Acoustic camouflage made of scales: Researchers have gained new insights into the natural patent that protects moths from echolocation by bats. The ingenious surface structures on the insect wings form what is known as an acoustic metamaterial with astonishing properties: It can absorb a wide range of sound, but it is thin and light. The concept could thus inspire the development of innovative soundproofing coatings, say the scientists.
The darkness protects moths from hungry glances, but this is of no use against the sophisticated hunters of the night: bats detect their prey using a highly developed echo-location system. They emit high-frequency screams, which are reflected by the body of the insect. Thanks to the fine echoes, the bats can then locate the prey and finally even snap them out of the air. But the nocturnal insects have upgraded with their enemies in the course of their joint evolution: They created sophisticated protective mechanisms.
It is known, for example, that some moths have developed a fine hearing and special flight skills in order to be able to react to the bats’ biosonar through clever evasive maneuvers. However, many other moths do not seem to use such active strategies – they are even deaf. As the research team headed by the University of Bristol recently reported, these species are not helpless victims either: As the scientists were able to show, these insects have developed acoustic camouflage methods to hide themselves from the calls of the bats: In contrast In relation to butterflies, the wings of some species of moths swallow the calls of bats surprisingly effectively, resulting in a reduced echo. 85 percent of the incident sound energy is therefore absorbed. This means that a bat has to get much closer to the insects in order to be able to locate them.
A refined material under the microscope
As the team has already shown, the surface features of the moth’s wings are responsible for the acoustic effect. As with all butterflies, they are covered by scale structures. In contrast to the butterflies, they not only serve optical purposes in the nocturnal insects. Based on analysis data, the researchers developed computer models of the scale structures. They already suggested that they react in complex ways to the incident biosonar and thereby absorb it. In their latest publication, Thomas Neil and his colleagues now present more detailed insights into the principles on which the acoustic concept is based. As part of the study, they examined the fine structure of the wings of two species of moths down to the last detail using state-of-the-art technology and subjected them to experimental sound for acoustic analyzes.
They come to the conclusion that the wing structures are the first known acoustic metamaterial from nature. The term metamaterial usually describes man-made structures with very unusual physical properties that exceed “normal” concepts. A metamaterial usually consists of small, periodically repeating units that can influence the propagation of light or sound in a complex manner. So far only optical metamaterials from nature are known, but according to the researchers’ analysis results, the concept of the moth wings is an acoustic example.
Sound absorption with ease
They were able to show that the features of the scales as well as their complex arrangement lead to a broad-based sound attenuation: the peak absorption is accordingly 78 kilohertz. This corresponds to the frequency that bats use particularly frequently for their biosonar. But the spectrum for sound attenuation is surprisingly large, the researchers found. However, as they point out, the most amazing aspect is the low weight of the system. Because a key feature of acoustic metamaterials is that their structures can be much smaller than the wavelength of the sound they act on, so that they can be significantly thinner than traditionally constructed sound absorbers.
As the researchers explain, the cuticle of the moth wings should actually be 1.5 millimeters thick in order to act as a conventional sound absorber. But of course the little insects couldn’t fly with it. But thanks to the special features of the scale structure, the moths have succeeded in creating a resonance absorber that is 100 times thinner than the wavelength of the absorbed sound. In this way, the insects can keep their lightness and at the same time hide from the bats’ biosonar, the scientists explain. “The system makes it possible to absorb all relevant bat frequencies. This ability is based on the effect and composition of units that are individually tuned to different frequencies. Together they create an acoustic metamaterial with broadband absorption, ”says senior author Marc Holderied. “Such broadband absorption is very difficult to achieve in the ultra-thin structures of the moth’s wings, which is what makes it so remarkable,” the researcher sums up.
As he and his colleagues point out, the natural patent now has considerable potential for the technology, because optimized sound absorption is required in various areas. So far, such structures are usually relatively thick, heavy and noticeable. The model of the moth wings, however, could now inspire the development of very thin and light coatings. “Perhaps one day, for example, there could be moth-wing type sound absorber wallpaper,” says Holderied.
Source: University of Bristol, Article: PNAS, doi: 10.1073 / pnas.2014531117