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rac_1974
01-14-2003, 02:17 PM
I'm thinking of building a box for a 15" and the size for a ported box is 3.4 cu. ft. I came up with 18" high-15" wide-and 22" long. Can anyone tell me if these measurements are right. I multiplied the above three numbers and diveded them by 1728 and came up with 3.4375 cu. ft. By the way, the port size is 4" * 10" long.

Randy Savage
01-14-2003, 02:54 PM
I don't feel like doing math, so here are the equations for everything, thanks to Jmac-

For a rectangular box ...
H = Height
W = Width
D = Depth
C = Thickness of MDF, fibreglass, or whatever the box is made of
V = Volume
P = Pi
R = Radius
S = Speaker displacement
Vport = port volume
Vslot = extra displacement of material used to create slot port
Vbrace = displacement of braces used

(H - 2C)(W - 2C)(D - 2C) - S - Vport - Vslot - Vbrace = Vbox
P*R^2 = Port Area of a circular port
Hport*Wport = Port area of a slot/rectangular port
P*R^2*Dport = Port volume of a circular port
Hport*Wport*Dport = Port volume of a slot/rectangular port
Hport*C*Dport = Added displacement of slot port if wall is used as one of the sides of the port.
Hport*2C*Dport = Added displacement of slot port if wall is not used as one of the sides of the port.

Savant
01-14-2003, 03:06 PM
Originally posted by rac_1974
I'm thinking of building a box for a 15" and the size for a ported box is 3.4 cu. ft. I came up with 18" high-15" wide-and 22" long. Can anyone tell me if these measurements are right. I multiplied the above three numbers and diveded them by 1728 and came up with 3.4375 cu. ft. By the way, the port size is 4" * 10" long.

If the measurements are inside you are probably ok.. if they are outside, you have to subtract the thickness of the wood to get the real measurements.. Don't forget the displacement of the speaker it's self..

Not sure about port volume, don't know if you have to account for all the air space in the port as 'not space' or if it's just the actaul material of the tube/port... Though, I thought I read someplace that the air volume of the port makes the woofer think the box is smaller?? which would mean subtract the air volume.. but like I said, I'm not sure there..

EDIT: From the above equation, the port volume subtracts from the box.. so if you had 1 ft^3 of port volume, your box would only be 2.4 ^3... make sense? (but your port isn't that big.. you just have to take it into account)..