Moth wings can inspire the next generation of sound-absorbing technology

The acoustic arms race between bats and moths has been going on for about 65 million years – since bats developed echolocation to find their prey.

Since then, moths have been under tremendous evolutionary pressure to develop defenses to survive, and one of these adaptations – the small scales on their wings – may contain the key to transforming future noise suppression technology.

This shows a new study published in Proceedings of the Royal Society A: Mathematical Physical and Engineering Sciences.

“Moths will inspire the next generation of sound-absorbing materials,” said senior author Mark Holderid, a professor of sensory biology at the School of Biological Sciences at the University of Bristol in the United Kingdom.

“New research shows that one day it will be possible to decorate the walls of your house with ultra-thin sound-absorbing wallpaper, using a design that mimics the mechanisms that give moths stealth acoustic camouflage.

The wings of a moth naturally absorb sound

Previously, these researchers found that the wings of moths provide protection against echolocation of bats through porous nanostructured scales on their surface, which absorb sound.

The scales of moth wings are about 100-200 microns long and only 1 to 2 microns thick (less than the wavelength of sound with the highest frequency used in bat echolocation). This means that they do not reflect sound waves back to the bat, instead they vibrate and convert sound into kinetic energy.

Scale of a moth’s wing in close-up. Credit: University of Bristol

Scientists have now studied whether this structure can provide information on the design of mounted sound absorption by examining the ability of moth wings attached to the surface to absorb sound.

“What we needed to know first was how well these moth scales would perform if they were in front of an acoustically reflective surface, such as a wall,” Holderid said. “We also had to understand how the mechanisms of absorption can change when the scales interact with this surface.”

They investigate this by placing small parts of the moth’s wings on an aluminum disk and then testing how the orientation of the wing (relative to the input sound) and the removal of layers of flakes affect sound absorption.

Remarkably, they found that the wings absorb up to 87% of the incoming sound energy when mounted on a hard surface, while absorbing a wide range of frequencies (broadband) coming from many different angles (omnidirectional).

“Even more impressive is that the wings do this while they are incredibly thin, with the rock layer being only 1/50 of the wavelength of the sound they absorb,” explains lead author Dr. Thomas Neal. researcher at the School of Biological Sciences at the University of Bristol.

“This exceptional performance qualifies the moth wing as a natural acoustically absorbent metasurface, a material that has unique properties and capabilities that cannot be created with conventional materials.

Consequences for construction and travel

Antheraea pernyi 1 1
The moth Antheraea pernyi. Credit: University of Bristol

The creation of ultra-thin sound-absorbing panels has implications for both the construction industry and travel.

As cities become noisier, the need for unobtrusive sound reduction increases, and light noise-absorbing panels can also have a huge impact on the tourism industry, where any weight saved on planes, cars and trains increases their efficiency.

So far, sound absorption has been studied at ultrasonic frequencies – which are above the range that humans can perceive – because bat echolocation uses sound waves in this range.

This is not practical for use in sound attenuation, as such technologies will have to reduce the noise pollution heard by humans.

Scientists now plan to meet the challenge of reproducing the sound-absorbing abilities of moth wings by designing and building prototypes that operate at lower frequencies – in the field of human hearing.

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