Nature has a way of putting our technology to shame. For decades now, scientists have been trying to manufacture the ultimate sound amplifier – a carefully engineered material that can control acoustic waves for stealth or simple motion and peace.
However, as it happens, that beloved ‘metastructure’ may already exist in the natural world, and has been hiding in the shaded wings of some earless moths.
The paddle-shaped scales that pattern some moth wings are so efficient at absorbing sound, a new study claims that they can be classified as a naturally occurring metamaterial. Structurally and conceptually, the authors say, they have the same core attributes.
Metamaterials usually refer to structures we engineer that cannot be found in nature. While we are occasionally surprised to discover biological structures that we wished we had thought of first, so include them as somewhat honorable metamaterials.
That’s an exciting find given how rare these metameterials are in nature. Although a few remarkable examples have been identified so far – silk worm, butterfly worm – this is the first acoustic metastructure ever discovered in the real world.
According to researchers, it is “unlike anything considered so far”.
Modeling the natural structure of the wings of two species of horned moth and two species of butterfly, researchers have shown that the way moth scales with a unique space and size give them a stealth cover a hundred times thinner than the longest wavelengths of sound which it can absorb.
This allows the insects to remain light and airy, while also providing acoustic camouflage of relocating bats – an achievement that engineers have longed for
“Our study shows that functional implementations of this existed in nature long before modern science,” the authors conclude.
“Understanding these structures and mechanisms offers the possibility in the future of developing thinner and lighter noise control materials and devices.”
Probably, this natural metamaterial has been sculpted by millions of years of evolution, so scientists shouldn’t take it too hard.
Unable to hear the high frequency calls that many bats use to locate their prey in the dark, earless species of moth that still venture out at night, ways are needed to avoid becoming dinner
Under such unique pressure, these insects have evolved a unique cover of stealth scales, not only on their hairy thorax and wings joints – as previous research has shown – but also on moth wings themselves.
The wing scales, however, are far more impressive. Although the body of an earless moth appears to be covered in a 1.5 millimeter shield of sound absorption, that flat thickness would weigh moth wings down far too much.
Under an electron microscope, researchers have now shown that the scales on moth wings create a soft cover less than 0.3 mm thick. For your information, the sound wave batteries used for echoes are approximately 17 mm.
Using ultrasound tomography, researchers tested how the wings of two species of earless moths captured sound compared to the wings of two butterfly species.
In measuring the impact on wings with and without scales, the team found that butterfly wings, which have not evolved under bat pressure, showed no sound-absorbing properties. Moth wings, on the other hand, have a soft carpet of scales shaped and patterned in a way that is tuned for all bat call frequencies.
These scales were able to reduce ultrasonic echoes and sound attenuation even at the lowest frequencies tested.
“Such broadband absorption in the ultrathin structures of moth wings is very difficult to achieve, and that’s what makes it so special,” explains sensory ecology expert Marc Holderied of the University of Bristol.
Other permeable amplifiers that achieve this level of absorption are much thicker, and tend to absorb sound in a narrow frequency range only.
The team hopes the discovery will help us design more efficient broadband light cancellation devices that are also lightweight.
“The pledge is one of much thinner sound amplifiers for our homes and offices, we would come close to a much more versatile and acceptable sound absorber ‘wallpaper’ rather than bulky absorption panels,” said Holderied.
All that from moth.
The study was published in PNAS.