"When moth wings are used to coat hard, artificial surfaces, they can significantly reduce the reflection of incoming ultrasound. The detailed mechanism of this sound absorption is still unclear. Still, it is likely to be a combination of the scales' mechanical absorption, coupled with some dissipation through thermal and viscous effects brought about by the interaction of the scales, wing membrane, and air movement through the scales. Moth wings evolved a way to assemble many resonators individually tuned to different bat frequencies into an array of absorbers, which together create broadband absorption by acting as acoustic metamaterial – the first known in nature. Such a broadband absorption is tough to achieve in the ultrathin structures of moths' wings, which makes it most remarkable!"

Conventional soundproofing materials tend to be porous. To be effective, they must be thicker than about 10 % of the wavelength of the sound they are blocking. Metamaterials made of specially designed structures can be thinner than 1 % of the wavelength they absorb, but these tend to operate over a very narrow band of frequencies. While broadband metamaterials have been created, they tend to be much thicker.

The moth wing scales come in many sizes, each with a characteristic resonance frequency. The different sizes absorb sound across a broad frequency range, making them far more effective than conventional sound-absorbing materials. Previous studies have shown how moth wings absorb sound waves as the insects travel through the air. In their research, Holderied's team looked at how the wings absorb sound when attached to an aluminum disk

Such a hard, manufactured surface will typically reflect most of the incoming sound. In contrast, the researchers observed that the moth-wing coating reduced this reflection by up to 87% at the lowest frequencies they tested. The ultrasound used by the team had wavelengths some 50 times longer than the thickness of the wings.

The team then looked closer at how the wings absorbed the sound. They removed the scales from the wing top and found this caused varying absorption with the orientation of the incoming sound. While its performance remained high when the bald side was facing the incoming sound, it almost completely collapsed in the opposite direction. By recreating this scenario in simulations, Holderied's team showed that the performance of the metamaterial depends strongly on the presence of scales in the air gap between the wing membrane and the hard surface below.

The sound waves absorbed by moth wings may be beyond the range of human hearing. However, by adapting the design to absorb lower frequencies, Holderied and his colleagues hope that new artificial metamaterials inspired by their structure could soon be developed. These structures could lead to breakthroughs in high-performance soundproofing: potentially leading to coatings for walls, vehicles, and noisy machinery that take up a fraction of the space required by existing materials.

Typical applications for our HEAD acoustics high-precision turntable HRT I are automated orientation-dependent acoustic tests where it rotates, e.g., telephony devices, smart home devices, (video-) conferencing systems, loudspeakers, microphones, or an artificial head. This time, helping Holderied and his team scrutinize its wings, a moth was pinned to the turntable (see picture 2) to position it at specific angles in the measurement field and be exposed to sound waves under laboratory conditions. The rotation unit of the turntable offers a 360° rotation range, which can be approached in 0.1° steps. The adjusted angles can be reproduced to an accuracy of 0.02°.

In addition to the control via the measurement and analysis software ACQUA or the software tool RC HRT I, users can also control the turntable stand-alone using programming languages like MATLAB or Python, making HRT I even more flexible to integrate into test setups.

"We are happy to run two HRT I turntables of HEAD acoustics for our moth test series, one placed on a table, the other mounted to the wall. The HRT I turntable operates precisely, robustly, and silently – even when at stillstand – and that is why it was the first choice for the University of Bristol", explains Prof Holderied.
"I hope that the new understanding of the absorption mechanisms of moth wing scales will inspire the next generation of acoustic metamaterial sound absorbers. The promise is one of much thinner sound absorbers for our homes and offices. Our new research paves the way for much more versatile and acceptable sound absorber wallpaper rather than bulky panels."

Thank you very much, Prof. Holderied. We wish you all the best in your further research work!