Abstract
An experimental study is described in which the sound power of six fans of different designs was measured using three different standardized measurement procedures, i.e., the free-field method ISO 3744, the reverberation room method ISO 3741/3742, and the in-duct method ISO/DIS 5136. The one-third-octave sound-power spectra obtained using the free-field method and the reverberation room method show excellent agreement in the frequency range from 100–10 000 Hz. The sound-power spectra determined according to the in-duct method show higher levels than the other two methods in the frequency region of plane-wave sound propagation in the ducts. This is due to the reflection of the sound waves at the fan inlet or outlet when the duct is removed. In the frequency range of high-order mode sound propagation, the in-duct sound-power levels are lower than the free-space levels. So far, there is no definite explanation for these differences; however, it is assumed that two frequency correction terms in the in-duct method need to be revised.
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