In-duct fan sound power measurement. Part I. Review of general problem

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 the paper the problems associated with such a method are discussed: Axial standing waves due to sound reflection 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 standarized in-duct method ISO 5136 (1990) is given.