Noise from continuous positive airway pressure (CPAP) flow generators is controlled using very small, irregularly shaped mufflers which must attenuate noise at frequencies up to 5 kHz. This thesis investigates computational and experimental techniques to predict the acoustic performance of these mufflers and presents a computational technique to efficiently optimise the muffler design in order to maximise the performance.The work presented in this thesis can be roughly grouped as (1) acoustic modelling, (2) experimental measurements and (3) parametric design optimisation. In the first group, three different analytical techniques were reviewed in the context of modelling the acoustic performance of a simple expansion chamber reactive muffler. Three-dimensional acoustic finite element analysis (FEA) models of simple geometries and complex CPAP device muffler geometries were then developed. Polyurethane foam inserts were incorporated into the FEA models of the muffler designs. The computational results of both the reactive and resistive designs were compared to the analytical results and were successfully validated against experimental data.The experimental component of the thesis includes measurement of the acoustic performance of mufflers and acoustic characterisation of polyurethane foams. The transmission loss of the simple and complex mufflers was measured using a two-microphone acoustic pulse method. The acoustic performance of the mufflers was also measured in the presence of mean flow to examine the influence of air flow on the transmission loss. The acoustic properties of two polyurethane foams were obtained in an impedance tube using a two cavity method. A rig for measuring the airflow resistivity of the porous materials was designed, constructed and commissioned. The combined acoustic characterisation of the foams was incorporated into the FEA models, enabling the modelling of muffler designs which contain absorptive elements.In the final part of the thesis, a methodology to efficiently optimise muffler configurations to maximise their acoustic performance was developed. This involved the development of an integrated model that couples various optimisation tools available through the MATLAB interface with the acoustic FEA capabilities of COMSOL. The reactive and resistive components of a prototype CPAP device muffler were independently optimised for acoustic performance. Results were also obtained for the transmission loss of the muffler for which the reactive and resistive components were simultaneously optimised. The merits of each approach are discussed.
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