LOW-FREQUENCY CHARACTERISTICS OF UNLINED END-IN SIDE-OUT RECTANGULAR PLENUM CHAMBERS

Abstract

Rectangular plena are extensively used in HVAC ducts to reduce propagated noise, and they normally include dissipative lining on the inner facings. However, the basic attenuation effectiveness in the low frequency region can be explained by reactive wave action inside the expansion chamber associated with inlet and outlet locations. By using the transfer matrix derived from the three dimensional analysis of plenum chamber, transmission losses are predicted for several configurations and they agree reasonably well with experimental ones. In addition, the acoustic characteristics of end-in/side-out rectangular chambers are compared with those of other inlet/outlet configurations to check the applicability. It is found that the end-in/side-out rectangular plenum chamber can be used as a very effective silencing element, especially in the low frequency region.