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Abstract

High efficient axial fans are widely used in tunnel ventilation and industry. Noise of these fans often becomes a crucial parameter during supplier selection because it influences people's health and safety especially in case of fire in the tunnel.
Different methods are used to reduce fans noise, but mainly with passive noise reduction – use of silencers on inlet and outlet of the fan, which in some cases can be not so efficient. In most cases, designers are using from two to four identical fans standing near each other at the same place. Apparently, this leads to an increase of sound power on 3 dB for example if two similar sources are standing near each other. However, there is an option to unphase sound signals coming from fans and reduce sound pressure levels on blade passing frequency (BPF). Sound levels on this frequency are making the main input in the total sound pressure.
For simplicity, in this work two fans were considered. To unphase signals from two sources on BPF, two options were used. First is physical change of mutual blade position for two impellers. Second is the change of mutual blade position for two impellers by changing electrical net phase. Using this method, sound levels from two fans could be reduced by up to 2 dB.