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@Emyr
Assuming a 25mm hub, and that the frame takes about 3mm off each side.
120mm Fan area = (pi x (120-6)^2)-(pi x 25^2)
= pi x (12996 - 625)
= pi x 12371
140mm Fan area = (pi x (140-6)^2)-(pi x 25^2)
= pi x (17956 - 625)
= pi x 17331
17331 / 12371 = 1.4009
so simplistically, a 14cm fan has 40% more driven area than a 12cm fan, so can spin even slower than a 12cm fan for the same CFM, causing less turbulence.