The acoustic arms race between bats and moths has been carried out for some 65 million years – ever since bats evolved echolocation to find their prey.
Moths have since been under massive evolutionary pressure to evolve defences in order to survive, 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 are going to inspire the next generation of sound-absorbing materials,” says 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 adorn the walls of your house with ultra-thin sound-absorbing wallpaper, using a design that copies the mechanisms that gives moths stealth acoustic camouflage.”
Moth wings naturally absorb sound
Previously, these researchers had discovered that the wings of moths provide protection from bat echolocation through porous nanostructure scales 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 echolocation by bats). This means they don’t reflect sound waves back to the bat, instead they vibrate and turn the sound into kinetic energy.
Now, scientists have studied whether this structure could inform the design of mounted sound absorption, by studying the ability of moth wings affixed 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,” says Holderied. “We also needed to find out how the mechanisms of absorption might change when the scales were interacting with this surface.”
They examined this by placing small sections of moth wings on an aluminium disc and then testing how the orientation of the wing (with respect to the incoming sound), and the removal of layers of scales, affected sound absorption.
Remarkably, they found that the wings absorbed as much as 87% of incoming sound energy when mounted on top of a solid surface, while also absorbing a wide range of frequencies (broadband) coming from many different angles (omnidirectional).
“What is even more impressive is that the wings are doing this whilst being incredibly thin, with the scale layer being only 1/50th of the thickness of the wavelength of the sound that they are absorbing,” explains lead author Dr Thomas Neil, a researcher in the School of Biological Sciences at the University of Bristol.
“This extraordinary performance qualifies the moth wing as a natural-occurring acoustic-absorbing metasurface, a material that has unique properties and capabilities that are not possible to create using conventional materials.”
Implications for building and travel
The creation of ultra-thin sound-absorbing panels has implications for both the building industry and travel.
As cities become louder, the need for non-intrusive sound mitigation grows, and lightweight sound-absorbing panels could also have huge impacts on the travel industry, where any weight saved in planes, cars and trains increases their efficiency.
Until now, the sound absorption studied has been at ultrasound frequencies – which are above the range that humans can perceive – since bats’ echolocation uses sound waves within that range.
This isn’t practical for use in sound mitigation, since such technologies would need to dampen the noise pollution that’s audible to humans.
Now, the scientists plan to tackle the challenge of replicating the moth wings’ sound-absorbing capabilities by designing and building prototypes that work at lower frequencies – within the realm of human hearing.