Study On Mechanism And Application Of Low-frequency And Broadband Sound Absorbers With Ultrathin Thickness

Highly efficient low-frequency broadband acoustic structures based on ultra-thin structures have important scientific research significance in wave physics and outstanding as well as extensive application value in noise control engineering.Based on the design mechanism of classical acoustic systems,conventional acoustic structures such as traditional porous sound absorbers and micro-perforated plate sound absorbers usually have the structural thickness of the same order of magnitude as the acoustic wave wavelength at the low-frequency cut-off frequency of their operating band,thus limiting their low-frequency absorption in the practical applications with limited installation space and giving rise to the challenges in noise control engineering for lowfrequency broadband noise reduction.In order to solve these challenges,this thesis employs the design concept of acoustic metasurfaces and systematically investigates the physical mechanisms of metasurface-based absorbers in terms of the thermalacoustic dissipation,the acoustic impedance modulation and the coherent coupling among multiple resonant components,and finally achieves low-frequency and broadband sound absorbers with ultrathin thickness and several highly efficient acoustic devices for engineering applications.This thesis consists of five chapters,including an introduction chapter(Chapter 1),a conclusion chapter(Chapter 5),and three main research chapters(Chapters 2-4).The three chapters focus on how this thesis presents research from narrow-band metasurface-based sound absorbers with high tunability to highly efficient ultrathin low-frequency broadband coupled sound absorbers and further to several highperformance acoustic devices for practical engineering applications.The main contents of this thesis are as follows:Chapter 1 introduces the research background about conventional sound absorbers,the research background about metasurface-based sound absorbers,and the measurement methods of acoustic impedance and absorption coefficients used in this thesis,and overviews the main research objects in this thesis.Chapter 2 begins with a theoretical study of the thermal-viscous effect of resonant sound-absorbing structures based on the narrow region acoustics theory,and then establishes a theoretical model for calculating the surface acoustic impedance of the basic acoustic resonant elements with the consideration of thermal-viscous losses.Based on the theoretical model,the chapter firstly designs a metasurface-based perfect acoustic absorber based on a curled Fabry-Pérot channel and performs experimental verification.Next,the chapter introduces an embedded neck to the curled Fabry-Pérot channel with the purpose of obtaining improved tunability in acoustic impedance and absorption performance.For further improving absorbers’ adjustability,a neckembedded Helmholtz resonator is proposed,which achieves efficient tunability of acoustic impedance and absorption performance under a constant structural shape and therefore provides an ideal building component for the development of broadband sound absorbers based on multiple coupled resonant components in subsequent chapters.Chapter 3 firstly investigates the physical mechanism of nonlocal coupling of sound-absorbing resonant components and establishes a theoretical model for analyzing the metasurface-based sound absorbers having multiple coupled components.Subsequently,the chapter proposes the design concept of constructing a high-efficiency broadband sound absorber with an ultra-thin thickness based on imperfect soundabsorbing components.Imperfect sound-absorbing components can generally have a thinner thickness than perfect sound-absorbing components,and they are also free of the strict limitation on the acoustic impedance that is required by perfect components,thus facilitating the realization of ultra-thin sound absorbers and more efficient modulation of the coherent coupling among multiple components.In order to demonstrate the proposed design concept,this chapter presents metasurface-based sound absorbers achieving high-efficiency and broadband sound absorption performances by suitable modulating the coherent coupling among multiple imperfect neck-embedded Helmholtz resonators.Although these neck-embedded Helmholtz resonators exhibit only weak absorption when operating individually,they can achieve a remarkable enhancement in absorption performance when operating as a whole.Specifically,this chapter shows three designs.The first multiple coupled metasurfacebased sound absorber is composed of 25 imperfect neck-embedded Helmholtz resonators with a unanimous thickness of 5 cm,which achieves highly efficient and broadband sound absorption with an average absorption coefficient of 0.95 in the frequency band from 297 to 475 Hz.The second design employs 20 imperfect neckembedded Helmholtz resonators and a micro-perforated panel as components and optimizes the series-parallel coupling among these components to achieve an average absorption coefficient of 0.957 over a wide frequency range from 870 Hz to 3224 Hz with a thickness of only 3.9 cm.The goal of the last design is to achieve the optimal thickness required by the causality constraint.By theoretically designing the nonlocal coupling among 36 double-layer neck-embedded Helmholtz resonators,an efficient absorption with an average absorption coefficient of 0.93 in the ultra-wide frequency range of 320 Hz to 6400 Hz is achieved with a thickness of 10 cm,which approaches the optimal thickness required by causality constraint.Chapter 4 further investigates functional acoustic devices that could be applied in noise control engineering and impedance engineering,including acoustic metaliners under grazing airflows,extreme asymmetric sound absorbers in duct systems,extreme acoustic wave trappers,and radiation-enhancement devices of sound sources.These high-performance acoustic devices utilize and extend the design concepts and impedance-modulation techniques presented in the related studies in Chapters 2 and 3,and have great potential for applications in noise control systems such as in large-scale wind tunnels and aero-engines,which also provide fresh ideas for the investigations of high-performance impedance-engineered acoustic devices.Finally,Chapter 5 summarizes the work of this thesis and provides a brief discussion on the direction of further work.