Biological metamaterials for enhanced noise control technology

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).

In our ever-noisier world, noise pollution leads to over 10000 premature human deaths every year and is the second largest environmental health risk in Western Europe, (European Environmental Agency, 2014, World Health Organisation 2011); the UK government estimates the annual social cost of just urban road noise in England as £7-10 billion. Noise control technology is thus increasingly deployed to provide a healthier living and working environment, but it will have to become ever lighter, thinner, and more effective for us to accept it into our offices and homes.
A key promise of the rapidly expanding field of acoustic metamaterials is to develop highly adaptable ultrathin subwavelength sound absorber technology that will substantially reduce the weight and space footprint of architectural acoustics solutions and thereby help providing a healthy living and working environment. There is indeed a revolution underway in acoustics technology with metamaterials at the fore, several start-ups (Sonobex, Metasonics) have emerged in the UK and many others overseas.
We recently have discovered that the 65 million year acoustic arms race between bats and moths has given moth scales the ability to absorb the biosonar sounds of their bat predators (Shen et al. 2018 PNAS; Neil et al. under revision), and their effectiveness far exceeds current technological solutions. These scales are indeed the first documented natural acoustic metamaterial, and their structure and resonant functionality is unlike anything currently known let alone utilized. In this proposal a deeply integrated multidisciplinary team including our industry project partner QinetiQ will close this knowledge gap and develop biological metamaterial principles into advanced noise control prototypes. Industry involvement ensures a continuing process to reach the next technological readiness levels towards a finished noise control product.
Immediate impact from this project pertains to three levels: First and foremost, development of technical and industrial noise control applications with our industry partner QinetiQ. Our work is therefore of direct relevance to national and global human health and wellbeing (details in National Importance in Case for Support) with substantial economic long-term potential. Second, scientific network generation by creating a Special Interest Group nucleus for the emerging field of biological acoustic metamaterials within the UK Acoustics Network (www.acoustics.ac.uk). And finally, media communication and direct public engagement, that are organised and delivered to maximise societal impact through communication to audiences that are larger than the scientific community. In the context of local, and national science and nature festivals, we will produce a hands-on transportable interactive display (BATtleships) that will contribute to University public engagement, e.g. local schools, and generate knowledge exchange and active public participation (see pathways to impact for details).
Impacting biology, our discoveries establish the new research field of Acoustic Camouflage. We have only started investigating biological acoustic metamaterials, and many more discoveries are undoubtedly waiting to be made. Understanding the biophysics will cross-inspire biology as it allows a targeted search for further biological structures, in particular adaptively tuneable absorbers.
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 placement activities. 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.