Sound absorbers in nature? 87 percent of the noise energy can be cut by moth wings

Bats first learned to use echolocation about 65 million years ago. They do this by making high-frequency clicks with their mouth or nose and listening for the echoes that come back from objects.

Since then, bats and moths have been in a “acoustic arms race” between predator and prey. Now, bats are a big threat to moths, so they have developed many ways to protect themselves. However, the moth’s main weapon—its wing—could be the key to improving noise-canceling technology.

Researchers recently found that moth wings do block echolocation calls from bats. They are now trying to figure out if their structure could be used to make sound-absorbing panels that work better when they are not moving.

Now, in a study published in Proceedings of the Royal Society A, scientists at the University of Bristol have found that the scales on moth wings could be good sound absorbers even when placed on an artificial surface.

Great at absorbing sound.
In a statement, Professor Marc Holderied of Bristol’s School of Biological Sciences said, “We needed to know first how well these moth scales would work in front of an acoustically highly reflective surface, like a wall.”

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

The Science Times says that the structure of an insect’s wings is the perfect pattern for a metasurface sound dampener that can be used in many places.

Prof. Holderied and his team tested this by putting small pieces of moth wings on an aluminum disc and then systematically testing how the orientation of the wing with respect to the sound coming in and the removal of scale layers affected absorption.

They had a point. Even when they were on top of a solid acoustic substrate, the wings of a moth were very good at absorbing sound. Up to 87 percent of the sound energy that came in was taken in by the wings.

Moth wing

They also said that the effect is wide-band and omnidirectional, meaning that it works for a wide range of frequencies and sound angles.

“What’s even more amazing is that the wings do this despite being so thin; the layer of scales is only 1/50th as thick as the wavelength of the sound they are absorbing,” said lead author Thomas Neil. “Because of this amazing ability, the moth wing is a naturally occurring acoustic absorbing metasurface, which is a material with unique properties and abilities that can’t be made with ordinary materials.” Creating and putting together prototypes
As cities get noisier, these kinds of effective, quiet ways to reduce noise become more important. These lightweight sound-absorbing panels could have a huge effect on the travel industry. Saving weight in planes, cars, and trains would make them run more efficiently, using less fuel and putting out less CO2.

Scientists want to copy the moth’s ability to block out noise by designing and building prototypes that are based on how the moth does it. At the moment, the range of frequencies where moth wings absorb light is in the ultrasonic range.

The biggest problem they face is making a structure that works at lower frequencies but still has the same ultrathin structure as the moth.

“The next generation of materials that absorb sound will be based on moths,” said Holderied.

“New research has shown that one day you’ll be able to cover the walls of your home with ultrathin sound-absorbing wallpaper. The design is based on the way moths hide their sounds to avoid being seen.”

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