Experimental Investigation of Reflection of Airborne Noise at Duct Terminations

Abstract

Noise between 25-500 Hz is a common problem in Heating, Ventilating, and Air Conditioning (HVAC) systems. The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) Handbook lists values of end reflection loss (ERL), a frequency dependent parameter describing energy reflected back up a duct at a termination impedance, to help engineers design and account for noise. The ASHRAE Handbook does not account for common termination variations and only lists ERL values using octave bands down to 63 Hz. This thesis experimentally determined the ERL of a variety of rectangular duct configurations and termination conditions between 25-500 Hz. This research also compared experimental ERL results with analytic predictions and ASHRAE Handbook values. Seven duct sizes were tested, from 6X6 to 18X54 inches. Duct termination baffle hardness was varied between acoustically hard (plywood) and soft (ceiling tiles) for the 6X6, 6X10, and 6X18 ducts. Five duct termination distances above the termination baffle were tested, between flush and 1D for the 6X10 and 6X18 ducts and between flush and 5D for the 6X6 duct, where D equals the duct s effective diameter. Diffusers and flex duct configurations were installed at the end of the rigid duct to test their effect on ERL on the 6X6, 6X10, and 6X18 ducts. ERL was determined using an adaptation of the ASTM E1050 Standard, an application of the two-microphone impedance tube method. Experimental results closely conformed to analytic predictions and are an improvement over ASHRAE Handbook ERL values. The results indicate that baffle hardness has a negligible impact on ERL, which contradicts the ASHRAE assumption that diffusers that terminate in a suspended lay-in acoustic ceiling can be treated as terminating in free space. Termination distance above the baffle has a negligible impact on ERL at distances less than six inches for the 6X6 duct. Termination distances above the baffle greater than six inches exhibit limited free space ERL behavior for the 6X6 duct. The use of flex duct greatly reduces low frequency ERL and this is not accounted for by the ASHRAE Handbook. The impact from flex duct usage also negates any influence from downstream termination variations.