Further Study of the Test Configuration for Axial Flow Fans Noise Measurements

The present paper concerns the effect of the test configuration on the noise generated by axial flow fans employed for automotive cooling systems. Different test configurations may be employed depending on the design phase. Discrepancies are often observed between the results of different tests, resulting in difficulties in transferring and generalizing the measured SPL. In the present case the effect of the mounting system (free-discharge wooden panel versus test plenum with zero and design downstream pressure) on the irregular growth of the tonal noise during speed ramps is considered. The spectral decomposition method applied to the measured SPL spectra has been used to obtain the propagation function which has been compared with the scaled SPL at the blade passage harmonics. The comparison shows that the fluctuations of the SPL during the speed ramps depend on the acoustics of the surrounding environment and not on variations in the incoming flow field.