Being an insect is a hard-fought struggle for constant survival. Predators lurk at every turn, looking for their next quick meal, and evading these hunters is no easy feat. Butterflies and moths have evolved impressive camouflage to avoid detection, but no form of visual camouflage can prevent bats – their top predator – from finding them with their echolocating abilities. However, after a 65 million-year-old evolutionary arms race, researchers believe they have discovered moths that can.
Researchers from the University of Bristol have identified an incredible layer of acoustic-dampening material that sits atop a moth’s wing that can absorb bats’ echolocating waves, according to a study published in PNAS. By absorbing the sound and preventing echoes from bouncing off their wings, the moth could stealthily avoid detection and survive where butterflies and other insects cannot. This represents the first time a naturally occurring acoustic metamaterial has been discovered.
The sound-absorbing material (resonant absorber) comprises an extremely thin layer of scales that lay across the wings, light enough to allow flight but dense enough to absorb sound and render their acoustic footprint almost invisible. Previous studies have shown that moths have a layer of sound-absorbing material over their body, but this layer is too thick to be functional on a wing. The researchers believe that by clever engineering, evolution has produced resonant absorbers atop their wings that are so thin and light that they don’t inhibit the moth’s flight.
Traditional soundproofing panels work by utilizing foam or wool peaks and valleys to absorb sound waves, making them ‘bounce’ around the surface to convert them into heat energy, to the point in which the sound is no longer detectable. However, this requires a large surface area and, in many cases, the material needs to be thick to absorb sound effectively. So how do moths compact this down into a layer so thin?
The acoustic metamaterial works by being smaller than the wavelength of sound it absorbs. If sound hits a material that is smaller than its wavelength, it cannot reflect off and, in the case of the moths, be detectable to the bats. Even more impressively, the researchers believe the moths have an array of different resonators tuned to different frequencies, so that a range of wavelengths can be absorbed by the material.
“Most amazingly, moth wings also evolved a way to make a resonant absorber absorb all bat frequencies, by adding another amazing feature – they assemble many of these resonators individually tuned to different frequencies into an array of absorbers, which together create broadband absorption by acting as an acoustic metamaterial – the first known in nature,” said head researcher Dr Holderied in a statement. “Such a broadband absorption is very hard to achieve in the ultrathin structures of moths’ wings, which is what makes it so remarkable.”
The researchers hope that the new discovery could be leveraged into creating thinner and more effective sound absorbers for studio and office use.
“The promise is one of much thinner sound absorbers for our homes and offices, we would be getting close to a much more versatile and acceptable sound absorber 'wallpaper' rather than bulky absorber panels,” said Dr Holderied.
[H/T: Chemistry World]