Moths’ amazing scaled wings inspire sound-absorbing wallpaper

Scientists say amazing ‘stealth acoustic camouflage’ could one day be used for ultra-light sound-proofing in cars and planes, reports Harry Cockburn

The Chinese oak silkmoth. Moths’ scaled wings offer acoustic protection from bats’ echolocation calls (University of Bristol)

The amazing sound-absorption abilities of moths’ wings, which helps them evade bats echolocation calls, has inspired new techniques for manufacturing ultra-thin sound absorbing panels.

Scientists at the University of Bristol discovered that the minute scales which cover moths’ wings can act as excellent sound absorbers even when placed on artificial surfaces – rather than when they are moving in free space.

They said the discovery could "hold the key to transforming noise-cancelling technology".

The team, led by Professor Marc Holderied at Bristol’s School of Biological Sciences, began the research on the unusual sound-absorption properties the wings have after discovering that the scaled structures of moths’ wings offer extra protection from hunting bats.

They noted that bats and moths have been involved in an "acoustic arms race between predator and prey ever since bats evolved echolocation some 65 million years ago".

Moths are under huge predation pressure from bats and have therefore evolved a range of defences in their efforts to survive. Among these are the the scales which make up their wings.

Professor Holderied said: “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.

“We also needed to find out how the mechanisms of absorption might change when the scales were interacting with this surface.”

The team tested this by placing small sections of moth wings on an aluminium disc, then systematically tested how orientation of the wing with respect to the incoming sound and the removal of scale layers affected sound absorption.

They said that "remarkably" moth wings proved to be excellent sound absorbers, with the wings absorbing as much as 87 per cent of the incoming sound energy.

They said the effect is omnidirectional and covers a wide range of frequencies and sound incident angles.

The scales on a moth’s wing seen under a microscope (University of Bristol)

Lead author Dr Thomas Neil, said: “What is even more impressive is that the wings are doing this whilst being incredibly thin, with the scale layer being only one fiftieth of the thickness of the wavelength of the sound that they are absorbing."

“This extraordinary performance qualifies the moth wing as a naturally occurring acoustic absorbing metasurface, a material that has unique properties and capabilities that are not possible to create using conventional materials.”

The team said the potential to create ultra-thin sound absorbing panels has huge implications in building acoustics.

Due to our cities getting louder, the need for efficient non-intrusive sound mitigation solutions is growing.

They also suggested that very lightweight sound absorbing panels could have "huge impacts on the travel industry", with any weight saving in planes, cars and trains increasing efficiency, reducing fuel use and CO2 emissions.

The team now plans to replicate the sound absorbing performance by designing and building prototypes based on the sound absorbing mechanisms of the moth.

The absorption that they have characterised in moth wing scales is all in the ultrasound frequency range, above that which humans can hear. Their next challenge is to design a structure that will work at lower frequencies whilst retaining the same ultrathin architecture employed by the moth.

Prof Holderied said: “Moths are going to inspire the next generation of sound absorbing materials.

“New research has shown that one day it will be possible to adorn the walls of your house with ultrathin sound absorbing wallpaper, using a design that copies the mechanisms that gives moths stealth acoustic camouflage.”

The research is published in the Proceedings of the Royal Society A.