Membrane-type Acoustic Metamaterials in Ventilated System

Abstract

In this thesis, membrane type metamaterials based on resonance have been investigated in ventilated system with glancing incidence of sound. Owing to hybrid membrane resonators (HMRs), additional impedances couple with the impedance of main tube that generate an equivalent impedance differs from air. As a result, the backscattering cannot be zero and leading to the transmission loss (TL) in outgoing side. However, the TL is related with the systems dissipations which decided by vibration modes. Therefore, not all of the transmission coefficients have the same value. The resonances of HMRs can be easily tuned by changing the weight and radius of central rigid disk. The negative bulk modulus is demonstrated here using a cylindrical chamber sealed by a membrane with a rigid disk attaching in its center. When the HMR resonant, a sharp dip happened in the transmission spectrum and the effective bulk modulus becomes negative, implying the volume change is out of phase with applied dynamic pressure. The process is also realized by theory using response function. A flow-through silence with high and broadband transmission loss in low frequency has been designed by using multiple units consisted of four rectangular membranes. In each rectangular membrane, two asymmetric rigid platelets attached so as to further increase the number of resonance. What is more, the high transmission loss mainly comes from absorption. By consisting of a dipole resonator and a monopole resonator, a perfect absorption is realized in ventilated system. The absorption functionality is independent of the incident direction and the size of absorber is at least 10 times smaller than the sound wavelength. A decorated membrane resonator (DMR) partially covers the tube is fixed in the center to supply the dipolar movement. And the hybrid membrane resonator (HMR) mounted on sidewall of tube offers the monopolar movement through compression and rarefaction. The responses of two resonators are in phase for the incident side and out of phase for the outgoing side. Therefore, the backscattering is eliminated through the impedance matching to air, while the transmission is also vanished by two responses cancelling each other through destructive interference. As a result, the total absorption is achieved. Apart from this, another ventilated composite absorber which comprising two similar HMRs mounted on the sidewall of tube is also demonstrated to have perfect absorption in low frequency. What is worth mentioning is that no air resistance exsit in this absorber. The air velocities in the vicinity of the unit1 have the opposite symmetry from that of unit2 that will cancel further away, and the equivalent surface impedance of whole system matches with air, so the total absorption is gained. However, reversing the incident direction, the asymmetric absorption and reflection are got due to mismatched impedance.