Box Calculation Help

**TheOtherSide** · 02-14-2007, 09:49 PM

Maybe I am just dumb tonight and can't figure out the darn formulas, but can someone help me here.

I am thinking of different box sizes and types to use. I already have the sub, Perfect 12VQ, and have come up with these 3 ideas. Only problem, I can't figure out port length to save my ass.

Scenario 1:
1 ft^3 sealed (I already have it in this config, I just need to make a truck specific box). It sounds good but I am wanting a little more out of it in terms of SPL. I am not shooting for high #'s but something a little better for tailgating that can be heard outside the cabin.

Scenario 2:
Vented box, 1.75 ft^3 tuned to 28 Hz.

Scenario 3:
Vented, 2.25 ft^3 tuned to 28 Hz.

I think I need a port with an area of about 20 in^2 but I am not 100% sure of my port calculations. I really need port length but if you think my area is wrong, let me know and what you recommend.

T/S for those who can use them:
Xmax = 11/16" or 16.75 mm
Fs=22.19 Hz
Qts=.34 (Scenario 2) .47 (Scenario 3)
Diam = 300 mm or 11.89"
Sd = .441 ft^2 or .0491 M^2
Vas = 3.38 ft^3 or 96.43 L
Vbasket=.108 ft^3 or 3.06 L

If anyone can help me out here I would appreciate it.

My calcs gave me a port diameter minimum of 3.42" (round) and a length of -5". That just doesn't sound right though.

If I plug in the 20 in^2 of surface area for a slot port, I get -4.7" for port length.

**///M5** · 02-15-2007, 06:54 AM

With 1.75 cubic feet NET, and a port of 2.5"x8" (20sqin) to get a 28hz tune the port will have to be 37". At 2.25cuft NET with a 2.5x8 port it will be about 28".

Note I yelled net assuming that is what you meant by your box size. Low tuning = long ports, especially in smaller boxes.

**TheOtherSide** · 02-15-2007, 07:08 AM

Thanks. I understand the concept, I can't figure out the math for some unknown reason.

Now I get to see if I can build it. Shouldn't be too tough.

**///M5** · 02-15-2007, 07:17 AM

I didn't do any end correction, just a pretty generic port length...

**TheOtherSide** · 02-15-2007, 07:34 AM

Yeah, I am working on that one. I did figure out my math error finally though. Damn conversions.

k is roughly .64 (1.75) and .5 (2.25)

**///M5** · 02-15-2007, 07:57 AM

28Hz is low, had you asked for port help at 38Hz I probably wouldn't have taken the time to do calc's for you. Go SQ!!

**TheOtherSide** · 02-15-2007, 08:53 AM

I had originally wanted to do 25Hz but modeling shows best transient response for that woofer at between 27.5-29.2 Hz in my size range.

Thanks for the help M5.

Now to take some measurements, figure out if I need to lift the seat, and design/build it. I will probably post all the development work.

**TheOtherSide** · 02-17-2007, 12:30 PM

Here is the initial design. The box based on quick measurements if built from seat bracket to bracket, would be bigger than the 2.25 ft^3 I was looking for so I designed in a partition that can be moved to change the net volume. Also in the design is a quick dirty brace that I need to find the volume of once I build it so I can figure out volume with it. Let me know what you think.

Vented box tuned at 28Hz with port length of 26.5" taking into account the 0.5" of end correction factor.
(click images for fullsize)

**///M5** · 02-17-2007, 05:33 PM

Box looks great, where are you going to put it?

**TheOtherSide** · 02-19-2007, 10:05 AM

I quickly threw that together to get an idea but it will be rear underseat venture. I have the side plates cut and I am working on the front and rear floor contours to hold the box up. I may need to take some of the middle out of the box for the center hump, I really didn't take that into consideration, and I think I may need a 1" seat lift. The way I see it, I need 1.5" of clearance for the sub (a little on the safe side and I can go to a different driver in the future) and 9.5" for the box height (includes internal dimensioning and each 3/4" side of mdf). I will get to building it and see how it turns out.

**///M5** · 02-19-2007, 10:32 AM

3/4" of seat raise doesn't require any extra work, just some ply cut to the size of the mounts. 1.5" and you will need some blocks. Regretably once you measure the hump you are going to have to completely redesign the box. Without doing anything tricky, my measurements put the gross box volume around 2 cubes with the hump taken out of the box and no seat lift. Of course with glass or using other cuts you can get more.