Sound and Vibration Absorption by Decorated Membrane Resonators

Abstract

Spongy and fabricated materials have dominated the market of sound absorption over a decade. However, they fail to tackle the noise below 500 Hz. Therefore, absorption of low frequency sound becomes one of the most challenging topics in the acoustic field. This thesis introduces the use of decorated membrane resonator (DMR) in shielding and absorbing of low frequency noise below 1000 Hz. The acoustic characteristics of the DMR are shown in its resonance and anti-resonance modes. Under resonance modes, the transmission and absorption reach local maximum. The absorption coefficient of single layer DMR is theoretically bounded by 0.5. Under anti-resonance modes, the average membrane displacement is almost zero, hence the transmission drops to local minimum. The acoustic characteristics of the DMR can be modified by adding a sealed gas layer and a hard plate behind. This composition is so-called hybridized membrane resonator (HMR). By tuning the thickness of the seal gas, the surface impedance of the HMR can match the impedance of air to achieve total absorption at anti-resonance. Moreover, the effective mass of a DMR grows significantly at its antiresonance frequencies. This characteristic helps DMRs behave as good vibration dampers.