TO-2703 (RP-264) -- Use of Acoustic Intensity Measurements to Evaluate the Sampling Tube Method of Measuring In-Duct Fan Sound Power PDF

A method using a single sampling tube for measuring the acoustic power radiated by a fan into a duct is described in ASHRAE Standard 68-78. Some disagreement has been reported between such in-duct fan sound power results and those obtained for the sound power when the same fan is ducted into a reverberation room. Above cutoff frequency, the in-duct fan sound power results have been reported to be below the reverberation room results and this difference has been said to increase with frequency. To confirm these trends and to find possible correction factors, the sampling tube approach was compared with a new two-microphone intensity technique to determine the absolute fan sound power in a duct fitted with an anechoic termination. Recommendations are made for revision of ASHRAE Standard 68-78 to include suitable correction factors.