New sound-sensing fabric - this "sweater" can hear your heartbeat

Recently, engineers from the Massachusetts Institute of Technology (MIT) and the Rhode Island School of Design have collaborated to develop a new "acoustic fabric" that can not only "hear" sounds like a microphone, but also "make" sounds like a speaker.

From a quiet library to the noise of busy traffic, the acoustic fabric can pick up sounds in these decibel ranges and can pinpoint the precise direction of a sound like a clapping hand. When woven into the lining of a shirt, the material can also detect the subtle signature of the wearer’s heartbeat.

A detailed introduction to this achievement was published in the latest issue of Nature magazine.

The first author of the paper, Wei Yan, a postdoctoral fellow at MIT (now an assistant professor at Nanyang Technological University in Singapore), said that the results of this research provide a new way to listen to our bodies and our surroundings, which will produce many unimaginable applications.

"Wearing voice-sensing clothing, you can use it to answer calls and communicate with others. In addition, this fabric can also be in contact with human skin, allowing the wearer to monitor their heart and breathing conditions in a comfortable, continuous, real-time and long-term manner," said Fink.

Acoustic materials inspired by the human ear

Traditional fabrics are often used to suppress or block sound. For example, the sound insulation layer in concert halls and the carpets in our lives. But the MIT research team has been working for many years to reshape the traditional role of fabrics. They focus on expanding the properties of materials, such as making fabrics more functional.

This time, when looking for ways to make sound-sensing fabrics, the researchers were greatly inspired by the "hearing principle" of the human ear.

Sound is actually a pressure wave that travels through the air. When these sound waves reach our ears, the extremely sensitive and complex three-dimensional organ, the eardrum, converts the pressure waves into mechanical vibrations, which then pass through the ossicles into the inner ear. The cochlea converts these vibration waves into electrical signals that can be sensed and processed by the brain.

Although some sounds are so small that we cannot perceive them normally, they will cause corresponding vibrations, but sometimes these vibrations are at the nanometer level. In order to capture these imperceptible vibration signals, the team tried to create a soft, durable, comfortable fabric "ear" that can detect sound, inspired by the human auditory system.

(Source: Nature)

To achieve this, they found that: the fabric had to incorporate stiff or "high modulus" fibers to effectively convert sound waves into vibrations; and secondly, they had to design a special fiber that could bend with the fabric and generate electrical output in the process.

With these prerequisites in mind, the team developed a layered block of material, called a preform, made of piezoelectric layers and reinforcement components that vibrate in response to sound waves. The resulting preform, about the size of a marker pen, was then heated and pulled, like fondant, into 40-meter-long thin fibers.

In fact, this fiber is designed from a "piezoelectric" material that generates electrical signals when bent or mechanically deformed, providing a way for fabrics to convert sound vibrations into electrical signals. When this flexible fiber is woven into fabric, it can also bend with the fabric like seaweed in the ocean.

Not only can you hear, but you can also make sounds

To test the sensitivity of the sound-sensing fibers, the researchers attached them to a suspended piece of polyester film and then used lasers to measure the vibrations of the sheet—and, in turn, the fiber—in response to sounds played through a nearby speaker.

Although the decibel level of sound varies between a quiet library and busy road traffic, the acoustic fibers are able to vibrate accordingly and generate an electrical current proportional to the sound played.

"This shows that the performance of acoustic sensing fibers on membranes is comparable to that of handheld microphones," says Grace Noel of MIT's Department of Chemical Engineering, one of the authors of the study.

Next, the team wove the custom acoustic fibers together with conventional yarns to produce a drapeable, machine-washable fabric that the researchers say feels like a lightweight jacket, lighter than denim but heavier than a dress shirt.

The researchers then sewed a piece of the fabric to the inside of a shirt and tested the fabric’s sensitivity to directional sound perception by clapping their hands while standing at different angles to the shirt. The results showed that the fabric was able to detect sound angles within 1 degree at a distance of 3 meters.

The researchers believe that this type of sensing fabric, which can sense sound in a direction, could help people with hearing loss hear external sounds in noisy environments.

(Source: MIT)

In addition, the research team also sewed a fiber to the lining of the shirt, roughly above the chest area, and found that the sound-sensing material could accurately detect the heartbeat of healthy volunteers, as well as subtle changes in heart beat characteristics.

In addition to monitoring heartbeats, researchers believe that incorporating sound-sensing fabrics into maternity clothing could also help monitor fetal heartbeats.

Even more interesting is that the researchers have come up with a new idea: what would happen if the function of the acoustic fibers were reversed, and they were no longer used as sound detectors but as loudspeakers?

To this end, the researchers first recorded a string of spoken words and fed the recording to the acoustic fibers in the form of applied voltage. They found that the acoustic fibers were able to convert electrical signals into audible vibrations, and then used another acoustic fiber as an "ear" to successfully detect the sound vibrations emitted by the acoustic fibers.

In addition to wearable hearing aids, clothing that can communicate with voice, and clothing that can track vital signs, the team sees many more possible applications.

“It could be integrated with spacecraft clothing to monitor space dust, or embedded in buildings to detect cracks or strain,” Yan suggested. “It could even be woven into a smart net to monitor fish in the ocean. Sound-sensing fibers are opening up a huge opportunity.”

References:

https://www.nature.com/articles/s41586-022-04476-9

https://news.mit.edu/2022/fabric-acoustic-microphone-0316

Source: Academic Headlines