Experimental determination of end reflection loss (ERL) over a range of common types and configurations of duct terminations

Abstract

The two-microphone impedance tube method ASTM E1050 was applied to directly measure the termination impedance of rectangular duct configurations of different aspect ratios and end conditions over a 25- to 500-Hz frequency band. Measured values of the acoustic impedance, and hence the reflection coefficient, of the duct termination permits computation of end reflection loss. ERL is essentially the insertion loss of duct termination. The measurement program is challenging, as the precision required for accurate ERL computation is much higher than that for accurate determination of the reflection coefficient. The measurement program is part of a project to assess the available published data on duct ERL. The ASHRAE Handbook contains a tabulation of ERL data derived from predictions of the radiation impedance of a circular plane piston, and there have been some validation measurements made on a limited set of circular duct configurations and orifice plate terminations. Results to date closely conform to published ERL values. [Work supported by ASHRAE.]