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Nature has a way to put our best technology to shame. For decades now, scientists have been trying to produce the ultimate acoustic absorber, a carefully designed material that can handle acoustic waves for stealthy movement or simple peace and quiet.
Apparently, however, that precious “metastructure” may already exist in the natural world, and has been hiding among the fluttering wings of some earless moths.
The shovel-shaped scales that shape some moth wings are so efficient at absorbing sound, a new study says they can be classified as a natural metamaterial. Structurally and conceptually, the authors say, they have the same fundamental attributes.
Metamaterials typically refer to structures we design that cannot be found in nature. While we are occasionally surprised to discover biological structures we wish we had thought of first, so include them as somewhat honorary metamaterials.
This is an exciting discovery given the rarity of these “metameters” in nature. Although some notable examples have been identified so far – the silk of silkworms, the iridescence of butterflies – this is the first acoustic metastructure ever discovered in the real world.
According to the researchers, it is “unlike anything considered to date”.
By modeling the natural structure of the wings of two earless moth species and two butterfly species, the researchers showed that the way the moth scales are uniquely spaced and sized gives them a stealthy coating a hundred times thinner than their lengths. wavelengths longer than the sound it can absorb.
This allows the insects to remain both light and suspended in the air, while providing acoustic camouflage from the bats they echo – an achievement that engineers have aspired to for years.
“Our study demonstrates that functional implementations of this existed in nature well before modern science,” the authors conclude.
“Understanding these structures and mechanisms offers the future possibility of developing thinner and lighter materials and noise control devices.”
In all likelihood, this natural metamaterial has been sculpted by millions of years of evolution, so scientists shouldn’t take it too much.
With no means of hearing the high-frequency calls that many bats use to locate their prey in the dark, earless moth species that still venture out at night need ways to avoid becoming a diner.
Under such unique forms of pressure, these insects have developed a unique coating of stealthy scales, not only on the hairy chest and wing joints – as previous research has shown – but also on the moth wings themselves.
The scales of the wings, however, are much more impressive. While the body of the earless moths appears to be lined with a 1.5mm sound-absorbing shield, that layer thickness would weigh the moth’s wings too much.
Under an electron microscope, the researchers have now shown that the scales on the moth’s wings create a soft coating less than 0.3mm thick. For reference, the sound waves used by bats for echolocation are about 17 mm.
Using ultrasound tomography, the researchers tested how the wings of two earless moth species captured sound compared to the wings of two butterfly species.
By measuring the effect on scaled and scaleless wings, the team found that butterfly wings, which did not evolve under the pressure of bats, did not show sound absorbing properties. On the other hand, the wings of the moths have a soft carpet of molded and molded scales tuned for each bat’s call frequency.
These scales were able to reduce ultrasonic echoes and dampen the sound even at the lowest frequencies tested.
“Such broadband absorption is very difficult to achieve in the ultra-thin wing structures of moths, which is what makes it so extraordinary,” says Marc Holderied, a sensory ecology expert at the University of Bristol.
Other porous absorbers that achieve this level of absorption are much thicker and tend to absorb sound only in a narrow frequency range.
The team hopes the discovery will help us design more efficient broadband noise canceling devices that are also lightweight.
“The Promise is one of the much thinnest sound absorbers for our homes and offices, we would be approaching a much more versatile and acceptable sound absorbing ‘wallpaper’ rather than bulky sound absorbing panels,” says Holderied.
All this from a moth.
The study was published in PNAS.
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