Membrane-type acoustic metamaterials in ventilated system

Abstract

In this thesis, membrane-type metamaterials based on resonance have been investigated in a ventilated system with glancing incidence of sound. Owing to hybrid membrane resonators (HMRs), additional impedances couple with the impedance of the main tube and generate an equivalent impedance that differs from air. As a result, the backscattering cannot be zero and leads to transmission loss (TL) in the outgoing side. However, the TL is related with the systems dissipations which are decided by the 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 the central rigid disk. The negative bulk modulus is demonstrated here using a cylindrical chamber sealed by a membrane with a rigid disk attached in its center. When the HMR resonates, a sharp dip happens in the transmission spectrum and the effective bulk modulus becomes negative, implying that the volume change is out of phase with the applied dynamic pressure. The process is also realized by theory using a response function. A flow-through silencer with high and broadband transmission loss in low frequency has been designed by using multiple units consisting of four rectangular membranes. In each rectangular membrane, two asymmetric rigid platelets are attached so as to further increase the number of resonances. 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 a 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 and is fixed in the center to supply the dipolar movement. And the hybrid membrane resonator (HMR) mounted on the sidewall of the tube offers monopolar movement through compression and rarefaction. The responses of the two resonators are in phase for the incident side and out of phase for the outgoing side. Therefore, backscattering is eliminated through the impedance matching to air, while the transmission also vanishes because the two responses cancel each other through destructive interference. As a result, total absorption is achieved. Apart from this, another ventilated composite absorber which comprises two similar HMRs mounted on the sidewall of the tube is also demonstrated to have perfect absorption at low frequency. It is worth mentioning that no air resistance exists in this absorber. The air velocities in the vicinity of Unit 1 have the opposite symmetry to that of Unit 2, which will further cancel each other, and the equivalent surface impedance of the whole system matches that of air, so total absorption is gained. However, when reversing the incident direction, asymmetric absorption and reflection are obtained due to mismatched impedance.

Date of Award	2016
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology