Multiplying resonances for attenuation in mechanical metamaterials: Part 1 – Concepts, initial validation and single layer structures

Abstract

Worldwide increases in noise levels due to growth in urban population, traffic and machinery have serious implications for health, productivity and quality of life. Prevention of sound transmission through walls and ceilings, particularly towards the lower frequency range of human hearing is important both because of its recent increase in levels and the challenges faced when providing sound insulation of long wavelength noise with existing construction methods. Mechanical metamaterials have the potential to address these challenges by enabling the creation of an artificial medium that generates far more attenuation of wave transmission than any existing material system. In part one of this work we design, model and test several local resonance structure (LRS) systems to provide the basis for future acoustic insulation systems. Three different LRS families were studied: spherically symmetric, flexural single-resonance and flexural multi-resonance. Some resonator elements showed a peak effective mass up to fifty times greater than their rest mass and achieved peak transmission losses 10s of dB greater than a non-resonant structure of equivalent surface density within the designated frequency range. By arranging sets of resonators with closely spaced resonance frequencies the transmission loss gains were spread over a wider frequency range and a reduction in the transmittance peak at the upper end of the band gap was achieved though variations in damping and mass of the resonators. A large (2.5m2) test article was constructed and tested under full scale diffuse field conditions such as are found in buildings. The results confirmed that the band gap observed in impedance tube measurements of small-scale LRS specimens survives. Modelling and testing results for more multilayer and multi-resonance systems are presented in part 2.