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Im speccing a extractor fan / air handling unit for a room.

Based on the calculations of air cycles per hour i need to use a bigger extractor fan. The problem is the smaller extractor fan which we where planning on using are $26\operatorname{dB}$ where as the larger extractor fan is $36\operatorname{dB}$. We want the extractor fans to be as quiet as possible.

If I was to have $2\times$ smaller $26\operatorname{dB}$ fans at either end of the room (the room is c. $\operatorname{L}4 \times \operatorname{W}2 \times \operatorname{H}3m^3$) would the sound increase to $2\times 26\operatorname{dB} = 52\operatorname{dB}$ or would they run both at $26\operatorname{dB}$ and the total audible noise be $26\operatorname{dB}$ ?

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29db

3db is doubling of sound power.Two sound sources = 2x sound power. I dont think two smaller fans can move same volume of air as single larger fan at significantly lower noise level.Is that db rating A weighted? Fans arent just pure white noise,the frequency spectrum of noise makes big difference on how annoying it is.

I think single bigger slower rotating fan would have deeper tone and be less audible at same db level than two faster fans.On other hand,higher frequency noise is easier to absorb or block,low tones go through walls easier.

And also,since you mentioned room dimensions,I will warn you,if your fan will have noise peak at same frequency as standing wave resonance of your room,then the noise will get much louder.

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