ASHRAE Standard 68-78 and the related ISO draft standard ISO DIS 5136 describe a method for measuring the acoustic power radiated by a fan into a duct, with or without flow, using a B & K UA 0436 sampling tube located at an optimum radial position. Since the publication of ASHRAE Standard 68-78, different laboratories have compared this method with more conventional ones, such as the AMCA 300, or reverberation room measurements utilizing ISO 3741. These laboratories have reported some disagreement between results particularly above the cut-off frequency of the duct, the results being somewhat lower with the sampling tube than with other methods, and the difference increasing with frequency.

In order to confirm these trends, and to work out possible correction factors, the sampling tube method has been compared with a new, recently developed 2-microphone intensity technique in the duct, with an anechoic termination, to determine the absolute sound power of different noise sources.