Towards moth wing inspired sound absorbing materials

Abstract

The 65 million year acoustic arms race between moths and bats has lead to the evolution of numerous different defence strategies amongst moths against bat echolocation. Some moths have evolved passive defences such as ultrasound absorbing scales, which diminish the strength of a moth generated echo returning to a bat. The scales work by resonant mechanisms and are much smaller than the wavelength of the sound which they absorb. This unique architecture opens avenues for bioinspired sound insulation solution that are much smaller than current technical sound absorbers. Previous works have looked at sound absorption by moth scales in free space, but to determine their effectiveness as a sound insulating solution for buildings, their performance must be measured when resting on a hard substrate. Here, we measure the potential for moth wings to act as a sound absorbing surface 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% despite a thickness to wavelength ratio of up to 1/50 at the lowest frequency tested (20 kHz). Remarkably, after 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. The finding that moth wings can act as a sound absorbing surface opens the door to the creation of biomimetic light-weight noise mitigation materials.