Multiplying resonances for attenuation in mechanical metamaterials: Part 2 - Multi-layer structures

Abstract

Reduction in sound transmission through walls and ceilings, particularly at lower frequencies, is important both because of ongoing growth in noise pollution and the challenges faced in providing good sound insulation with existing construction methods. Mechanical metamaterials can help address these challenges by enabling the creation of an artificial medium that produces significantly greater attenuation than existing passive lightweight material constructions. In part one of this work we designed, modelled and tested simple local resonance structures (LRS) to investigate their potential for future acoustic insulation systems. In part two we extend this work to multilayer and multi-resonance systems.Three LRS families have been studied: multilayer with single resonance, multilayer with multiple resonances and intermediate layer with single resonance. Comparisons are presented based on lumped parameter modelling and transmission loss (TL) measurements under plane wave acoustic excitation. The LRS designs achieved peak transmission losses up to 40 dB greater than non-resonant structures of equivalent surface density within a specified frequency range, and exhibited gains having bandwidths up to 300 Hz. The depth and width of the attenuation bands were found to be controlled by different design parameters, so systems with appropriately tuned interlayer couplings and resonator stiffness exhibited large increases in magnitude and bandwidth of the attenuation. Furthermore, the distribution of stiffness, damping and mass in the resonators powerfully affected the shape of the TL spectrum, and could be used to keep TL at or above mass law levels throughout. LRS systems have the potential to provide significantly higher transmission loss at low frequencies than conventional wall systems of similar size and weight. This is a step towards a locally resonant architecture that can be incorporated into a practical insulation system.