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Nature has a way of shaming our best technology. For decades, scientists have attempted to craft the ultimate sound absorber – a carefully designed material that can handle sound waves for stealthy movement or simple peace and quiet.
It turns out, however, that this cherished “ metastructure ” may already exist in the natural world, and is hiding in the floating wings of some earless butterflies.
The paddle-shaped scales that outline some butterfly wings are so effective at absorbing sound, a new study claims they can be classified as a natural metamaterial. Structurally and conceptually, say the authors, they have the same basic attributes.
Metamaterials generally refer to structures that we design that we cannot find in nature. Although every now and then we are surprised to discover biological structures that we wish we had thought of first, so include them as somewhat honorific metamaterials.
This is an exciting discovery given the rarity of these “metameters” in nature. While there are a few notable examples 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 that has been seen to date.
By modeling the natural wing structure of two species of earless butterflies and two species of butterflies, the researchers showed that the way the butterfly’s scales are uniquely spaced and sized gives them a stealthy coating that is a hundred times thinner than the longest wavelengths of sound it can absorb.
This allows insects to remain both light and airy, while providing acoustic camouflage against echolocated bats – an achievement engineers have aspired to for years.
“Our study demonstrates that functional implementations of this existed in nature long before modern science,” the authors conclude.
“Understanding these structures and mechanisms offers the future possibility of developing thinner and lighter noise control materials and devices.”
In all likelihood, this natural metamaterial has been sculpted through millions of years of evolution, so scientists shouldn’t take it too hard.
Without a way to hear 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 these unique forms of pressure, these insects developed a unique coating of stealthy scales, not only on their thorax and furry wings – as previous research has shown – but also on the wings of the butterflies themselves.
The wing scales are much more impressive, however. While the bodies of butterflies without ears appear to be covered with a 1.5-millimeter sound-absorbing screen, this level thickness would make the butterfly wings much too heavy.
Under an electron microscope, researchers have now shown that the scales on butterfly wings create a soft coating less than 0.3mm thick. For reference, the sound waves that bats use for echolocations are around 17mm.
Using ultrasound tomography, the researchers tested how the wings of two species of earless butterflies picked up sound compared to the wings of two species of butterflies.
By measuring the effect on wings with and without scales, the team found that butterfly wings, which had not evolved under pressure from bats, exhibited no sound absorbing properties. On the other hand, the wings of moths have a soft carpet of shaped and patterned scales in a way that is tuned for each bat call frequency.
These scales were able to reduce ultrasonic echoes and dampen sound even at the lowest frequencies tested.
“Such broadband absorption is very difficult to achieve in the ultra-thin structures of the wings of moths, which makes it so remarkable,” says sensory ecology expert Marc Holderied of the University of Bristol.
Other porous absorbers that achieve this absorption level are much thicker and tend to absorb sound only in a narrow frequency range.
The team hopes this discovery will help us design more efficient and lightweight broadband noise canceling devices.
“Promise is one of the much thinner sound absorbers for our homes and offices, we’d be getting closer to a much more versatile and acceptable ‘wallpaper’ rather than bulky sound absorbers,” says Holderied.
All of this from a moth.
The study was published in PNAS.
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