Abstract
Duct end reflection (ER) is the apparent loss of sound power resulting from an abrupt change in a cross-sectional area of the duct. In most references, the magnitude of ER is only a function of frequency and duct size, because of assumed plane-wave propagation. If, between the source and duct termination, there are short duct runs, 90-deg duct bends, offset ducts, T-sections, various end diffusers, high aspect ratio cross sections, the presence of airflow, etc., the relative effects on ER are unknown. The American Society of Heating, Refrigeration, and Air-Conditioning Engineer’s Applications Handbook presents tables that provide the power loss due to ER. However, the assumptions used and the limitations of the table are lacking. This paper describes results of laboratory-measured changes in reverberation room sound-pressure level (SPL) created by common system variables that are associated with ER: duct length between noise source and reverb room termination, and several duct termination conditions were investigated using small diameter duct. In a controlled laboratory environment, these system conditions were found to create an apparent random variability (up to 10 dB) in the resulting SPL, for both a high impedance fan reference sound source and a low impedance loudspeaker source.
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