Abstract
A method has been developed for measuring the acoustic power output in watts and the frequency spectra of ventilating fans. Both the inlet and the exhaust noise are determined as a function of back-pressure. The fan is ideally connected to a circular section of duct about equal in diameter to the fan diameter and five to ten diameters in length. Three to six diameters from the input end of this duct an air straightener is located. The acoustic measurements are made with a condenser microphone enclosed in wind screen located four to eight diameters from the inlet. The outlet end of the duct couples through an exponential connector to an anechoic wedge structure two feet square. This is backed by a plenum chamber containing an array of holes that can be progressively closed to produce increased back-pressure. Sound pressure level measurements are made in frequency bands and converted to acoustic watts per band. The inlet noise is measured outdoors on a hemispherical surface to get the directivity pattern and the acoustic power for each frequency band. Engineering formulas for applying the data to prediction of the noise produced in rooms are presented, including the effects of duct bends, duct losses, and the distance from the ventilating grille in the room.
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