Noise produced by a small variable speed centrifugal fan used in sleep apnoea assisted breathing devices can be annoying to the user and others. Mufflers are incorporated into the devices to reduce the noise in the air paths to and from the fan. Noise reduction is an important issue in the development of this product since these devices usually operate on a bedside table in a quiet environment. The mufflers are very small, irregularly shaped, and must attenuate noise up to high frequencies. It is important that the acoustic performance of these mufflers is reliably predicted and optimised, in order to improve the quality of the well-being of the user. The mufflers used in respiratory equipment are predominantly reactive mufflers, although their acoustic performance can be improved with the inclusion of dissipative materials. In this preliminary study, analytical and computational results of a simple reactive expansion chamber muffler are presented and compared. Three different analytical modelling techniques have been investigated corresponding to (a) the continuity of pressure and volume velocity method, (b) the impedance method (also known as transmission line theory), and (c) the transfer matrix method (also known as the 2-port approach or 4-pole parameter method). A finite element model of a simple reactive expansion chamber muffler has also been developed and the results obtained compared with those from the analytical methods. Future work will involve developing an understanding of the construction (geometry, manufacture and assembly) of the current sleep apnoea devices and the constraints that this places on acoustic design, the sources and characteristics of noise both from laboratory measurements and patient perception, and the methodologies available to optimise acoustic design.
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