Diffraction of Life - biosonar camouflage, cloaking and concealment

Invisibility cloaks are fantastic devices in popular culture from Harry Potter to Star Wars. The science behind cloaking has been advanced to a level that brings a future real-life invisibility cloak within our reach. In fact, several cloaks with partial functionality have already been realised with so-called metamaterials - assemblies of multiple elements engineered to have properties not yet found in nature.
An even more promising field for the development of a functional cloaking device is not for light, but for sound - acoustic cloaks. Because the wavelengths of sound are longer than those of light waves, it is easier to design and build acoustic metamaterials and hence effective cloaks. Indeed, the most advanced acoustic cloak can now completely hide an object on a surface - a so-called carpet cloak. As a metamaterial it consists of partly overlapping perforated plates, arranged much like roof tiles.
While we know of no metamaterials for light in nature, is this also true for acoustic metamaterials? Which organism would need such a device to hide itself acoustically? We propose the answer lies in the 65MY old arms race between echolocating bats and their moth prey. A 'biosonar cloak' against bats would reduce predation pressure on the moths and therefore offer substantial evolutionary benefits. Interestingly, the layers of scales on a moth's body surfaces bear remarkable structural resemblance to an acoustic carpet cloak.
We hypothesise that moth wings are an acoustic metamaterial engineered by nature. We will investigate whether the scales on the moths have acoustic properties that hide the moth from an echolocating bat.
In our pilot study, we have developed a 'biosonar visualizer' that creates acoustic images revealing the reflective nature of body parts. This technique is closely related to medical ultrasound imaging (tomography). We also use a laser scanner to measure how the layer of scales vibrates in response to ultrasound. From these preliminary data we find a surprising range of interesting adaptations: First, scales on a (dead and dried) moth wing change wing reflectivity by a factor of four. In another very exciting discovery, we find that the long tails of Luna moths reflect strong echoes such that they attract the bat's attention and attack away from the moth's body. We also find that the eye spots, used in a visual display to startle an approaching predator, also stand out acoustically. Finally, we have evidence that moths choose the places to rest and adjust their wing position to reduce contrast and blend into the substrate acoustically. This pilot data make clear that there is a wide and promising unstudied field of echoacoustic adaptations.

In analogy to visual camouflage, we introduce an entirely novel field of research - biosonar camouflage.

We identify several possible strategies for camouflage. One strategy is for the moth to reflect very little ultrasound (cloaking) when in flight, thus reducing the distance over which a bat can detect it. Another strategy involves mimicking the echoes of a resting surface. In this scenario, the moth resting on the bark of a tree is acoustically blending with its environment.
Our research will establish what acoustic properties and sound processing mechanisms have evolved in moth scales in response to bat biosonar.
Since the industrial revolution, the world has become a noisy place where man-made sounds are pervasive throughout our living habitats. Acoustic pollution is a source of discomfort and stress for humans and animals. We will use our understanding of moth wings to 3D print scaled prototypes with acoustic metamaterial properties at audible frequencies. Thereby we contribute new bio-inspired solutions to current noise control challenges at the low frequencies so important to human speech and comfort.

In the 65MY old arms race between echolocating bats and moths the strength of the moth's echo determines the distance over which bats can detect it. Our pilot data now make clear that -in analogy to visual camouflage- there is a wide unstudied field of echoacoustic adaptations, which we introduce here as 'biosonar camouflage'.
Our central hypothesis is that the layers of scales on the moths' body act as acoustically active metamaterials with echo-reflective properties that reduce predation risk by biosonar.
One camouflage strategy is efficient absorption of ultrasound (cloaking) in flight to reduce the bat's detection range. Another involves matching absorption to that of the resting substrate to acoustically blend with it.
Our pilot data show that fresh scales affect ultrasound absorbance, revealing biosonar 'cloaking', 'matching to background', 'shadow elimination', 'deceptive false targets' and 'eye spot startling signals'. None of these mechanisms have been studied, but they only work at frequencies used by bats, which is direct evidence for a role against biosonar.
Our innovative set of approaches, mainly developed and exclusively available in Bristol, offers a unique opportunity to discover novel near-field and boundary layer mechanisms of sound manipulation.
We hypothesize that the actual mechanism is a combination of static absorptive, dissipative, refractive and active mechanical interactions between structure and ultrasound. Using acoustomechanical characterisation and mathematical modelling we will test five different absorptive processes. We will manufacture and test plastic replicas of wing structures scaled up to frequencies relevant to humans. Humans do not use or are sensitive to ultrasonic biosonar, yet our research will develop novel ways to visualise and understand this elusive part of the natural world, and aim to inspire man-made radar and sonar camouflage and ultra-thin absorptive structures for architectural acoustics and noise control.

The planned research will benefit researchers in the field of bioacoustics, including the sensory ecology and biophysics of moths and bats. Establishing the echo acoustics of moths will further inform the constraints set upon bat echolocation strategies, and therefore the evolution of hearing in bats. Several other disciplines related to the studies of environmental acoustics, architectural acoustics, noise control and abatement will also be interested by our approach and research rationale. In particular, the identification and characterization of structurally complicated metamaterials -the scaled wings of moths- at the length scale of centimetres to micrometers, including nanoscale mechanical responses will spark the interest of designers and developers of acoustic foams and panels. In addition, our finite element modeling work will constitute a novel basis for the "in silico" design of adapted, hybrid alternatives to conventional alveolar foams.
Perhaps starting at the times of the industrial revolution, the world has become a noisy place. Machinery, vehicle and human-generated sounds are pervasive throughout our living habitats, and the living habitat. Our research is expected to impact on our fundamental understanding of acoustic ecology, and provide further tools to mitigate the transmission of sounds where they are unwanted. Acoustic pollution is a source of discomfort for humans and animals. In effect, sound pollution affects not only hearing, but is a source of stress, annoyance and affects sleep and communication. Noise at the work place, home and learning environments has been shown to affect cardiovascular health, attention and academic performance.
Research outcomes have direct impact on how technology will contribute to improve the quality of the human and natural environment. Our work is therefore of direct relevance to national and global human health and human comfort.
In terms of public engagement, the research planned will appeal to a broad cross-section of the public as part of an increasing awareness of the role of noise in everyone's life and the importance of quality of life in the urban environment. Our findings will contribute to current efforts to improve noise control, especially at low frequencies (around 200 Hz), which are notoriously difficult to control. Such contribution will be achieved by direct engagement and collaboration with noise control professionals and manufacturers.
As detailed in our Pathways to Impact document, we will also directly engage with the media, science festivals, environmental agencies and other public organisations, particularly through our interactive biosonar imaging exhibit (tomograph). The public will thus actively benefit from our activities through the electronic media (web pages, twitter, YouTube channel), but also through activities in science festivals, contributions to the general press, and television and radio interviews.
Finally, in conducting this research programme, the team (PDRAs) will gain and benefit from further training and experience in project and personnel management, as well as developing strong communication skills through public engagement and industry and policy-driven knowledge exchange activities. Importantly, we will ensure that training is delivered to the entire team, and that of volunteers, enhancing the educational value of impact, and generating increased opportunities for science to engage with the public and policy makers, teachers, school children, industrial partners and fellow academic researchers.