A 250-million-year-old insect could inspire an ultra-thin wallpaper that

About 65 million years ago, right around the time dinosaurs went extinct, bats developed the ability to echolocate. They produced clicks with their mouths or noses and listened to the echoes of those clicks bouncing off surfaces and animals in the dark. Bats were large predators of moths, so many moths developed ears that are sensitive to that echolocation, but not all of them.

Some species, such as hairy silk moths, never grew ears. Instead, they used another defense mechanism they already had at their disposal: sound-absorbing wings.

[Image: courtesy University of Bristol]The existence of what is now known as “acoustic camouflage” was discovered in 2018, when a team of researchers from the University of Bristol discovered that hairy moths have a coating on their wings that can absorb up to 85% of the sound hitting them. absorb. , making it difficult for bats to locate them because their calls would be poorly reflected.

Now, a new study from the same team suggests these moth wings could inspire a new type of ultra-thin acoustic material that would act like wallpaper and be 10 times thinner than what’s currently on the market.

This isn’t the first time people have turned to nature for solutions. While groundbreaking, wallpaper design is part of an existing field known as biomimicry, or the act of imitating nature’s design and processes in man-made systems. Examples include bird-safe glass inspired by the UV-reflecting strands in cobwebs; cooling vents inspired by termite mounds; and of course airplane wings modeled after birds.

Moths evolved 250 million years ago, or about 200 million years for bats. According to Marc Holderied, a professor of sensory biology at the University of Bristol and co-author of the new study with Thomas Neil, deaf hairy moths had scales on their wings even before the bats arrived — perhaps to protect themselves from sticky cobwebs. So all they had to do was adjust their function.

[Image: courtesy University of Bristol]The scientists knew that earless moths had to rely on their acoustic wings to survive, but exactly how that works took them six years to figure out. The trick, Holderied says, is in a fine dust that sits on their wings.

Under a microscope, that dust looks like a collection of tiny overlapping scales, similar to a complex roof tile pattern. When a sound hits those scales at the right frequency (more on that later), the scales start to vibrate. By vibrating, they extract the sound energy from the air and convert it into mechanical energy that is eventually damped and converted into heat. This process is known as resonant absorption.

[Image: courtesy University of Bristol]This is where sound frequency comes into play. Every sound frequency has a wavelength. For traditional acoustic materials to work, they need to be as thick as one-tenth the wavelength, Holderied says. So for long wavelength sounds, like the constant hum of an airplane or the background noise in a loud restaurant, we need acoustic materials that are thick (think several inches) and porous enough to disperse a sound wave when it hits them. . †

[Image: courtesy University of Bristol]In contrast, resonant absorbers can be as thick as one-hundredth of the wavelength they are trying to stop. “That opens up a lot of opportunities for us as humans to try to understand the mechanism that has given evolution to this moth, and turn it into something useful for us in our hearing range,” Holderied says.

The new material could act a bit like an ultra-thin wallpaper fabric applied to a variety of surfaces, from office walls to the seats of an airplane.

“We’re talking about something that measures millimeters rather than centimeters,” Holderied says, noting that resonant materials would be much more efficient than the porous materials used in traditional acoustics. And if a material is thinner, he says, it would also be lighter, which could help, for example, reduce the aircraft’s overall weight and save fuel.

[Image: courtesy University of Bristol]Holderied’s team is working on several prototypes made from something called a metamaterial. These engineered materials are made of a large number of cells; in this case, scales that would mimic how moth wings work. He couldn’t share the exact materials that would make up the larger metamaterial, but says the first prototypes, which are 4 inches by 4 inches, are made up of materials like silicon and other things that are easy to manipulate.

The challenge now is to translate the concept so that it works for the human audible range. In this particular study, the scientists used ultrasonic signals that are above the range humans can hear, but Holderied says the size of each shell on the metamaterial can help dictate the kinds of sounds it can absorb.

It will probably take a few years for new material to hit the market, but if and when it does, it will be the culmination of 250 million years of work.


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