Smart acoustic meta-structure with magnetic-reactive porous medium for noise control

Abstract

Porous materials have been widely used in many fields for their advantage on reducing noise by simple attachment or application to a noise source. In order to control the absorption characteristics, an in-depth analysis of the acoustic characteristics of the porous structure should be progressed. The acoustic bulk properties, which are the complex characteristic impedance and wavenumber, are important factors for the evaluation the acoustic characteristic of the porous structure, determined by effective density and bulk modulus of its medium. For the accurate experimental derivation of bulk properties, in-duct method have been widely used. There are two mainly performed method with in-duct system, two and four microphone impedance tube method. With two microphone method, it is possible to predict the acoustic macro-properties, surface acoustic impedance and absorption coefficient. Four microphone impedance tube method estimates the acoustic macro and bulk properties simultaneously, such as characteristic impedance and wavenumber of homogeneous materials. Acoustic bulk properties can be measured with two microphone system using Two-thickness method, but the thickness of samples should have a ratio of two each other. Due to the sound absorption characteristics of the porous material, noise reducing ability in the low frequency is comparatively lower than the characteristics of high frequency band. Porous medium has the most effective sound absorption with maximum particle velocity, and when the thickness of porous absorber is quarter wavelength of propagating wave, maximum particle velocity is exhibited at the frequency of quarter wavelength of sound wave. Solutions such as increasing the thickness and applying air cavity layer behind the porous medium were used to overcome the low frequency problem, but there is a limit to the thickness increase in practical application that can be applied. In the case of noise sources such as automobiles, refrigerators and washing machines, etc., the noise characteristics changes with internal and external factors. Conventional passive sound absorbers cannot actively deal with such changes, and to realize a sound absorbing material corresponding to characteristics of a time-varying noise sources, various studies have been carried out through the active control system using speaker or piezoelectric material, such as PVDF (Polyvinylidene fluoride) film. In this study, A sound absorption structure constructed using multiple layers of fibrous papers in helical shape was presented. The acoustic properties changes depending on the thickness of the air cavity between thin layers and porosity of the layer itself. Sound absorption coefficients were measured with two-microphone impedance tube to investigate the acoustic properties variation with porosity. The measured results were compared to predictions utilizing wave propagations in the slit for verification of the sound absorbing mechanism. A simple method of improving the sound absorption with less amount of materials through composite structure is proposed and verified through the experiment. Dynamic stiffness and loss factor of helical-shaped sound absorbers were measured and compared for each samples through analyzing supporting properties to the vibrating beam. Depending on the construction method, the frame dynamic properties showed variation even when the sound absorption is identical. These information is required for application to specific noise control applications. A modified two-thickness method to measure acoustic properties of porous materials using surface acoustic impedance was also proposed in this study and compared to those obtained by the four-microphone impedance tube and traditional two-thickness methods. In the two-thickness method, the complex propagation constant and characteristic impedance are estimated from the measured surface acoustic impedances, and the sample thicknesses should have a ratio of two. Through the proposed method, the characteristic impedance of a porous material is calculated using arbitrary sample thicknesses. The surface acoustic impedances for samples of different thicknesses are measured via the two-microphone impedance tube method to estimate their complex acoustic properties. Sensitivity analysis is performed to analyze the measurement uncertainty on the resulting values. The sensitivity depended on the sample thickness and mount edge constraint in the tube. Using the method proposed in this study, acoustic properties are measured accurately with small differences in thickness and surface acoustic impedance. Multi band sound absorbing structure with slit porous medium, double resonant porous structure (DRPS), was proposed and its acoustic characteristics was investigated. Proposed double resonant porous structure was composed of slit absorber in helical shape attached to perforated membrane backed by air medium. It showed significant sound absorption improvement at the low frequency and still had good performance in mid and high frequency band. Its acoustic characteristics was controlled by adjustment of design parameters for specific noise control applications. To confirm the proposed structure’s acoustic characteristics, three different samples were made and comparison analysis was performed. Predicted results showed the proposed structure’s sound absorption mechanism was composed by Helmholtz resonator, perforated membrane and slit porous medium’s effect. Parametric study of design parameter was conducted for optimal sound absorption efficiency. Magnetic smart porous material which have noise reduction ability for broad frequency band with active control was also presented by applying the magnetic reactive material to the porous medium. Two magnetic-reactive porous structure were presented in this study, and to perform adaptive-passive noise control, proposed smart foam structure in this study is able to reduce the various noise sources according to the magnitude, polarity and adaptive control of the magnetic field, and measurement and evaluation of the noise reduction ability was performed with impedance tube method and transfer matrix method. We experimentally conducted active noise control using smart foam for in duct noise and confirmed that effective absorption characteristic control is possible. It was confirmed through experiments that sound absorption characteristics can be controlled efficiently with relatively low electric power. Theoretical model to predict the absorption characteristic was proposed through transfer-matrix method, and experimental and theoretical results showed good agreement. For reduction of low frequency noise with active noise control, an active foam composed with soaked melamine foam with MR fluid was prepared and experimental validation was performed with feed-forward filtered-x LMS algorithm. Proposed magnetic active foam showed excellent noise control performance in duct acoustic condition.