The HVAC plenum is commonly applied as a means to reduce problematic low-frequency noise produced by mechanical equipment in ducted systems. The predicted acoustical performance has been derived from an earlier investigation (Wells 1958). This early work did not focus on low-frequency performance, the effects of wall linings, or typical geometric variables. Through the ASHRAE-sponsored research project RP-1026, conducted by Vibro-Acoustics, rigorous testing enabled the identification of comprehensive attenuation properties for common HVAC plena. A two-stage prediction approach resulted, which applies an improved version of the Wells equation for mid- to high-frequency attenuation (5000 Hz upper frequency limit), quantified as a transmission loss (TL), and a surface area and wall characteristic model for the critical low-frequency TL (50 Hz lower frequency limit). This method has improved the prediction accuracy of flow-through type HVAC plena with a volume range of 20–1100 ft3 (0.5-30 m3). In addition, this work has identified acoustical effects for seven different wall-lining systems and typical geometric variables, such as opening offsets, opening ratio variables, and multiple openings.

Units: Dual