A sound-suppressing silk engineered to fight noise pollution could also prevent eavesdropping on private conversations, according to an acoustics expert.

The fabric, which contains a vibrating fibre that suppresses sound to create quiet spaces, was recently revealed by researchers at the Massachusetts Institute of Technology (MIT).

Barely thicker than a human hair, the fabric vibrates when a voltage is applied to the fibre, which works in two different ways. In one, the vibrating fabric generates soundwaves that interfere with an unwanted noise to cancel it out, similar to noise-cancelling headphones.

In the other technique, the fabric is held still to suppress vibrations. This prevents noise from being transmitted, reducing the volume on the other side. This second approach could enable noise reduction in large spaces such as rooms or cars, the MIT announcement said.

Made of common materials including silk, canvas and muslin, the fabrics could be used in real-world spaces, the researchers claimed. Applications could include thin fabric ‘walls’ to prevent sound getting from one part of a room to another.

Silencing noisy neighbours

We need methods to reduce noise, said acoustic engineering professor Trevor Cox from the University of Salford, who was not involved in the research. “It's a huge societal problem. Whether that’s coming from environmental sources like planes, or from our noisy neighbours… it [causes] problems like cardiovascular diseases,” he said to Professional Engineering.

“It's good to see more research, and it fits into the trend now of trying to make stuff which is lighter and thinner… You can achieve what they want from traditional materials, but traditionally if you want to stop something going through a wall, [the walls] are big, or they're heavy, or they've got big cavities. So trying to do it with a thin, lightweight material opens up some interesting possibilities.”

So far, the researchers have only tested the sound suppression using 8cm pieces of fabric in lab experiments.

Scaling up could be a challenge, said Professor Cox. “How can you make these patches bigger? Then the sound field won’t be nice and neat, as it is when it's a nice little, round patch,” he said. “As soon as you have a big fabric, you've got many more modes to deal with, and it becomes much more complicated.”

Maintenance could be another challenge in the built environment, which Professor Cox said has held back other noise suppression projects using active control.

He suggested there could be some novel applications, however. These could include aerospace, where the cost of lightweight materials for noise cancellation could be outweighed by a potential reduction in fuel consumption.

The fabric could have even more unusual applications. “There are places where you want to have secret conversations,” he said. “You’re in a foreign embassy, how do you protect your speech and make sure it's private? Put a tent up, which has this, to stop speech being overheard.”

Music festivals, which use a lot of tents and have associated noise issues, could be another option if costs and logistics can be solved, Professor Cox added.

Fabric ‘loudspeakers’

The sound-suppressing silk builds on the MIT researchers’ prior work creating fabric microphones. In that research, they sewed a single strand of piezoelectric fibre – which produces an electrical signal when squeezed or bent – into fabric. When a nearby noise causes the fabric to vibrate, the piezoelectric fibre converts those vibrations into an electrical signal, which can capture the sound.

In the new work, the team flipped that idea to create a fabric ‘loudspeaker’ that can be used to cancel out sound waves.

“While we can use fabric to create sound, there is already so much noise in our world. We thought creating silence could be even more valuable,” said Grace (Noel) Yang, lead author of the new study.

To enable direct sound suppression, the researchers used a silk fabric loudspeaker to emit sound waves that destructively interfere with unwanted sound waves. This technique is only effective over a small area, however, so they also developed a technique that uses fabric vibrations to suppress sound in much larger areas.

In that technique, vibrations in the fibre are controlled to force the fabric to remain still. Installed on a shared wall, the vibration-mediated suppression could prevent sound from being transmitted through the fabric.

When they tested the silk fabric in direct suppression mode, the researchers found that it could significantly reduce the volume of sounds up to 65 decibels, roughly as loud as enthusiastic human conversation. In vibration-mediated suppression mode, the fabric reduced sound transmission by up to 75%.

Next, the researchers hope to use the fabric to block sound of multiple frequencies. This would likely require complex signal processing and additional electronics.

They also want to further study the architecture of the fabric, to see how changing things like the number of piezoelectric fibres, the direction in which they are sewn, or the applied voltages could improve performance.

The project also involved the Rhode Island School of Design, the University of Wisconsin at Madison, Case Western Reserve University in Ohio, and chemical engineers in the Smith Group at MIT

An open-access paper about the research appeared recently in Advanced Materials.

Extracted from IMechE website, read more here

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