5cu ft sealed enclosure

Personally I would lose the divider in the center. You can use it as a brace if you want. Cut a 6-inch to 8-inch round hole in it. Size is what gives us a really low hit. Small ported boxes tend to be effective in a narrow band while larger ported boxes seem to play across a wide range.

Now for the ports, two 4-inch ports around 9.5-inches long should do the trick.

 

No, but you could easily modify those dimensions...
As it stands, that thing has a 2.75" wide slot vent, which is a huge amount of area - I'd be worried that would be tuned closer to 40hz than it would be 30hz (surely it's somewhere in that range, based on that port length).

All you need to do is change three dimensions on that box to suit two subs:
** Make the overall depth 20.5 (rather than 11)
** Make the slot vent width 5" (in keeping with my comment above... just a touch more conservative - 'street tuning')
** make the last wall of that port 4.5" rather than 14" (to keep the port length correct)

And, of course, cut two sub holes rather than one. You might think about also mounting a strip of MDF edgewise between the subs, as a brace inside the box. Keeping things stiffer on the baffle will make those subs a touch louder. Doesn't need to be big, maybe a 2" strip of MDF, extending from the top to bottom of the box.

 

I didn't see three pages, lol.

A few worries with this design IMO:
Round ports... easy to buy at Home Depot, but other than that, no benefits, and not much port area.
Bracing wall - good idea, in theory, but then why bother with round ports?
Also, you need at least a port-diameter's worth of clearance behind your port. A 4" round port needs 4" of clearance behind it and the back wall, or else it won't function as designed at all.

It would be MUCH more practical to just build a single slot vent between the two subs, which would ALSO function as a center brace wall. With that design, the port walls extend all the way from the top to the bottom of the box, inherently bracing the baffle, top, and bottom of the box just like your brace does in your design - but no PVC tubes required, and you get more port area to boot. And, they are more resistant to port noise like chuffing. And, the clearance requirement is simply the narrowest dimension of the port (for example, if you had a 3"x12" slot vent, you'd need 3" of rear clearance). And, it's easy to bend them if your port dimensions end up longer than your interior depth will allow - just like the box I made the comments on above.

That's one of my absolute favorite dual-sub designs, the inherent bracing of a center slot port makes that baffle uber-rigid, and the port is ideally placed between the two subs, and the port area is as generous as you want.

 

The problems are many if you have two different subs in one install...

You don't get the best of both subs, you get the worst. ;-)

Reason is, when you crank it up, one of the subs will reach it's limits before the other one might even be warmed up - and that will limit how far you can turn up the volume.

Also, more dynamically, the same thing happens really at every frequency in the spectrum (especially if they are in different boxes) because two subs will have different frequency responses... so, it's more complex than what I said above, even.

And, different subs have different efficiency levels as well, meaning one will be louder than the other, dominating the sound characteristics of the install compared to the other - and, almost inherently, that will also be the sub that reaches it's limits first (more efficient subs happen in part because of lighter-weight, lower-powerhandling components used in lesser designs). So, your cheaper sub will be the one you really hear, over your "good sub".

It's a bad idea all around.

 

Good idea, but if we are gonna use a slot in the middle lets get a little more than 3-inches of clearence in the rear. I really would like to see the woofers share the same airspace. I don't typically like to see a long center port that effectively acts as a divider. When woofers (ported Boxes) are in too small a box, they tend to be peaky.

Personally I would use a single 6-inch port in the center.

92civicproblems, geolemon knows his stuff. The differences between us is our personnal styles. His recommendation is a valid and well thought out idea.

 

different subwoofers have different group delays and different response curves...leading to muddy undefined bass.

One reason why having good midbass up front can drastically improve the transient response in a system.

Also there is nothing wrong with round ports. People are biased. Round ports have better laminar flow than rectangular ports. Rectangular ports are easier to build with a lot of port area.

 

A 6" port would be even better, but then the box might be too small, and your port length might exceed the length of the box... complicating things just a bit (even though you could "T" it into two 3" ports). But same situation - definitely dividing the box, unless your port is pretty short. I like that though, because it braces the hell out of the box... some guys will actually run a wall down the center of the port, all the way, front to back, to REALLY divide the box in two. I'm usually more of a fan of skipping it to gain just a bit more airspace in the box, but I'm an efficiency kind of guy...

And I'm not saying there's anything wrong with round ports, btw... just that there's no advantage to them, and you are pretty well locked into 3" or 4" if you are shopping Home Depot. I just saw that dividing wall, and ports, and said "why not kill two birds with one stone?".

 

geolemon, two 3" round ports do not have the same area as a 6" round port.

Technically two 4.2" diameter ports would be needed to T off the 6" port.

 

how long depends on where you want to tune it at and how much port area vs net volume.

 

3.5 cu ft...can you find any 6" diameter PVC piping??

 

I said nothing of the sort. I haven't suggested using round ports at any point.

If you have a 6" wide slot vent and it exceeds the length of your box, you can split it into two 3" wide slot vents extending into each chamber to make up the remainder of the difference.

For example, if you had a 20" deep box but needed 30" of port length, you could run a 6" wide slot vent all the way back to within 3" of the back wall, and then have two 10" long wing walls to T the port into two 3" slot vents.

That actually works just as well as if you built the box 3/4" wider with a dividing wall going straight down the middle [giving you two 3" slot vents].

 

I don't think you'd need a 6" vent though, although Ranger suggested it. A slot vent has a lot of area to begin with, a 4" one is still relatively big for two 12" subs.

Like any port, if you get down too small, it'll start to chuff and make noises because of the increased air velocity (the cone displacement being squeezed through a small area turns into velocity).

Square ports are a little better than round ports in this regard too, because with round ports, the velocity is uniform all around the perimeter of the port... so when you reach that critical velocity where you start getting noise across the lip at the openings of the port, you get noise. With a square port, the port air velocity is slower in the corners of the port than it is on the center of each straight wall, so port noise comes on more progressively, and doesn't resonate the entire lip of the port when it does happen. It's also easier to round over 3/4" wood than it is 1/4" PVC, and you can create a bigger roundover... because rounding the port openings can help avoid port noise too. ;-)