Im building a woofer box for my car and I have some 12 inch abs pipe. It measures 11 inches i.d. . I need 1.25 cf of air for my woofer. How long would it need to be? I understand your formula ok but how would I find the length it needs to be?
11" = 5.5" radius = 0.458' 3.1416 X .458'² X L = 1.25'³ 1.439' L = 1.25'³ L = 1.897' or 22.76" The volume of 2' long pipe at 11" width is. CF = π R² L = 3.1416 X 5.5"² X 24" = 2280.80"³ or 1.32'³ Be aware though, with any tube of air comes resonance.
Bill, look at your notation. If 5.5"² means to square 5.5" and not 5.5 square inches, then doesn't 2280.80"³ have to mean to cube 2280.80" and not 2280.80" cubic inches? Might I suggest: CF = π R² L = 3.1416 X (5.5")² X 24" = (2280.80)"³ or (1.32)'³ or CF = π R² L = 3.1416 X (5.5")² X 24" = 2280.80("³) or 1.32('³) Probably the best would be to not use the " and ', despite how convenient they are. CF = π R² L = 3.1416 X (5.5 in)² X 24 in = 2280.80 in³ or 1.32 ft³