Engineers at the MIT designed a fabric that is converting sound first into mechanical vibrations, then into electrical signals, similarly to how our ears hear. This fabric is inspired by the human ear and works like a microphone.
All fabrics vibrate in response to audible sounds, though these vibrations are on the scale of nanometers — far too small to ordinarily be sensed. To capture these imperceptible signals, the researchers created a flexible fiber that, when woven into a fabric, bends with the fabric like seaweed on the ocean’s surface.
The fiber is designed from a “piezoelectric” material that produces an electrical signal when bent or mechanically deformed, providing a means for the fabric to convert sound vibrations into electrical signals.
The fabric can capture sounds ranging in decibel from a quiet library to heavy road traffic and determine the precise direction of sudden sounds like handclaps. When woven into a shirt’s lining, the fabric can detect a wearer’s subtle heartbeat features. The fibers can also be made to generate sound, such as a recording of spoken words, that another fabric can detect.
In looking for ways to make sound-sensing fabrics, the team of the Massachusetts Institute of Technology (MIT), Cambridge, MA/USA, took inspiration from the human ear and sought to create a fabric “ear” that would be soft, durable, comfortable, and able to detect sound. Their research led to 2 important discoveries: Such a fabric would have to incorporate stiff, or “high-modulus,” fibers to effectively convert sound waves into vibrations. Furthermore, the team would have to design a fiber that could bend with the fabric and produce an electrical output in the process.
With these guidelines in mind, a layered block of materials called a preform, made from a piezoelectric layer as well as ingredients to enhance the material’s vibrations in response to sound waves has been developed. The resulting pre-form, about the size of a thick marker, was then heated and pulled like taffy into thin, 40-m-long fibers.
The researchers tested the fiber’s sensitivity to sound by attaching it to a suspended sheet of mylar. They used a laser to measure the vibration of the sheet — and by extension, the fiber — in response to sound played through a nearby speaker. The sound varied in decibels between a quiet library and heavy road traffic. In response, the fiber vibrated and generated an electric current proportional to the sound played.
Next, the team wove the fiber with conventional yarns to produce panels of drapable, machine-washable fabric. One panel was sewed to the back of a shirt, and the team tested the fabric’s sensitivity to directional sound by clapping their hands while standing at various angles to the shirt.
The researchers envision that a directional sound-sensing fabric could help those with hearing loss to tune in to a speaker amid noisy surroundings.
The team also stitched a single fiber to a shirt’s inner lining, just over the chest region, and found it accurately detected the heartbeat of a healthy volunteer, along with subtle variations in the heart’s S1 and S2, or “lub-dub” features. In addition to monitoring, one’s own heartbeat, Fink sees possibilities for incorporating the acoustic fabric into maternity wear to help monitor a baby’s fetal heartbeat.
Finally, the researchers reversed the fiber’s function to serve not as a sound-detector but as a speaker. They recorded a string of spoken words and fed the recording to the fiber in the form of an applied voltage. The fiber converted the electrical signals to audible vibrations, which a second fiber was able to detect.
In addition to wearable hearing aids, clothes that communicate, and garments that track vital signs, the team sees applications beyond clothing.