ON A GOSSAMER VEIL OF SILENCE

IT SOUNDS TOO GOOD TO BE TRUE: MIT researchers have developed a silk fabric that can suppress unwanted noise and reduce noise transmission in a large room. Adam Zewe reports this in MIT News, May 7, 2024. Here are tidbits on this gossamer veil of silence. 

The Challenge. “We are living in a very noisy world,” Zewe writes. “From the hum of traffic outside your window to the next-door neighbor’s blaring TV to sounds from a co-worker’s cubicle, unwanted noise remains a resounding problem. To cut through the din, an interdisciplinary collaboration of researchers from MIT and elsewhere developed a sound-suppressing silk fabric that could be used to create quiet spaces.”

A Gossamer Veil. Zewe describes, “The fabric, which is barely thicker than a human hair, contains a special fiber that vibrates when a voltage is applied. The researchers leveraged those vibrations to suppress sound in two different ways.”

Akin to Noise-cancelling Headphones. “In one,” Zewe says, “the vibrating fabric generates sound waves that interfere with an unwanted noise to cancel it out, similar to noise-canceling headphones, which work well in a small space like your ears but do not work in large enclosures like rooms or planes.”

And More Surprising. Zewe continues, “In the other, more surprising technique, the fabric is held still to suppress vibrations that are key to the transmission of sound. This prevents noise from being transmitted through the fabric and quiets the volume beyond. This second approach allows for noise reduction in much larger spaces like rooms or cars.”

What a Talented Team! Lead author Grace H. Yang earned an SM in 2021 and completed her Ph.D. this year, both from MIT. Co-authors include four grad students and a chem engineering professor also at MIT; a grad student and professor from Case Western Reserve (yah! whence my math Ph.D.); a grad student and professor from the University of Wisconsin at Madison; and a grad student at the Rhode Island School of Design. Full details are at “Single Layer Silk and Cotton Woven Fabrics for Acoustic Emission and Active Sound Suppression,” Wiley Online Library Advanced Materials, first published April 1, 2024. 

From the Abstract. Yang et al. report, “Sound suppression is demonstrated using two distinct mechanisms. In the first, direct acoustic interference is shown to reduce sound by up to 37 dB. The second relies on pacifying the fabric vibrations by the piezoelectric fiber, reducing the amplitude of vibration waves by 95% and attenuating the transmitted sound by up to 75%. Interestingly, this vibration-mediated suppression in principle reduces sound in an unlimited volume. It also allows the acoustic reflectivity of the fabric to be dynamically controlled, increasing by up to 68%.”

From the Paper: “Active noise reduction realizes suppression of sound in a small volume and has become a significant area of application for headphones and earbuds, where only a small quiet zone near the eardrum is needed. Larger scale volumetric sound suppression, e.g., in a room, is primarily done through passive approaches, such as reflection of sound using materials with high acoustic impedance or absorption of sound typically done with fibrous structures or foams. Despite their ubiquity, these materials suffer from low performance at low frequencies, often necessitating layers of soundproofing material that are 15–100 mm thick. Moreover, the aesthetic and environmental implications are significant as many of these materials are made of hydrocarbons.”

“It is estimated,” the researchers note, “that the volume of fibrous soundproofing is more than 5 billion cubic meters and is an ≈$12 billion market. The absence of compact, lightweight, and efficient sound suppression materials constitutes a limitation, motivating an unmet need for thin and visually aesthetic sound barriers.”

Direct Acoustic Suppression. Examples from the paper: “In addition to transmitting sound (Figure a) and emitting sound (Figure b) for propagation, the fabric emitter can be used for direct acoustic suppression, also known traditionally as ‘active noise cancellation.’”

 “In many instances, direct acoustic suppression can save material costs and space while providing greater attenuation, particularly at lower frequencies compared to passive noise reduction using sound-absorbing panels. The fabric emitter can function as a direct acoustic suppression device by emitting sound waves that destructively interfere with the sound waves of the unwanted noise (Figure c).”

Image from Yang et al.

Details from the paper: “a) In transmission mode, incoming sound waves give rise to vibrations in a passive fabric. These vibrations transfer energy to the air in the form of sound. b) The piezoelectric fiber in the fabric causes the fabric to vibrate, leading to the emission of sound. c) Sound waves resulting from the sound which is transmitted through the fabric interfere with sound waves induced by the piezoelectric fiber vibrating the fabric. This interference can cause the cancellation of the sound at a particular point in space. d) The piezoelectric fiber induces mechanical vibrations that destructively interfere on the surface of the fabric with those induced by the incoming sound waves, subduing the fabric surface and precluding sound transmission through it.”

Future Work. The researchers note, “In order to suppress multifrequency sound waves originating from multiple sources, advanced signal processing and the development of complex control algorithms will be needed for the applications of both direct acoustic suppression and vibration-mediated suppression to be feasible in real-world scenarios.”

“There is opportunity to face this challenge,” they write, “by using the additional degrees of freedom that the fabric emitter system offers, as multiple fibers and fabrics of various sizes and orientations could be used in a single system to suppress more complex sound.” 

All fascinating technology, and significant advances from traditional methods. ds

© Dennis Simanaitis, SimanaitisSays.com, 2024