The acoustic arms race between bats and moths has been going on for some 65 million years – since bats evolved echolocation to find their prey.
Moths have since been under massive evolutionary pressure to evolve their defenses in order to survive, and one of those adaptations – the tiny scales on their wings – may hold the key to transforming future noise-canceling 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,” says lead 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, using a design that copies the mechanisms that give moths stealthy acoustic camouflage.”
Butterfly wings naturally absorb sound
Previously, these researchers found that moth wings provide protection against bat echolocation through porous nanostructure scales on their surface that absorb sound.
Moth wing scales are about 100-200 microns long and only 1-2 microns thick (smaller than the highest frequency sound wavelength used for echolocation by bats). mouse). This means that they do not reflect sound waves back to the bat, but vibrate and transform the sound into kinetic energy.
Now, scientists have investigated whether this structure could inform the design of mounted sound absorption, by studying the ability of butterfly wings attached to a surface to absorb sound.
“What we first needed to know was how well these moth scales would perform if they were in front of an acoustically highly reflective surface, like a wall,” says Holderied. “We also needed to find out how absorption mechanisms might change when scales interacted with this surface.”
They examined this by placing small sections of moth wings on an aluminum disc and then testing how the orientation of the wing (relative to incoming sound) and the removal of scale layers affected the sound absorption.
Remarkably, they found that the fenders absorbed up to 87% of incoming sound energy when mounted on a solid surface, while absorbing a wide range of frequencies (broadband) from many different angles (omnidirectional) .
“What’s even more impressive is that the wings do this while being incredibly thin, with the layer of scales being only 1/50th the thickness of the wavelength of sound they make. absorb,” says lead author Dr Thomas Neil, a research scientist at the University of Bristol’s School of Biological Sciences.
“This extraordinary performance qualifies the moth wing as a natural, acoustically absorbing metasurface, a material that possesses unique properties and capabilities not possible with conventional materials.”
Implications for construction and travel
Creating ultra-thin sound absorbing panels has implications for both the construction industry and travel.
As cities get noisier, the need for non-intrusive sound attenuation increases, and lightweight sound deadening panels could also have huge impacts on the travel industry, where any weight saved in 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 bat echolocation uses sound waves in this range.
This is impractical for use in sound attenuation, as such technologies are expected to attenuate noise pollution audible to humans.
Now, scientists plan to take on the challenge of replicating the sound-absorbing abilities of moth wings by designing and building prototypes that operate at lower frequencies – within the realm of human hearing.