Designing my first enclosure, want to bounce ideas off you guys

[philrab]

I'm sure plenty of you guys could get this done faster than me, but I always enjoy figuring things out for myself hence I want to design this enclosure myself.

The particulars:

2007 Mazda 3 hatchback/wagon

500 watts rms (amp already in the vehicle)

max dimensions 36W - 17H - 24" Deep, front of the box will have a 20 degree angle to closely fit the lean of the rear seats. I want to keep the depth as minimal as feasible to keep as much hatch space as possible, so Height and Width can max out as necessary to keep depth minimal.

1 Obsidian Audio 12" dual 4 ohm coils (amp isn't rated for 1 ohm, will wire for 2 ohm)

Obsidian Audio recomends 1.75 - 2.00 cu ft. (shouldn't be a problem with my max dimensions) tuned to 32-35 hz and they recommend a single 4" or 6" (round) port.

Now, the questions before I sink a lot of time into designing an enclosure that's a flawed concept.

1. Conventional wisdom for a hatch is sub up port back from what I've read. In my old build (in a sedan), I had two 10's subs forward ports sideways (two seperate enclosures) and that sounded good. What's the wisdom behind subs up ports back?

1a. Persuaint to the above, I have a back package tray (just really thick cardboard) I'd like to keep in the vehicle as it keeps people from peaking into my hatch and it's a convenient spot to put lightweight stuff. Sub up sounds like a recipe for a rattle trap with this.

2. I was toying with the idea of designing the enclosure to put both the woofer and port (square port) on the driver's side of the enclosure, sub and port forward side by side, that way I can ride around with that side of the 60/40 split seats down and not be firing everything through the seat. It's probably a stupid idea, talk me out of it.

3. Since the OA 12 is rated for 600 rms, and I'm looking at only 500 rms, would it be advisable to run on the higher end of the cubic feet recomendation for more efficiency?

4. Tuning frequency. My amp (Kenwood) doesn't have a subsonic filter. I know playing below tuning freq is a good way to blow a woofer (been there, done that) so should I tune for 32 hz (bottom of their recommended range) to be safe?

5. Figuring the size of a slot port, would the square area be the same as the square area of the round port they're recommending?

I've played around with Torres calculator a bit, need to really dig in and commit to learning it, but that's again part of the fun here. I'm curious if anyone can point me to a good resource for all the math behind the box calculators, just out of curiosity. My job involves designing some fairly complicated Excel spreadsheets for financial calculations, and if I knew the math I'm sure I could build my own sheet (just as a pet project.)

I'm open to criticism, and ideas. That's why I'm posting here. Feel free to (gently and respectfully please) tell me if my head is wedged up my behind, but I'm here to step up my knowledge and learn how to do this for myself rather than depend on someone else for box designs.

[Hugh G. Rection]

1. Conventional wisdom for a hatch is sub up port back from what I've read. In my old build (in a sedan), I had two 10's subs forward ports sideways (two seperate enclosures) and that sounded good. What's the wisdom behind subs up ports back?

quarter wave theory

just a note- im all jacked up on pain killers because im broken badly from a motorcycle wreck. hopefully im thinking straight.

[philrab]

1. Conventional wisdom for a hatch is sub up port back from what I've read. In my old build (in a sedan), I had two 10's subs forward ports sideways (two seperate enclosures) and that sounded good. What's the wisdom behind subs up ports back?

quarter wave theory

just a note- im all jacked up on pain killers because im broken badly from a motorcycle wreck. hopefully im thinking straight.

Read about your accident man, hope you heal quick. Motorcycles are no joke, lost a friend of mine's younger brother (kid was 17) on a bike and just lost a 28 year old friend of mine from highschool to an ATV accident. You're alive, you'll heal, nothing else matters.

Can you expand on quarter wave theory? Suppose I'll let my fingers do some walking around Google and see what I come up with.

Preliminary measurements and calculations are as follows. Have to wait till I can get home to my laptop before I can plug this into Winisd and Google Sketchup and start getting this together, but so far:

Trapezoidal box

14" high x 34" long x 10" deep on the top and 15" deep on the bottom. That gives me a 20 degree front angle where it'll butt up against my seats, 1.84 net air space after the port and sub displacement. I can squeeze that net displacement down a little (and will, cause this bastard is so long) with bracing. May take another swing at it and make the box taller and take some depth out, but the numbers worked out so damned nicely.

Slot port 12.5" high (inside measurement of the box figuring .75" thick mdf, I know it isn't perfect, MDF is usually .765") x 2.25" wide. Does that HxW ratio look a little off, or good?

Slot port will need to be 36" long for 32hz tuning frequency. I already know I'm going to have a corner in this thing, figure I'll shave a little 45 off for the corner to try and keep things even, and round off the other edge. All this is based on OA's rec to use a 4-6" port. With 20mm of Xmax on that sub, I'm figuring more port area is cheap insurance against port noise.

If I'm barking up the wrong tree, feel free to point it out to me.

I could also use less port area (the equivalent of a 5" port, dimensions 12.5" x 1.5") and have a MUCH shorter port of 20" for the same tuning frequency. That's an option, just not sure which is the right one.

BTW, all the above assumes I'd be doing a front firing sub. Obviously, if you guys talk me out of it and I go top firing, this box design is going to change quite substantially, but these are the things I'm trying to work out before I tackle Google Sketchup.

[Soccerballzs]

I like sub up and port backward use the curve of the roof line for sound transmission of the bass wave.

[philrab]

I like sub up and port backward use the curve of the roof line for sound transmission of the bass wave.

Like I keep asking though; is there any quantifiable/mathematical reason for that, or just based on what everyone's ears like? I'm at that point in designing this where I can certainly about face and do that, but I'm wondering what the madness behind the method is.

You never learn if you don't question.

[philrab]

Alright, pitched what I had worked up the first time. Going to model a traditional hatch box, sub up port back. I'll make the enclosure a little shorter to hopefully not rattle my back package tray too much, we'll see. Ultimately deadening isn't going to help much if it's just air pushing on it, so it'll either work or I'll have to remove it when I run this box.

Trapezoidal box: 12" H x 32" W x 14" top depth and 18" bottom depth (20 degree angle to fit against the seat back. 1.989 cu ft net volume, so right at the top of OA's specs for a ported OA12. Also leaves me plenty of space for some 1" dowel to brace this long bastard.

Port will be a single slot port, equivalent airspace to a 6" round port (again, top of OA's recomended specs, assuming this big a port won't have any issue with port noise vs. a smaller port) 10.5" tall (as tall as the inner dimension of the box) and 2 11/16's of an inch wide (fucked up math just worked out that way converting decimal to fraction) 32 7/8's of an inch long. I'll have to run the full length of the box plus a little turn at the end, 45 in the outside corner and roundover the inner corner to smooth airflow. That'll tune me to 32 hz.

Thinking if I'm going to sink this time and money into this enclosure, going to do it right. Will double baffle and countersink the sub (probably don't have to double baffle for a single 12", but it sure looks nice as shit.) Undecided if I'll carpet the box black to match my hatch trim, or bedline it to kinda go with the weatherproof matt in the trunk it'll be resting on.

Now, new questions:

1. Do I need to worry about putting some dowel in the port itself to keep things stable, since the box is so long?

2. If I do the above, how does that play into port area? Any adjustments I need to make, or will it matter that much in the long run?

3. Was planning to put the sub and port on the same side of the box, just have the port run behind the sub and all the way to the other side (it'll make sense once I get it into Google Sketchup). I've heard that having the sub too close to the port opening can cause issues. Any wisdom to pass along for that?

Anything you veteran box builders want to pass along would be welcomed. The last time I did any of this, I knew jack shit and got lucky. Literally, had no idea what airspace requirements I needed for the RF subs I was running, no clue about port tuning, was completely clueless. Out of curiosity I ran the specs from what I could remember through a couple of enclosure calculators and realized I just got dumbass lucky and it actually worked decently (coulda gone bigger on the enclosures, longer on the port tubes, ect.) This time around, I want to learn the science behind it, plan for the box to fit this sub, learn the process, do the build. I want this to be all my work, but I'm always open to constructive criticism hence I ditched the original idea in lieu of a sub up/port back box as the conventional wisdom seams to work well in hatchbacks.

[philrab]

No one else wants to chime in or critique? I'd welcome some opinions at least on the basic design. I'm used to seeing boxes that are more square than this long rectangle. Not sure if that'll cause issues, but I'm wanting to utilize length to minimize depth.

Also wondering if there's a point where not having enough depth behind the magnet will cause an issue. Box will be 12" high and the installed depth on the sub is under 7", so I'm thinking it'll be fine, but like I keep saying first ported box so I'm in uncharted territory for me.

[Hugh G. Rection]

Also wondering if there's a point where not having enough depth behind the magnet will cause an issue. Box will be 12" high and the installed depth on the sub is under 7", so I'm thinking it'll be fine, but like I keep saying first ported box so I'm in uncharted territory for me.

the depth of the box does have some effect as the box will have it own wavelength to an extent...

[philrab]

Also wondering if there's a point where not having enough depth behind the magnet will cause an issue. Box will be 12" high and the installed depth on the sub is under 7", so I'm thinking it'll be fine, but like I keep saying first ported box so I'm in uncharted territory for me.

the depth of the box does have some effect as the box will have it own wavelength to an extent...

Any advice on where that breakeven point is? I'll have a bit more than 4" of space behind the magnet in this case, think I'm okay, but wanting to know for the next time I build a box. This isn't going to be my last one.

[philrab]

Even though no one is commenting, I'm keeping tentative box designs here. Comment if you want, or not.

14" H x 34" W x 10" deep top and 15" deep bottom; Trapezoidal box with a 70 degree forward angle to match up with the seat backs

1.989 ^3 ft

Slot port 12.5" x 2.25" (roughly 6" equivalent) 26.75" long, tuned to 35 hz. Sub and Port racing the rear to load off the hatch. Figure I'll have about 2' of space to the hatch, and 5' from the hatch to the head rests to give the bass time and space to propogate. Also didn't want to face the sub up and have it rattle the back package tray more than it already will.

3/4" MDF, double baffle flush mount (will have to remember to take that extra 1.5" into account on port length from two layers of 3/4 mdf making up the last bit of the port).

Will also have some 1" dowel to brace front to back in two places along the box due to it's long length, but it's necessary to keep the depth as minimal as possible and conserve hatch space. Looking like my box design is starting to stabilize, now just got to go equipment shopping with the budget I have in mind.

Shopping list:

Table Saw

Router

Router bits (Downward cut, roundovers, rabbet)

Jigsaw

Pneumatic Nailer

Sheet of 3/4 MDF (maybe two, gotta make a cut sheet from final plans)

2 tubes of Tightbond 2

Pack of stables

length of 1" dowel

binding posts

carpet

another can of spray adhesive (have half of one left from another project.)

Gotta get the mortgage and insurance paid end of this month, then see what and where I have the money. Trying not to tap savings (I hate that shit) but I won't have my feelings hurt if I need to since I just dropped an extra $500 in there this month on top of my usual.