The acoustic arms race between bats and moths has been going on for some 65 million years — ever since bats evolved echolocation to find their prey.
Moths have since been under tremendous evolutionary pressure to develop their defenses for survival, and one of these adaptations — the tiny scales on their wings — could hold the key to transforming future noise-cancelling technology.
This is according to 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 Marc Holderied, professor of sensory biology in the School of Biological Sciences at the University of Bristol, UK.
“New research has shown that one day it will be possible to decorate the walls of your home with ultra-thin sound-absorbing wallpaper, with a design that mimics the mechanisms that give moths stealth acoustic camouflage.”
Moth wings naturally absorb sound
Previously, these researchers had found that moths’ wings provide protection against bat echolocation through porous nanostructured shells on their surface that absorb sound.
The scales on moth wings are about 100-200 microns long and only 1 to 2 microns thick (smaller than the wavelength of the highest frequency sound used in bat echolocation). This means that they do not reflect sound waves back to the bat, but vibrate and convert the sound into kinetic energy.
Now scientists have investigated whether this structure could inform mounted sound absorption design, by studying the ability of moth wings attached to a 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 highly reflective surface, such as a wall,” Holderied says. “We also had to figure out how the absorption mechanisms might change when the shells interacted with this surface.”
They investigated this by placing small pieces of moth wings on an aluminum disc and then testing how the orientation of the wing (relative to the incoming sound) and the removal of scales affected sound absorption.
Remarkably, they found that the wings absorbed a whopping 87% of the incoming sound energy when mounted on a solid surface, while also absorbing a wide range of frequencies (broadband) from many different angles (omnidirectional).
“What’s more impressive is that the wings do this while being incredibly thin, with the shell layer being only 1/50th the thickness of the wavelength of the sound they absorb,” explains lead author Dr. Thomas Neil, a researcher at the University of Bristol’s School of Biological Sciences.
“This extraordinary performance qualifies the moth wing as a naturally occurring acoustically absorbent meta-surface, a material with unique properties and capabilities not possible with conventional materials.”
Consequences for building and travel
Making ultra-thin sound-absorbing panels has implications for both the construction industry and travel.
As cities get louder, the need for non-intrusive noise mitigation grows, and lightweight sound-absorbing panels can also have huge implications for the travel industry, where any weight saved on planes, cars and trains increases their efficiency.
So far, the sound absorption studied has been at ultrasonic frequencies — which are above the range that humans can perceive — since bats echolocation uses sound waves within that range.
This is not practical for use in noise mitigation, as such technologies would need to attenuate the noise pollution audible to humans.
Now the scientists plan to take on the challenge of replicating the sound-absorbing capabilities of the butterfly wings by designing and building prototypes that operate at lower frequencies — within the range of human hearing.