Measurement of sound power radiated by fans: The revised in-duct method DIN ISO 5136

Many technical fan installations involve a duct on the inlet side and/or outlet side of a fan. Therefore the need arises for a measurement method for determination of the sound power radiated by fans or other fluid handling sources into a duct. In this paper the problems associated with such a method are discussed: Axial standing waves due to sound reflections from the duct end, acoustic loading of the source, turbulent flow pressures superimposed on the sound field, flow noise generated by flow/microphone probe interaction (self noise), discrimination between sound pressures and turbulent flow pressures, measurement position in the duct in view of higher-order mode sound propagation and directional characteristic of the microphone probe used, and modal distribution of sound power. Early and recent work on the above topics is reviewed. A brief description of the standardized in-duct method ISO 5136:1990 [1] as well as the revised version ISO/DIS 5136:1999 [2] is given.