Clothing That Can Hear? MIT Researchers Create Acoustic Fabric
Taking inspiration from the biological auditory system, MIT researchers have created a fabric that senses sound signals and converts them to electrical signals, analogous to how a microphone works.
It's now common practice to use fibers in electronic systems and devices. Thanks to its flexibility, fiber can be twisted into yarns to make fabrics used to make rollable and bendable electronics.
Building on such e-textile research, MIT researchers recently found that fabric can be used as a sound-sensing device to detect sounds ranging in decibel from a quiet library to heavy road traffic. This new development could eventually advance hearing aid systems and devices, according to the researchers.
Close-up view of MIT’s acoustic fabric. Image used courtesy of MIT
Transforming Sound to Electricity
Noise pollution is almost everywhere. This sound, however, can be used to produce electricity. The MIT researchers did just that by looking to the human auditory system as inspiration.
The eardrum is known to transduce sound pressures into mechanical vibrations. The sound pressures (or sound waves) from the eardrum travel to the cochlea, the spiral cavity of the inner ear. In the cochlea, short cilia fibers convert sound waves into electrical signals that are carried to the brain by the auditory nerve.
Like in the human ear, high Young’s modulus fibers transduce sound waves to mechanical vibrations. Image used courtesy of Nature
Analogous to this process in the biological auditory system, engineers have developed ways to convert vibrations from sound to useful electricity by employing electronic components like induction coils. Through electromagnetic induction, vibrations caused by sound can be converted into electrical energy. By using a suitable transducer connected to an induction coil, electromotive force (EMF) is produced as a varying magnetic field around the induction coil.
In the past, researchers at the Jaypee Institute of Information Technology, India, employed the principle of electromagnetic induction to generate electrical energy from sound energy. The researchers connected the diaphragm of a speaker to an induction coil in the presence of a permanent magnet. The diaphragm vibrated when in contact with sound waves coming from a car horn and the exhaust pipe of a motorcycle. The coil moved between the north pole and the south pole of the permanent magnet to create a varying magnetic field around the induction coil when the diaphragm vibrated.
By Faraday’s law of electromagnetic induction, an electromotive force—dependent on the velocity of relative motion between the coil and the permanent magnet, strength of the magnetic field, and length of conductor—was generated.
Composite Piezoelectric Fiber Converts Sound to Electricity
Though fabrics are normally used to dampen sound, the researchers at MIT have successfully developed an acoustic fabric that operates as a sensitive audible microphone. Owing to the high Young’s modulus of a 40-meter-long fiber, the fabric efficiently transduces sound waves into mechanical vibrations.
Elastomer cladding was adopted during the thermal drawing process of the piezoelectric material to heighten fiber sensitivity. Image used courtesy of Nature
The researchers wove thermally-drawn composite piezoelectric elastomeric fiber into the fabric. When bent or deformed, the piezoelectric fiber produced an electrical signal. Though the fiber made up less than 0.1% of the fabric by volume, a single draw from the fiber equaled tens of square meters of fabric microphone.
Upon examining the fabric’s sensitivity to directional sound, Grace Noel, a co-researcher in the project, remarked that the fabric could detect the angle of the sound coming from a hand clap to within one degree at a distance of three meters away.
This research promises to open a range of opportunities for fibers in e-textiles. For instance, this fabric might one day be useful in monitoring heart and respiratory conditions. Because the fabric supports directional detection, it could also one day enhance hearing aids, the researchers claim.