Biological metamaterials for enhanced noise control technology

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Invisibility cloaks are fantastic devices in popular culture from Harry Potter to Star Trek. But even in the real world so-called metamaterials (synthetic composite materials with emergent new properties) can act as (partial) cloaks both against light (vision) and sound (acoustics). We recently discovered that the 65MY old arms race with their echolocating bat predators has equipped moths with remarkable acoustic metamaterials on their wings and bodies (e.g. Shen et al. 2018 PNAS). The strength of a moth's echo determines the distance over which bats can detect it. Fur on bodies and scales on wings of moths have broadband absorptive properties that each outperform current sound absorber technology. While moth fur is a fibrous porous absorber almost twice as efficient as comparable technical solutions, the scales on moth wings have an even more exciting functional principle: Each scale resonates and together they create efficient broadband absorption of bat ultrasound. In contrast to technical solutions, these scales best absorb low frequencies, and show an unparalleled deep-subwavelength (<1% of wavelength) functionality. Their structure and (postulated) functionality make moth wings the first documented biological acoustic metamaterial - a discovery as transformative as nanoscale photonic crystals creating structural colour in butterfly scales. Our objective is to reveal the, as yet unknown, biophysics behind these evolved metamaterial absorbers and translate them into the human hearing range. In collaboration with our industry partner we will then develop prototypes for the next generation of more efficient bio-inspired noise control devices (biology-push). In return, understanding the biophysics will cross-inspire biology, as it allows us to look for and identify further acoustic metamaterials with different adaptiveness (i.e. tuneable metasurfaces; technology-pull).Unlocking the potential of evolved deeply subwavelength sound absorber metamaterials requires a coordinated, multidisciplinary, world-leading team of researchers; it is not possible to disassociate the biology from the mechanical modelling and treat the problem piecemeal. The assembled team of researchers has complementary expertise ranging from structural analysis of scales created by epidermal cells, acoustomechanical characterisation, and absorptive index assessment (lead Biology, Holderied, Robert), to theoretical biophysics of metamaterial properties (lead Applied Mathematics, Craster), to computational biophysics, modelling, and prototyping (lead Ultrasonics Engineering, Drinkwater with industry partner) and product development and commercialisation (industry partner). A range of cutting-edge technologies and methodologies (some of which pioneered in the applicants' labs exclusively) are required for this research including Dynamic Acoustic 3D imaging, Scanning Laser Doppler Vibrometry and Refractometry, X-ray nanoCT (successful Diamond synchrotron light source bid 2018), COMSOL multiphysics modelling, 3D lithography and nanoScribe 3D fabrication.Promisingly, our first lithographically produced scale replicas indeed resonate at the most important frequency for human communication (4 kHz). The outcome of our iterative effort will be novel broadband sound absorbers, that are much thinner and lighter than existing systems. These bioinspired absorbers not only have substantial economic potential (as evidenced by the commitment of our industry partner), their lower space and weight footprint promises more flexible and acceptable noise control solutions for our offices and homes. They will help in our fight against acoustic pollution (e.g. cost to the NHS of hearing loss is estimated to be 450M per year), which is the 2nd largest environmental health risk in Western Europe leading to over 10000 premature deaths every year (EEA, 2014; WHO, 2011).