SUMMARY

A change in acoustic impedance at duct terminations causes some of the sound to propagate back into the duct. The amount of sound reflected back into the duct is called the “End Reflection Loss” (ERL). This thesis experimentally determines the ERL of a variety of duct configurations. ASTM E1050 Standard Test Method for Impedance and Absorption of Acoustical Materials Using A Tube, Two Microphones and A Digital Frequency Analysis System employs the “two-microphone” method of Chung and Blaser and provides the basic methodology implemented for determining ERL. ASTM E1050 was applied to directly measure the termination impedance of rectangular duct configurations of different aspect ratios and end conditions over a frequency band of 25 to 500 Hz. ERL between 25-500 Hz is a common problem in Heating, Ventilating, and Air Conditioning (HVAC) system and building applications. The measurement program is challenging, as the precision required for accurate computation of the ERL is much higher than that for accurate determination of the reflection coefficient. This research focused on determining the impact that termination variations have on ERL and how experimental results correspond with analytic predictions and current ERL values in the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) Handbook. The ASHRAE Handbook contains a tabulation of ERL data derived form predictions of the radiation impedance of a circular plane piston, and there has been some past validation measurements made on a limited set of circular duct configurations and with a limited set of orifice plate terminations. The ASHRAE Handbook does not take into account rectangular ducts or other termination variations and lists ERL values down to 63 Hz. The results closely conform to analytic predictions and published ERL values. [Work supported by ASHRAE.]