Index of content:
Volume 118, Issue 4, October 2005
- NOISE: ITS EFFECTS AND CONTROL 
118(2005); http://dx.doi.org/10.1121/1.2010267View Description Hide Description
A numerical model based on the finite element method is developed for a finite length, HVAC splitter silencer. The model includes an arbitrary number of bulk-reacting splitters separated from the airway by a thin perforated metal sheet and accommodates higher-order modes in the incident sound field. Each perforated sheet is joined to rigid, impervious, metallic fairing situated at either end of a splitter. The transmission loss for the silencer is quantified by application of the point collocation technique, and predictions are compared to experimental measurements reported in the literature. The splitter fairing is shown to significantly affect silencer performance, especially when higher-order incident modes are present. It is concluded that laboratory measurements, and theoretical predictions, that are based on a predominantly plane wave sound source are unlikely to reflect accurately the true performance of an HVACsilencer in a real ducting system.
118(2005); http://dx.doi.org/10.1121/1.2010353View Description Hide Description
When a segment of a rigid duct wall is replaced by a membrane and is backed by a cavity, incident noise induces membrane vibrations and causes noise reflection. The reflection is effective over a broad band in the low-frequency region when a certain high tension is applied on the membrane in the axial direction of the duct. The device is thus called a drumlike silencer. The existing vibroacoustic theory is based on a two-dimensional duct model and the membrane is reduced to a one-dimensional string. This study extends the theory to three dimensions for the duct and two dimensions for the membrane which has all four edges fixed. It is shown, analytically, that the lateral tension is always detrimental to the silencing performance. However, the optimal performance of the one-dimensional string is recovered exactly when the lateral tension on the two-dimensional membrane vanishes despite the very different boundary conditions. The conclusion is validated experimentally, paving the way for the application of the drum silencer in which the cavity is completely separated from the gas flow in the duct.