Moth wings could inspire a new generation of soundproofing material

Scientists at the University of Bristol in England have found that moth wings are excellent absorbers of sound in ways that could have a huge impact on soundproofing technologies for buildings and transportation.

Moths can be masters of camouflage, many with markings on their wings that blend in with the colour of tree bark, making them difficult for predators to spot.

Now researchers have found that another way they hide — by absorbing the sounds emitted by bats, one of their main predators, using their wings — could offer clues for reducing sound that bounces off of solid surfaces.

The moth's sound-absorbing wings make it practically invisible to the bat because far less sound is returned when the bat uses echolocation to find prey. This effect is similar to radar absorbing coatings used on stealth aircraft.

Under the microscope, a moth's wing is made up of tiny scales with holes that absorb ultrasonic sound waves, these sounds are emitted by bats at a higher frequency than humans can detect.

In an experiment, a small section of a wing was placed over a highly reflective aluminum disc then sound waves were aimed at the disc to see how much sound was reflected back. To their amazement, the scientists found the moth wing absorbed up to 87 per cent of the sound that struck the surface over a wide range of frequencies and angles at which the sound waves struck the wing.

The remarkable aspect is how thin the wing scales are — only two per cent as wide as the width of the wavelength of sound they are absorbing, which is much thinner than conventional sound proofing material.

The scientists also tested stripped down versions of the moth wing for their sound absorbing abilities. The wings' cross section has scales above and below the membrane they're connected to. When the scales on the top were removed, leaving the membrane layer exposed to the sound source with the scales underneath, the researchers unexpectedly found it absorbed sound almost as well as the intact wing. 

They predict that if the microscopic structure of a moth wing could be emulated in manufacturing and scaled up to industrial size, and tuned to the lower sound frequencies we hear in everyday life, it could have a huge impact on buildings, homes and transportation, especially aircraft, which must be soundproofed against propeller and jet noise. Ultra-thin sound absorbers would save weight on aircraft and, in turn, save on fuel.

City buildings could be insulated against traffic noise and it may even be possible to design moth-inspired wallpaper to make your home a quieter place. 

This is not the first time scientists and engineers have taken cues from nature to make technology quieter. Owls are silent fliers and part of the secret to their stealth is the serrated shape formed by the edges of their feathers.

The serrations on the leading edge of the wing help to smooth out the airflow, reducing the turbulence that would otherwise create sound, which is then further dampened by the wings' velvety down feather texture and soft fringe along the feathers' trailing edges.

Designers of modern jet engines have taken tips from the owl and designed serrated edges on the back of the engines and wings to make the jets quieter. Research has also shown that adding serrations to the trailing edges of wind turbine blades reduces noise as well.

We live in an increasingly noisy world, and sound can be detrimental to health, such as when it disturbs sleep or causes stress. Nature has had millions of years of evolution to find elegant solutions to this loud problem.