Moth wings as sound absorber metasurface

Abstract

In noise control applications, a perfect metasurface absorber would have the desirable traits of not only mitigating unwanted sound, but also being much thinner than the wavelengths of interest. Such deep-subwavelength performance is difficult to achieve technologically, yet moth wings, as natural metamaterials, offer functionality as efficient sound absorbers through the action of the numerous resonant scales that decorate their wing membrane. Here, we quantify the potential for moth wings to act as a sound-absorbing metasurface coating for acoustically reflective substrates. Moth wings were found to be efficient sound absorbers, reducing reflection from an acoustically hard surface by up to 87% at the lowest frequency tested (20 kHz), despite a thickness to wavelength ratio of up to 1/50. Remarkably, after the removal of the scales from the dorsal surface the wing's orientation on the surface changed its absorptive performance: absorption remains high when the bald wing membrane faces the sound but breaks down almost completely in the reverse orientation. Numerical simulations confirm the strong influence of the air gap below the wing membrane but only when it is adorned with scales. The finding that moth wings act as deep-subwavelength sound-absorbing metasurfaces opens the door to bioinspired, high-performance sound mitigation solutions.

Keywords: acoustic metamaterial; bioinspired metamaterials; biological sound absorber; deep-subwavelength.

Figure 1.

Location of wing punch taken from the moth species Antheraea pernyi (a). Experimental set-up for characterizing the angular distribution of RC of the wing sample and metal disc (b). Workflow of the six experimental treatments (c). (Online version in colour.)

Figure 2.

Scanning electron microscopy image of (a) Cross-section through the target wing section highlighting two base scales (pink, white dashed outline) and one cover scale (yellow, white dotted outline). (b) Single base scale in top view. (c) Microstructure of a base scale showing the parallel ridges and cross-ribs. Layer thickness (LT), scale length (SL), scale width (SW), inter-ridge distance (IR) and cross-rib distance (CR). (Online version in colour.)

Figure 3.

Spectral target strength (a,b) and RC ((c,d); mean and standard error as shaded area; n = 5) of a metal disc covered by a wing sample for three experimental treatments (Intact, Dorsal bald and Both bald) when ensonifying either the dorsal surface (a,c) or the ventral surface (b,d). Horizontal lines near abscissa indicate significant pairwise differences (thin lines p ≤ 0.05; thick lines p ≤ 0.01). (Online version in colour.)

Figure 4.

Calculated spectral RCs of a wing segment decorated with (a) base scales and (b) cover scales of average dimensions (see electronic supplementary material, table S1) in four treatments: ‘Intact’; ‘Dorsal bald, dorsal surface ensonified’; ‘Dorsal bald, ventral surface ensonified’; ‘Both bald’ (see schematics in legends; note: Both bald curve covers Dorsal bald, ventral surface ensonified curve in (a)). (Online version in colour.)

Figure 5.

Effect of the depth of an air gap below a cover scale array on spectral RC for the ‘Dorsal bald, ventral surface ensonified’ treatment. (Online version in colour.)

Figure 6.

Directionality of target strength at 20, 60 and 100 kHz (n = 5) between a metal disc (black) and a wing-covered disc, comparing (a–c) ‘Intact’, (d–f) ‘Dorsal bald, dorsal surface ensonified’, (g–i) ‘Dorsal bald, ventral surface ensonified’ and (j–l) ‘Both bald’ wing samples. Shaded areas represent standard error. Coloured lines near edge of angle axis indicate significant pairwise differences (p ≤ 0.05; green lines = treatment is significantly less than metal disc, red line = metal disc is significantly less than treatment). (Online version in colour.)

Figure 7.

Directionality comparison showing the difference in target strength between the reflection from a metal disc and reflection from a scale array on a metal disk at 64 kHz for both measured (gold) and modelled (orange) data in the dorsal bald, dorsal ensonified treatment. Note that negative dB means the moth wing reduces the reflections of the metal disc. (Online version in colour.)