Nature has a way of embarrassing our best technology. For decades, scientists have been trying to create the ultimate sound absorber – a carefully engineered material that can handle sound waves for stealth movement or smooth peace and quiet.
It turns out, however, that the beloved ‘metastructure’ may already exist in the natural world, and it is hiding in the fluttering wings of some earless moth.
Pedal-shaped scales or some moth wings are so efficient at absorbing sound, a new study claims, that they can be classified as naturally occurring metamaterials. Constructively and conceptually, the authors say, they have similar original features.
Metamaterials usually refer to those structures of the engineer that cannot be found in nature. However, after each and every time we are surprised to find biological structures that we wish we had thought of before, so include them as somewhat honorary metamaterials.
Given how rare these ‘metamaterials’ are in nature it is an exciting discovery. While there are still some notable examples identified – silk of silk rays, ego of butterflies – this is the first acoustic metastructure discovered in the real world.
According to the researchers, this is “contrary to anything believed to date”.
Modeling the natural structure of the wings of two species of mothless moths and butterfly species, researchers have shown that moth scales are uniquely spaced and the size makes them a hundred times thinner than they can absorb the long wavelengths of a single sound.
This allows the organism to remain both light and aerated, while providing an acoustic camouflage from the ecolocating bat – an achievement engineers have been working on for years.
“Our study shows that its functional implementations exist well in nature before modern science,” the authors conclude.
“Understanding these compositions and mechanisms provides the future potential for developing thinner and lighter sound control materials and devices.”
In all likelihood, this natural metamaterial has been created by millions of years of evolution, so scientists should not take it too seriously.
Without any means of listening to high frequency calls, many bats use their prey to find prey in the dark, without prey, which still ventures out at night, needing ways to avoid becoming dinner.
Under such a unique kind of pressure, these insects have developed a unique coating of stiltty scales not only on their furry breasts and wing joints, as previous research shows – but also on moth wings.
The wing scales, however, are more impressive. While the earless moth’s body appears to be coated in a 1.5 mm shield of sound absorption, the thickness of that layer will make the moth’s wings heavier.
Under the electron microscope, researchers have now shown that the scales on moth wings form a soft coating less than 0.3 mm thick. For reference, the use of bat of sound waves for echolocations is about 17 mm.
Using ultrasound tomography, the researchers tested how the wings of two earless moths received sound compared to the wings of two butterfly species.
Measuring the effect on the wings with and without scales, the team found butterfly wings, which did not develop under bat pressure, did not show sound absorbing properties. Moth wings, on the other hand, have soft carpets with scales and are arranged that way for each frequency of the bat call.
These scales were capable of reducing ultrasonic echoes and were able to dampen sound even at the lowest frequency tested.
“Such broadband absorption is very difficult in the ultrathin structures of moth wings, which makes it very noticeable,” said Mark Holdry, a sensory ecology expert at the University of Bristol.
Other porous absorbers that achieve this level of absorption are more ger, and tend to absorb sound only in a narrower frequency range.
The team hopes that this discovery will help us design more efficient broadband voice-canceling devices that are also lightweight.
“Promise is a very thin sound absorber for our homes and offices fees, we will move closer to a more versatile and acceptable sound absorber‘ W wallp wrapper ’instead of wide absorber panels.
All from one moth.
The study was published in P.N.A.S..
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