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

Rectangular plena are extensively used in HVAC ducts to reduce propaga ted noise, and they normally include dissipative lining on the inner f acings. However, the basic attenuation effectiveness in the low freque ncy region can be explained by reactive wave action inside the expansi on chamber associated with inlet and outlet locations. By using the tr ansfer matrix derived from the three dimensional analysis of plenum ch amber, transmission losses are predicted for several configurations an d they agree reasonably well with experimental ones. In addition, the acoustic characteristics of end-in/side-out rectangular chambers are c ompared with those of other inlet/outlet configurations to check the a pplicability. 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.