Locally resonant structures for sound shielding applications

Abstract

The intrusion of unwanted sound is a matter of increasing concern worldwide. Growing sound pollution has significant implications for human health, productivity and wellbeing in industrial, educational, and residential settings. Lower frequency noise in particular is subjectively important, as this is the range in which the human ear is most sensitive and irritating acoustic intrusion frequently occurs, yet sound insulation at low frequencies is technically challenging and expensive, specially for lighter construction. Meta-materials theory offers a relatively new approach to achieving high levels of acoustic transmission loss (TL) through the generation of band gaps where wave propagation is forbidden. This theory is being applied to develop panels with internal resonant structures that generate low-frequency acoustic band gaps yet are only a few centimetres thick. These resonant structures may exhibit negative mass behaviour, but it has been found that the key performance indicator is the ratio of the absolute value of the effective mass to the rest mass near resonance. This paper focuses on the behaviour of single and multi-layer systems with a multiplicity of closely spaced resonances, and examines the effect of resonator variability resulting from material and manufacturing imperfections. The acoustic response of these systems can be significantly richer than that of the individual resonant elements. This allows the system to, for example, compensate for dips in TL associated with Mass-Air-Mass resonances in double layer walls. Tailoring the individual resonator properties was shown to enable the maximisation of performance while minimising any detrimental effects. Scalability of the resonant elements was found to be a major issue at lower frequencies due to fundamental characteristics of mechanical oscillators. A combination of analytical and finite element modelling was used to explore the system design space, evaluate and rank the impact of design parameters. The models were verified by dynamic and acoustic measurements of small-scale specimens containing limited numbers of resonant elements. Various resonator elements have shown a peak effective mass up to fifty times greater than their rest mass. When incorporated into multi-layer structures, some of these local resonances have resulted in transmission loss peaks approaching 80dB, tens of decibels higher than that of non-resonant structures of equivalent area density.