of the fiber on the membrane is comparable to a handheld microphone," Grace Noel, co-author of the study, says.
The team created drapable, machine-washable fabric by weaving the fiber with standard yarns. "It feels almost like a lightweight jacket — lighter than denim, but heavier than a dress shirt," says Elizabeth Meiklejohn, who used a traditional loom to weave the fabric. She then sewed a panel to the shirt's back. Next, the team clapped their hands in varying directions to determine if the acoustic fiber could detect the sound's direction. "The fabric was able to detect the angle of the sound to within 1 degree at a distance of 3 meters away," Noel notes.
The team created drapable, machine-washable fabric panels by weaving the fiber into yarns. (Image Credit: Greg Hren/MIT)
Also, the team stitched an individual fiber to a shirt's inner lining over the chest area. They discovered that it detected a wearer's heartbeat and slight changes in the heart's S1 and S2 features. This can also be incorporated into maternity clothing to monitor a baby's heartbeat. In addition, people with hearing loss can benefit from wearing this sound-sensing fabric since they can listen intently to a speaker in a noisy room.
Lastly, the team turned the microphone fiber into a speaker by reversing its functionality. In doing so, they captured spoken words and input the recording into the fiber as a voltage. The fiber then transformed the electrical signals into audible vibration that an additional fiber detected.
There are various applications besides clothing, hearing aids, communicative clothing, and vital sign monitoring garments for this sound-sensing fabric. "It can be integrated with spacecraft skin to listen to (accumulating) space dust, or embedded into buildings to detect cracks or strains," Wei Yan, lead author, proposes. "It can even be woven into a smart net to monitor fish in the ocean. The fiber is opening widespread opportunities."
"The learnings of this research offers quite literally a new way for fabrics to listen to our body and to the surrounding environment," Yoel Fink, MIT professor, says. "The dedication of our students, postdocs and staff to advancing research which has always marveled me is especially relevant to this work, which was carried out during the pandemic."
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