Experimental study of high velocity air discharge noise from some ventilating ducts and elbows

Abstract

The air discharged by a 12-in ‘Aphonic’ (quiet) radial flow fan, further quietened by long oversize felt-lined ducts and package attenuators, was passed into a large acoustically-deadened test chamber through straight round (4–10 in diam.) ducts and rectangular ducts of equivalent cross-sectional areas. The ducts were open-ended and, in the case of the round ducts, also fitted with 90° short and long elbows with and without terminating ducts. Duct velocities up to 4000 ft/min were attained in some cases by controlling fan speed. The A-weighted sound levels were measured in each case at 2 ft from the center of the exit section at an angle of 45°, and in some cases also at 3 ft for comparison. The object was to make measurements in the direct sound field of the exit plane. Relations between duct velocities and measured sound levels are evaluated over the ranges tested.