Check my math...

[mbacis 10+ year member]

I'd like to make a custom ported box for two RE8s. This will be a fiberglass enclosure that will fit over the rear wheel hub on my Saturn Vue. Since it will be an odd shape I plan to overshoot 1cuft and then add bracing to get exactly the 1 cube. Now, I would like to tune it to 30Hz and came up with a port that is 2" dia. and is effectively 7.93"long, therefore physically 6.93" long.

Please tell me if I've done this right, this will be my first ported enclosure attempt as well as my first fiberglassing attempt. Also, my plan was to have the two subs facing across the vehicle with the port facing the front of the vehicle. Would this be a good setup?

And....do I need a subsonic filter if I tune this to 30Hz?

Thanks.

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[Moe Lester 10+ year member]

youve got the length correct, but a 2" port isnt nearly large enough, id go with a 4" minimum, maybe multiple 3" or 4"

 

[brandontw 10+ year member]

a 4 inch port for a 1 cube box, tuned to 30 would have to be 34.7 inches long, just to let you know, and 2 3"ers would be 20 inches each, but i may be wrong on that

 

[mbacis 10+ year member]

OK, so I'm using the following formula...

Lv = Av*1.84*10^8/[Vb*1728*(Fb/0.159)^2] - 0.823*sqrt(Av)

Lv is port length (what your solving for)

Av is your port area (height x width of the port face) pi*r^2 = 3.14*1^2

Vb is the internal volume of your box 1cuft

Fb is the tuning frequency you want 30Hz

I come up with 7.93358 for the length of my port. If I calculate with a 6 inch port (r=3) I get a length of 80.1524 in. So my length increases with diameter? Why wouldn't I just use a short 1" or .5" port? I must be missing something.