Duct directivity index applications

Abstract

This paper concerns the application of a recently developed chart for determining the directional properties of sound emitted from the open end of a ventilation duct. When designing a duct silencer to reduce noise from a large vertical discharge duct, it is useful to note that the first 5 to 10 dBA noise reduction may result from directivity losses at 90 degrees and can be accurately predicted. In 1971 the first author conducted sound directivity tests with 300 and 600 mm diameter ducts and the results were made into a rough chart of Duct Directivity Losses that ultimately found its way into the NSW EPA Environmental Noise Control Manual (5 June 1985, page 207.1). It is wrong in principle and rather inaccurate, but some users are unaware of its failings. Over the last 13 years further duct directivity testing has been conducted and a new duct directivity chart drawn. It is based on sound directivity testing on ducts of 305, 400, 610, 915 and 1220 mm diameter. The directivity data has been related to the sound power level of noise emitted from the duct and the spherical dispersion of sound energy. The new Duct Directivity Chart allows the directivity gain or loss to be obtained for any diameter from 100 mm to 10 metres, at angles from zero to135 degrees without the need for complex calculations.