A generic approach for the study of higher-order mode propagation in circular ducts with simple aperture devices

The aim of the work reported in this thesis is to establish a generic approach for the study of the propagation of higher-order modes in ducts of circular cross section when a simple aperture device is installed in the duct. Additionally to describe the effects of the simple aperture device on the whole in duct acoustic field, especially in the frequency range after the first higher-order mode cuts on. The approach, which is based on approximating each higher-order mode as an uncoupled mode, requires an accurate and effective decomposition of the in duct field in the higher frequency range. In the theoretical work for the propagation of the higher-order modes in circular ducts, one established model to describe the open end of the duct is considered. Another model to describe the sound source (both plane wave source and point source) and the boundaries between the aperture device and the main duct is proposed. Combining the two models together, a revised model to describe the whole acoustic system is obtained and used to carry out the in duct field decomposition in the higher frequency range. From this the amplitudes of various higher-order modes are obtained. Experimental investigations have also been carried out to determine the applicability of this model. The experimental work can be divided into two groups: reference measurements for the inputs to the models and direct measurements of the sound fields. The reference measurements are used to obtain the strength of the sound source. The direct measurements are used to compare with the results obtained from the theoretical calculations. Through the comparison, the errors and the applicability of the theoretical model are established. It is shown that this approach to the problem may be used in the normalized wave number region up to k * R -< 7 , which is nearly four times the plane wave region and includes sixteen propagating higher-order modes. By determining the amplitudes of the higher-order modes, the whole in-duct acoustic field is fully decomposed into individual model contributions and can be reconstructed in detail. In order to get a complete and coordinate-independent description of the effects of installing different simple aperture devices, the power radiated out of the open end of - i - the duct is calculated in the fonn of different single modes. Additionally the insertion losses for different aperture device situations are also obtained. Then through the comparison of the powers and the insertion losses, it is possible to find an effective and direct way to express the effects of installing different simple aperture devices in a duct.