home theater sub enclosure

Okay, to start, this sub project is actually for my home theater system, using car subs.  I have built many boxes for cars before, but this is the first for the house.

Short version, I have two main questions.  The first relates to the 'maximum recommended enclosure' via manufacturer specs.  The second relates to channeling the bass through a slot in the wall, and how large the slot will have to be, to not affect the tunning of the box.

The problem originates from the combination of low budget, and lack of subwoofer placement options.  I knocked out a wall, and I'm going to be mounting the TV and surround in the wall, flush.  Being as the room doesn't really have anywhere to put the sub (trust me =P), I thought hey, I could actually build it in behind the TV, before I put it all together. *shrug*  I also happen to have an old school 2-channel amplifier to power the sub(s), so that will help in the budget end.

Since the amp only pushes 100w/ch, I decided to use 2 subs to maximze usage of the amp.  Now, using only 100w, I can buy fairly cheap (low power) subs, but I still want something decent, and maybe upgradeable later for when I get a monoblock home amplifier. 

I came across a deal on some Polk Momo 12" subs, I'm certainly not set on these, and welcome recommendations that might be better for my application at about $100 or less (for 2).  But I will stick with the Polks for my examples.

This brings me to the first issue.  These subs are (according to manufacturer) designed for sealed enclosures, and the recommended enclosure is 0.88ft^3.  Now, to me, that is just crazy, the smallest box I ever built was 1ft, and that was for a 10", taking about 800w like 10 years ago =D.  I have tons of air space to play with behind this wall, so, Out of curiousity, I loaded T/S into BassBox Pro, and started playing around with ported enclosures.  In the end I came up with an enclosure that was a little short of 4ft^3, tuned to 25hz, and had an f3 of 24hz, response doesn't drop below 6db until about 22hz.  Again, I thought this was crazy, I don't think I've ever built a box that could perform like this down to 22-24hz.  Now I'm wondering, how accurate are these 'enclosure design' programs.  With the box design being dramatically larger than what the manufacturer recommends, can I really expect the subwoofer to perform like is does on the graph?  Or is it just going to sound like its sitting in the middle of the room, free-air?  Originally, I figured I could make it a little larger than recommended, since I'm only pushing 1/4 of its RMS wattage capability, and have space to spare.  So, ultimately, the question is, how accurate are these programs?  Also, are there any experienced people out there that could give me an opinion on how such an enclosure might sound?

Secondly, while I have tons of room behing the wall, the left-over space I have on the face of the wall is only about 7"x 20" on each side.  I was tuning the ported box independently, but when I started looking at how it was going to have to be channeled to the front, through a large slot, I started wondering how/if that would affect the tuning of the actual enclosure.  This slot is obviously much larger than the actual port, but I have never tried anything like this, so I have no clue what it will sound like.  So again, ultimately, the question would be, will this behave like a simple ported enclosure, or, will it behave like a 'dual reflex bandpass' that has horrible tuning?  And at what point do the 2 seperate from each other, meaning, how large would that slot have to be, to not affect the tuning?

At this point, I can only hope, anyone has even read through this novel, more or less want to leave a reply =D, so I will leave it as is.  I actually have drawings of it, and if interested, I could maybe upload a couple if anyone needs more of a visual of what I'm saying..

Thanks ahead for anyone who decides to venture into this post =D.

The thought of infinite baffle did cross my mind, but I'm afraid there's going to be to much vibration/rattling to deal with, where the actual enclosure will allow me to direct all the bass where i want it to go, but I will take a look at the link anyway for the time being.

Thanks for your response.

Honestly, your concerns are not valid.  A properly built IB introduces no more vibration into a room than any other subwoofer, and "directing the bass where you want it to go" is not how bass frequencies work.  They go everywhere, no matter what you do.  An IB will be much more feasible in your situation than trying to build a band pass with a vent in the wall, which is what you describe.  That idea would be likely create an utterly unusable system.

Maybe you are misunderstanding... 

First, I'm not trying to build a bandpass, but rather just suggested that the 'ported enclosure' might behave like a bandpass as a result of being routed through a slot in the wall.

Second, the 'infinite baffle', wouldn't truly be an 'infinite baffle', but rather, would use the 'closet like' area as a large box.  Would this not create substantially more pressure/vibration 'inside' the 'closet like' area, as compared to a  ported box, being channeled through the wall, into the room. 

Thanks again!

I an certain I understand what you're talking about quite well.  I've built literally hundreds of installations just like what you describe.  What you describe would indeed "function like" a bandpass because it would indeed BE a bandpass enclosure.  Secondly, if the enclosure size is 10X the Vas of the drivers (actually more like 4X but that's picking nits) then it is indeed an IB.  Your statement about pressurization and vibration demonstrates that you really don't understand how speakers work (and this is not a slam, it is a common misconception.)  Like I said before, your concerns about "pressurization/vibration" are unfounded.  The exact same thing happens inside a sealed or IB or vented enclosure.  The function of a vent is to create a Helmholtz resonator that reinforces a specific frequency, it is not to relieve pressure or reduce vibration or direct the bass or anything even remotely like that.  :)

Thank you Steven.

I don't doubt DYohn's expertise, I am hoping for people to argue with me =).   I just wasn't sure if I had explained myself well enough.  I tried to model the response I would get from the IB enclosure, and it has nothing near the performance of the ported enclosure.  So that is one concern.  Another problem is, from the sound of it, you basically want me to mount the woofer on a board, flush with the wall, correct?  Well, if I had the space to do that I would just mount the ported enclosure flush with the wall.  The only space I have on the face of the wall, is 2 seperate areas about 7"x20".  So I can't really do that, and if mounted the speaker inside that slot, in IB fashion, it seems it would still be acting like a bandpass. 

I am aware that this is a tricky situation, that is why I am hear, to listen to suggestions, and learn what I can, so I don't waste time/money.  On the other hand, this is as much about having fun with the build, and conquering the problem, as it is about the end result.  I enjoy projects like this, and I am open to suggestions.

Concerning the 'pressurization' thing , I didn't think the port was there to 'relieve pressure'.  But, are you are telling me there is no pressure on the inside walls of the box, which is what it sounds like you are saying?  People put bracing in their boxes for this exact reason, there are large amounts of 'pressure' in side the box, this is the purpose of the box, to push back against the speaker.  In most of the boxes I've built for car audio, you can feel the walls of the box wanting to push out, even with bracing, this is because of the massive amounts of pressure the speaker is pushing against it.  I'm sure you know this, I don't doubt even that you are much more experienced than me, thats what I'm here for, not to teach, but to learn.  It just seems for this particular example we seem to be misunderstanding each other.

I know what an infinite baffle is, but it cant really work, unless I mount it on a different wall, because this wall is small, and after installing the tv, and component rack, their will only be 7" on each side, which doesn't give enough space to mount a 12", or even 10" woofer, so maybe I'm just misunderstanding you, rather than you misunderstanding me.  Again, I want to point out  the dramatic difference I see concerning frequency response in the simulation with the IB, compared to vented enclosure.

Thanks for your Input DYohn, I do appreciate it, and would like to continue this discussion, so maybe I can have a decent sounding sub in the end.

I will try to upload a drawing of the area I have to work with.

When I said that the closet would not really be an infinite baffle, you are right, by definition it will be, but in reality, the drywall will become the baffle, and the speaker will put pressure on this drywall, which isn't going to take it as well as 3/4mdf, thats all I was trying to say.