In-duct fan sound power measurement. Part II. Revision of ISO 5136

Abstract

In recent investigations problems with the present version of the in-duct method ISO 5136 (1990) became apparent which indicate that a revision of the standard is necessary. The two major problem areas are: (1) Swirl flow existing in the test duct may interfere with the microphone probe to give too high sound power levels. Farzami and Guedel [1992 Proc. Fan Noise, CETIM Senlis, France, pp. 375–380] showed that this problem can be solved by placing a flow straightener between the fan outlet and measurement plane. (2) By comparison with other methods of sound power determination [Holste and Neise, JSV 152, 1–26 (1992)], the sound power levels obtained by using the in-duct method are too low by several decibels in the frequency region of higher-order mode sound propagation. It is shown that the discrepancy between the results of the in-duct method and other methods is caused by the so-called modal correction C4 which is to account for the directivity characteristic of the microphone fitted with a turbulence screen in view of the propagation angle of the higher-order duct modes. Experimental and theoretical results are presented for a new combined frequency correction C34 which includes the effects of the superimposed mean flow on the sensitivity and directivity of the microphone probe as well as on the modal sound propagation and characteristics in the duct.