Author: Iljae Lee

Publisher:

Published: 2005

Total Pages:

ISBN-13:

Abstract: Acoustic characteristics of silencers filled with fibrous material (hence dissipative) are investigated. Following a theoretical and experimental analysis of a single-pass, perforated, dissipative concentric silencer, the study is extended to a hybrid silencer designed by combining dissipative and reflective (Helmholtz resonator) components. The ability to model these silencers relies heavily on the understanding of the acoustic behavior of the fibrous material and the perforations. Therefore, the present study has developed two experimental setups to measure: (a) the complex characteristic impedance and the wavenumber of the fibrous material with varying filling density and texturization conditions, and (b) the acoustic impedance of perforations in contact with the fibrous material, and with and without the mean flow. New empirical expressions are then provided for the acoustic impedance of perforations with varying porosity, hole diameter, wall thickness, mean flow rate, and the fiber characteristics. The experimental results illustrate that the presence of absorbent significantly increases both the resistance and reactance of the perforation impedance. The addition of mean flow is also shown in general to increase the resistance, while decreasing the reactance particularly at low porosities. The empirical expressions for the fiber acoustic properties and the perforation impedance are then integrated into the predictions of transmission loss. These predictions are based primarily on a three-dimensional boundary element method (BEM) developed in the present study, due to its ability to treat silencers with complex internals, in addition to one- and two-dimensional analytical approaches also introduced. Comparisons of predictions with the acoustic attenuation experiments support the proposed relationships for the properties of fibrous material and the perforation impedance. The influence of the internal geometry modifications of dissipative silencers, such as baffles and extended inlet/outlet, and the impact of connecting duct length between a pair of silencers, are investigated with BEM. Hence, the contributions of the present study include the development of methodologies for the measurement of acoustic properties of the fibrous material and the impedance of perforations, particularly in contact with the absorbent along with the resulting empirical expressions, thereby assisting towards analytical and computational design tools for the dissipative and hybrid silencers.