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.

vapor602 wrote:

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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.

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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!

vapor602 wrote:

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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!

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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.

Did you even look into the site I linked for you?

How much space is behind this wall?  Meaning, starting from the wall with your 7X20" opening, how far back can you go?

The slot' for each sub would be at the bottom left and bottom right, the actual enclosure would be behind the TV, since the compenent rack is filling up the bottom portion of the area.  I realize this is a somewhat radical idea, but I'm just trying to make do with the extra space I have back there.

Here is a see through of the subenclosure, and the slot the will have to channel the bass into the room.

Okay, so where are you telling me to mount the speaker?  I don't think I understand what you are saying.  The way I see it, the bass has to be fed into the listening environment via the 'slot' or, "port" either way.  So regardless, it seems even if the other side of the woofer in in an IB, facing it into the port, it will still be a bandpass, just single reflex, vs dual reflex.

As this website explains it.

Using IB would result in this...

Using my ported enclosure would result in this...

If it really is going to behave like a bandpass, my only option is to try to design a bandpass on purpose, so maybe I can actually tune it as best possible.

This is the main reason I would rather use ported vs IB.  The red line is my vented enclosure, the orage line is IB.

Well, you are all right, I tried playing around with the bandpass tuning out of curiosity, and it doesn't look too good.  Originally I was designing it thinking it would just be a ported enclosure, not being affected by the 'slot' but I guess this is not the case.  I never actually wanted to use a bandpass of any sort.

As for the use of 8" subs, I would be fine with that, but there is only about 4.5" as is, those grills are 7.5" wide, and for the front speakers I was already cutting about half way through a 2x4 on each side.  I was stretching at the idea of even having 7" width for the sub, plus the grills would have to be bigger.  I'm looking at it now, and I'm thinking, I might be able to fit some 8s if I angle them at 45 degrees or something, not sure how that would affect the sound.  But if it could sound decent, and I could get it to fit physically, I'm open to it.

As for the TL, I haven't actually considered it, and have little knowledge in that area.  I'm not even sure what benefit that would provide, or how it would be applied here, I'm open to suggestions as the idea for the sub behind the TV seems to be a no go, be it IB or otherwise.

Another part of the problem here is that I live with these complete wierdos, whom have their priorities all mixed up, and can't seem to grasp the fact that loud and clear rumbling throughout the entire house is truley important to the continuation of life as we know it, and we will all seize to exist if we don't feel T-rex coming 5 minutes before he gets here. =D

Now I'm thinking even 3 6" on each side might fit.

Yeah, I realize that is a fairly difficult budget.  Especially for the bass output I'm imagining in my head.  I was gong to get the 12"s super cheap, I might have to toss in a little more money.  Another thought just crossed my mind, 4 6x9s. 

I'm not really sure how any of these 'small sub' combinations will turn out, so if anyone has experience using smaller subs in the home, feel free to comment.

dont use 6x9's. seriously

ckeeler wrote:

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dont use 6x9's. seriously

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vapor602 wrote:

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This is the main reason I would rather use ported vs IB.  The red line is my vented enclosure, the orage line is IB.

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Actually, the orange line is just right.  That is the response you want in your room.  The sub will be filtered at about 80 Hz and will slope sharply on the high freq side, so what you are looking at is a classic 6 db / octave slope from 100 Hz down.

Take a look at this Tang Band or some of the other offerings at Parts Express.

Edited to say, I just looked at the link in haemphyst's post...TB as well.  How 'bout that?

haemphyst wrote:

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Ahh... you're just sayin' that... I have one question for you:

Why not?

You'd be VERY surprised.

Another interesting option...

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Funny, thats exactly what I was looking at when I made that post. =P 4 of them together might have decent output, but I don't think I can get myself to except those $ for some 6x9 subs, so its not likely to happen.

stevdart wrote:

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Actually, the orange line is just right.  That is the response you want in your room.  The sub will be filtered at about 80 Hz and will slope sharply on the high freq side, so what you are looking at is a classic 6 db / octave slope from 100 Hz down.

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I don't understand why I would want a 6 db / octave slope from 100 Hz down on the subwoofer.  Am I missing something here? =D

stevdart wrote:

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Take a look at this Tang Band or some of the other offerings at Parts Express.

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My problem is more about the width of the area, not the depth.

Yes, what he said.  If you were to complete the design and build with such a slope and then tested response with an SPL meter, you'd likely find somewhat of an increase at the ~40 Hz region and probably less than -6 db/oct below that.  Your room size, being small, will provide more gain than a large room would.

Keep your eye on woofers designed for home use, not car subs.  There are bargains to be found.

I can see the logic in that.  Thanks for the input.

Now, I'm about to argue a couple points, not because I think you guys are wrong, but I'm always up for learning something new, and this brain of mine always has plenty of questions to go around.

I guess the only real 'arugement' I have to that, is the fact that it seems the ideal enclosure for HT would then be sealed.  From the little I've read about it, it seems ported is usually desired, and most of the home subs I've seen are in fact ported.  Should I just be tuning it different, with more slope, rather than a flat line?  I thought the flat line was desired, and the port does help take that 'flat line' a little farther to the left. but like I said, I do understand what you guys are saying.  I guess without measuring the acoustics of the room it will be somewhat difficult to accurately predict the bass response. 

Concerning the size size of my room, and general expectations for room gain, well, I think this room is fairly unique and I dont really know what to expect.  3 of the 4 corners are basically hallways.  The fourth corner is a door way.  Looking at the wall with the front speakers, immediately to the left is a bar, that is open to the kitchen, then a hallway, and to the right is an opening that leads to the kitchen.  Basically my point is, this room is full of holes, and I'm having this feeling all the bass is going to 'leak' out, and I might not benefit much from the room gain.  Of course, maybe it won't make a difference and I'm making this all up in my head!  I wouldn't mind hearing some thoughts on how these 'holes' in the room might affect the bass response, or room gain.