Accuracy Considerations in Fan Sound Measurements

Existing fan-sound power data appear reasonably adequate for air-conditioning system design purposes, provided that the fan does not generate any discrete frequency sound. One of the main reasons for conducting fan-noise tests is, therefore, to get accurate data on any discrete-frequency components. Such measurements tend to be subject to large errors owing to standing waves in the duct. Longitudinal standing waves not only affect the pressure field in the duct but also the sound-power output of the fan and must be eliminated by an anechoic termination. Crosswise standing waves pose problems for in-duct measurements, which can be overcome by averaging over the duct cross section or by the use of an acoustic wattmeter. Accurate measurement of the broad-band component is much simpler. Microphone windscreens for in-duct measurements differ greatly in their ability to reject near-field turbulence created by the fan. For this, too, an acoustic wattmeter should be of great help.