In anechoic chambers, the flat-walled multilayered acoustic lining systems are cheaper and easier to install than conventional wedge type systems. Sequence, material and thickness of layers are all design variables. By choosing the right configuration, the acoustical performance and overall thickness of flat-walled multilayered systems can be comparable to conventional wedge systems. In this study, the materials considered include air and poroelastic materials such as polyurethane foams, melamine foams, and glass wool. In order to evaluate acoustical performance of a given configuration, the poroelastic materials are modeled using Biot’s formulations instead of a simpler impedance method to get more accurate results. A genetic algorithm implemented within ModeFRONTIER optimization software was used to select configurations which have a cut-off frequency of 100 Hz or less. The configuration that met this requirement with the smallest overall thickness was determined optimal. This configuration has an overall thickness of 65.8 cm and is composed of 4 different polyurethane foams. Since a considerable difference was observed between the cut-off frequencies obtained using Biot’s model and the simpler impedance method, this justifies the use of Biot’s model in the optimization.

Sound transmission in a finite-length lined annular duct with mean swirling flow is investigated using mode-matching methods. The main application of this work is the acoustics of aeroengine duct systems, especially the prediction of sound transmission behind a fan/rotor stage where the swirl velocity could be comparable to the axial velocity. First, a spectral collocation technique is used to determine the eigenmodes of three-dimensional linearized Euler equations, then two mode-matching schemes are utilized for calculating the sound transmission in ducts. The two schemes are compared with each other and also with the finite element method. The modified matching scheme is believed to deal with the impedance discontinuity more accurately at the interface. A sound power definition for high frequencies in the presence of mean swirling flow is used for the analysis of sound transmission characteristics. The modified matching scheme is then used to analyze the effect of mean swirling flow on the sound power transmission loss and conduct liner optimization in the impedance plane, compared with the uniform axial mean flow case. Finally, sound attenuation due to inner wall lining, outer wall lining, and combined inner and outer wall lining, respectively, is investigated.

A theory based on cross-sectional averaging is developed to analyze quasi-one-dimensional acoustic propagation in hybrid ducts with two propagation media in the cross-section. Specifically, ducts lined with a thick layer of porous material are considered. The porous material makes the duct wavenumber complex, changing the phase speed and introducing attenuation. To lowest order, the wavenumber depends only on the ratio of cross-sectional areas and the properties of the constituent media, and surprisingly not on the material configuration in the cross-section. High frequency accuracy can be improved by using a small correction that includes shape coefficients that depend on the cross-sectional configurations. If the propagation wavenumber is measured experimentally in a hybrid duct, the complex effective sound speed and density, fundamental porous material properties, can be extracted relatively easily. Experimentally, open cell foam samples line the sides of a tube closed at one end, and the complex wavenumber is determined from standing wave measurements. The cross-sectional averaging theory is then used to determine the acoustic properties of the open-cell foam. Results are compared for various lining configurations to assess the accuracy of the method. Another application of this work is the theoretical and experimental study of the propagation of quasi one-dimensional acoustic waves through a duct with spatially periodic area changes. This configuration exhibits stop-band and pass-band behavior, with substantially reduced sound transmission in stop bands, but little effect in pass bands. The regions of the duct with larger cross-sectional area are partially filled with an annular region of porous material to provide pass-band attenuation, leaving a constant area passage for airflow. Predictions and measurements for hybrid ducts with periodic area changes are presented. A muffler designed to place engine harmonics in targeted stop-bands is described.