Better explicating the strengths and shortcomings of these models will help refine projections and improve transparency in the years ahead.
Image credit: Witsawat.S.
Edited by Katia Bertoldi, Harvard University, Cambridge, MA, and accepted by Editorial Board Member Evelyn L. Hu October 4, 2020 (received for review July 10, 2020)
Significance
Bats and moths are embroiled in an evolutionary arms race. Using ultrasonic biosonar, bats detect their insect prey, which in turn deploy diverse strategies to avoid predation. Here, we show that some moth species evolved wings covered with a canopy of scales that reduces ultrasonic echoes. Our empirical and mathematical analysis together show that moth wings exhibit key features of a desirable technological acoustic metamaterial. This work enriches our understanding of the structural and functional complexity of lepidopteran wings and reveals enticing new ways to design, using bioinspired metamaterial properties, high-performance acoustic panels and noise mitigation devices.
Abstract
Metamaterials assemble multiple subwavelength elements to create structures with extraordinary physical properties (1⇓⇓–4). Optical metamaterials are rare in nature and no natural acoustic metamaterials are known. Here, we reveal that the intricate scale layer on moth wings forms a metamaterial ultrasound absorber (peak absorption = 72% of sound intensity at 78 kHz) that is 111 times thinner than the longest absorbed wavelength. Individual scales act as resonant (5) unit cells that are linked via a shared wing membrane to form this metamaterial, and collectively they generate hard-to-attain broadband deep-subwavelength absorption. Their collective absorption exceeds the sum of their individual contributions. This sound absorber provides moth wings with acoustic camouflage (6) against echolocating bats. It combines broadband absorption of all frequencies used by bats with light and ultrathin structures that meet aerodynamic constraints on wing weight and thickness. The morphological implementation seen in this evolved acoustic metamaterial reveals enticing ways to design high-performance noise mitigation devices.
Footnotes
↵1T.R.N. and Z.S. contributed equally to this work.
- ↵2To whom correspondence may be addressed. Email: marc.holderied{at}bristol.ac.uk.
Author contributions: T.R.N. and M.W.H. designed research; T.R.N., Z.S., and M.W.H. performed research; D.R., B.W.D., and M.W.H. contributed new reagents/analytic tools; T.R.N., Z.S., D.R., B.W.D., and M.W.H. analyzed data; and T.R.N., Z.S., D.R., B.W.D., and M.W.H. wrote the paper.
The authors declare no competing interest.
This article is a PNAS Direct Submission. K.B. is a guest editor invited by the Editorial Board.
This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2014531117/-/DCSupplemental.
Data Availability.
Echo spectra data have been deposited in the University of Bristol research data repository, http://data.bris.ac.uk/data/dataset/l7qg341nfpe92uy4qp9pxjqwj (33).
Published under the PNAS license.
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Moth wings are acoustic metamaterials
Article Classifications
- Biological Sciences
- Biophysics and Computational Biology
- Physical Sciences
- Applied Mathematics
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- Abstract
- Moth Wings Are Deep-Subwavelength Broadband Sound Absorbers
- Moth Wings as Broadband Resonant Acoustic Metamaterial
- Scale Layers as Resonator Arrays: Measurements
- Scale Layers as Resonator Arrays: Numerical Modeling
- Mixed Scale Arrays as Acoustic Metamaterials
- From BOMA to BioMA
- Materials and Methods
- Data Availability.
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