Acoustic Impedance and the Control of Sound Waves

Abstract

Acoustic impedance is one of the most important parameters for fluid media and is determined by effective mass and elastic modulus. For an acoustic absorber with a shallow depth, the system has high characteristic resonance frequency and its performance is controlled by the system stiffness which is inversely proportional to the depth. This study begins with the analysis of a common porous material as the benchmark and compares with the traditional micro-perforated panels (MPP) before introducing advanced designs. It is shown that MPP can indeed yield similar performance as the porous material when the aperture is very small hence high cost. However, the performance at very low frequencies remains poor. In order to achieve a broadband performance, parallel resonator array is required. This system is shown to reduce system stiffness significantly while the overall mass increase is much smaller than that of a single MPP resonator. Advanced means of reducing system stiffness by electromagnetic forces is discussed, and the physics of a negative dynamic mass is also analyzed.