Abstract
The low frequency airborne noise pollution has always been a difficult issue dictated by the causal constraint, which guarantees an extremely large minimum absorber thickness for low frequency absorption. In this thesis, we show that by varying the boundary condition on the backside of an absorber from hard-reflecting to soft-reflecting, with the former being an implicit assumption in the derivation of the causal constraint, we can circumvent the stringent constraint imposed by the causality principle. As a result, a broadband near-total absorption in the low to ultra-low frequency range can be achieved with sample thickness much smaller than that imposed by the causal minimum.
The acoustic soft boundary condition is a complementary condition (ASBC) to the acoustic hard boundary condition. It is very common for the underwater acoustics, since the liquid-air interface represents an ASBC. However, for the airborne sound ASBC is not commonly encountered. In this thesis we introduce three approaches to realize the ASBC for the airborne sound, with potential applications to the remediation of low to ultra-low frequency noise.
The first approach relies on controlling the resonance behavior of a metamaterial, so that the resulting impedance can be smaller than that of air. As an example, three configurations of membrane resonators are proposed. Each of them presents a fairly well ASBC at resonance. The second approach utilizes the open boundary condition, which is capable of realizing broadband ASBC in the low frequency range. Such an ASBC is verified by simulations and impedance tube measurements. Putting different kinds of absorber in front of the opening, a significant absorption improvement is found for different absorbers, compared to the one backed by acoustic hard boundary (AHBC). The best absorption spectrum comes from a self-fabricated metallic porous absorber of 0.5ð‘ð‘š thickness. Over 0.9 flat absorption from 50Hz to 500Hz was observed. The minimum absorber thickness for such an absorption spectrum, as suggested by the causal constraint, is 160 times larger than that of the actual absorber. Inspired by the ASBC created by the open boundary condition, we discovered that by opening small holes on the backside of a hybrid membrane resonator (HMR), the total absorption peak can be widened and shifted from the original frequency that is near the anti-resonance, to a frequency below the membrane’s first resonance. Joining four such open HMR together, on average 94% absorption from 80Hz to 110Hz was observed with a sample thickness 7.5 times smaller than the causal minimum. In the last approach, an active metamaterial is discussed. We show that by carefully controlling the amplitude and phase of a vibrating panel, it can not only realize the ASBC but also the AHBC as well as total absorption, by simply tuning the amplitude of the vibrating panel.
To request for meeting link, please write to phjacma@ust.hk.