Moth scales act as acoustic metamaterials

Bats track prey using ultrasound-mediated echolocation, and moth species without ultrasound-sensitive ears must rely on other strategies to avoid predation. Thomas Neil, Zhiyuan Shen, et al. (pp. 31134–31141) report a form of acoustic camouflage in two species of earless moths—Antheraea pernyi and Dactyloceras lucina—that allows them to evade echolocating bats. A thin layer of paddle-shaped scales that reduce ultrasonic echoes and absorb impinging sound at the relevant frequencies carpets the moths’ wings. Unlike butterfly scales, the moth scales reduce the average target strength across the frequency range bats use to detect flying prey and absorb up to 72% of sound intensity at 78 kHz. The scales dampen sound at frequencies as low as 20 kHz, acting as ultrathin, deep-subwavelength metamaterial absorbers; porous absorbers used in sound insulation technology are typically much thicker.

Analysis of moth scale models borne on stalks inserted into a wing membrane revealed that the scales owe their sound-absorbing properties to a unique resonance geometry resulting from a combination of scale morphologies and tiling patterns. In contrast, the authors note, a metamaterial absorber built using butterfly scales, which are arrayed uniformly and exhibit clustered resonances, would not dampen sound over a similarly broad range. According to the authors, the findings could inspire the design of high-performance acoustic panels and ultrathin, lightweight noise-canceling devices. — P.N.

Article from PNAS December 8, 2020 117 (49) 30861-30863;