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post #1 of 17 Old 06-26-2020, 02:03 PM - Thread Starter
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DIY Sub under stairs in dedicated home theater

Hi Folks. I'm building out a home theater in my basement (equipment list and description here). I already have two in-wall Wisdom S90i subs which should home fully produce good sound but not get exceptionally deep.

I have room for a third sub underneath the stairway leading up to the main floor of the house. The area will be used to house my equipment rack, bu there is lots of additional volume for a sub. See the attached pic, the space is about 38" deep (44" including stud depth).

I originally planned to put a monolith 15" sub there or something similar with a custom-built grill on the wall, but I think it would be fun and I could get great results with a DIY sub here. I'm willing to spend a $1-3k on it. What should I do?

Three ideas I've had:

1. Use the plans for the devastator with a 18" or 21" woofer, having the front face flush with the wall.
2. Build a cylindrical subwoofer positioned horizontally. Maybe I could even get a custom 3/8" walled aluminum cylinder fabricated to minimize resonance. This could also accommodate an 18" or 21" driver.
3. Do an infinite baffle sub. The total volume of the space is closet is about 2,200 liters. And there is enough wall space for 2x 15" or 3x 18" drivers. The front baffle could be custom fabricated from a thick aluminum sheet to control resonance.

Thoughts?

Thank you in advance!
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post #2 of 17 Old 06-26-2020, 02:12 PM - Thread Starter
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Quote:
Originally Posted by matt-sf View Post
Hi Folks. I'm building out a home theater in my basement (equipment list and description here). I already have two in-wall Wisdom S90i subs which should home fully produce good sound but not get exceptionally deep.

I have room for a third sub underneath the stairway leading up to the main floor of the house. The area will be used to house my equipment rack, bu there is lots of additional volume for a sub. See the attached pic, the space is about 38" deep (44" including stud depth).

I originally planned to put a monolith 15" sub there or something similar with a custom-built grill on the wall, but I think it would be fun and I could get great results with a DIY sub here. I'm willing to spend a $1-3k on it. What should I do?

Three ideas I've had:

1. Use the plans for the devastator with a 18" or 21" woofer, having the front face flush with the wall.
2. Build a cylindrical subwoofer positioned horizontally. Maybe I could even get a custom 3/8" walled aluminum cylinder fabricated to minimize resonance. This could also accommodate an 18" or 21" driver.
3. Do an infinite baffle sub. The total volume of the space is closet is about 2,200 liters. And there is enough wall space for 2x 15" or 3x 18" drivers. The front baffle could be custom fabricated from a thick aluminum sheet to control resonance.

Thoughts?

Thank you in advance!
Here are some quick drawings showing the wall with 15" and 18" drivers in an IB array.
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post #3 of 17 Old 06-26-2020, 06:12 PM
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With that amount of volume available, and the phase response of your existing subs, I'd build that space into a huge ported box and throw those 3 18"s you speak of in there. It should make integration/summation of your current subs and the new sub so much easier/better, while also giving you a ton of output.

As just one option, you could easily port the space as low as 11 or 12Hz with a couple square ports of 9"x9" or 10"x10" and have port velocity be a non-issue while having ridiculous output all the way down to 10Hz. With a HPF at 10Hz, the phase response should be in a pretty good place to integrate with your existing subs.
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post #4 of 17 Old 06-27-2020, 04:08 AM
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I can only imagine that trying to completely seal up the area under the stairs would be a nightmare... I'd go with IB as it might be easier.
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post #5 of 17 Old 06-27-2020, 04:07 PM
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Quote:
Originally Posted by mumbles60 View Post
I can only imagine that trying to completely seal up the area under the stairs would be a nightmare... I'd go with IB as it might be easier.
You want to do your best sealing it up whether it's ported or not.

In any case, modeling even an extraordinarily leaky ported box (Ql=5) in WinISD shows it still retains a huge advantage over IB, and my reasoning in OP's case of going ported for better integration with his existing subs still stands.
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post #6 of 17 Old 06-30-2020, 11:19 AM
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Quote:
Originally Posted by aron7awol View Post
You want to do your best sealing it up whether it's ported or not.

In any case, modeling even an extraordinarily leaky ported box (Ql=5) in WinISD shows it still retains a huge advantage over IB, and my reasoning in OP's case of going ported for better integration with his existing subs still stands.
My response was not to contradict yours, I was merely offering another idea, which is what the OP asked for... sorry if you took offense. My original point was that you would have to seal up some portion of the under stair space on 6 sides for a typical enclosure while you would only need to separate the front wave from the back wave in an IB config which might be easier. It's very difficult to say since all we have is a 2D drawing.
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post #7 of 17 Old 06-30-2020, 11:55 AM
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Originally Posted by mumbles60 View Post
My response was not to contradict yours, I was merely offering another idea, which is what the OP asked for... sorry if you took offense. My original point was that you would have to seal up some portion of the under stair space on 6 sides for a typical enclosure while you would only need to separate the front wave from the back wave in an IB config which might be easier. It's very difficult to say since all we have is a 2D drawing.
No offense taken whatsoever.

IB is a reasonable approach for OP, I just think in this case it's going to create some real difficulty in integrating with his existing in-wall subs and their phase response. So considering that and the additional output he would get going ported (even if he does a pretty poor job sealing up leaks) that is my recommendation in this case. Even if he said he wasn't confident he could seal it up at all, I'd still then recommend he build ported boxes and stick them inside that space. I just feel like if he's going to go through all the trouble to build something, it might as well be something that will integrate/sum well with his existing subs, otherwise it was a lot of work for very little benefit. I've been in that boat, and it's not enjoyable.

If he didn't have those funky in-wall subs already in his system, I'd probably be recommending he go IB along with you.

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post #8 of 17 Old 06-30-2020, 05:38 PM - Thread Starter
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Thanks both for your thoughts and input.

If we go with a ported design we'll end up with a bunch less volume. The original volume measurements I posted accounted from the equipment closet which is located under the taller area of the stairs. Integrating with the other subwoofers in the room is extremely important, and I plan to use Dirac base management

My carpenter cut out an opening for the a driver today and not surprisingly there is less wall area than I originally thought (without doing some major surgery on the wall). The square-ish area in the photo in the space for the driver. It is ~ 28" wide and 28" tall (subtracting out the 4" require for a baseboard). Ignore the drywall on the back side, that will be removed.

So because there is really only space for one driver, I think a sealed / ported design makes sense. I completely agree with @mumbles60 that sealing the actual carpentry under the wall would be difficult. So instead we build a box inside the area, and have the ports on the in the stud bay to the left of the driver. That should yield a box about 36" wide by 28" tall, and 40" deep, for a total of about 1600 L of volume (accounting for the corner of the box being beveled on the left due to the stair height), and it could accept an 18-24" driver.
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post #9 of 17 Old 06-30-2020, 06:01 PM
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Originally Posted by matt-sf View Post
So instead we build a box inside the area, and have the ports on the in the stud bay to the left of the driver. That should yield a box about 36" wide by 28" tall, and 40" deep, for a total of about 1600 L of volume (accounting for the corner of the box being beveled on the left due to the stair height), and it could accept an 18-24" driver.
If you're willing to buy an HS-24, you actually have the volume to build a pretty much perfect ported box for that driver that you could tune pretty much as low as you want, have crazy output across the board, and have enough port area that you can squeeze everything out that ridiculously great driver without the port being much of a limiting factor. IMO it would immediately be the best ported sub on the entire forum.

Here's an example, 50cf net with a 12"x12"x44" port tuned at 12Hz, with 2000W and a 10Hz BW4 HPF, which should yield a phase response that plays very nicely with your existing subs. Capable of 120dB+ natively from 11Hz up! That is an absolutely stellar native response! Even if you wanted to power it with a cheap NX3000 amp, you only lose ~2dB.



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post #10 of 17 Old 06-30-2020, 10:46 PM - Thread Starter
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Originally Posted by aron7awol View Post
If you're willing to buy an HS-24, you actually have the volume to build a pretty much perfect ported box for that driver that you could tune pretty much as low as you want, have crazy output across the board, and have enough port area that you can squeeze everything out that ridiculously great driver without the port being much of a limiting factor. IMO it would immediately be the best ported sub on the entire forum.
You had me at “best ported sun on the entire forum.” I’m definitely down with the HS-24.

What would be the trade-off in using a sealed design instead of ported???


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post #11 of 17 Old Yesterday, 04:49 AM
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Quote:
Originally Posted by aron7awol View Post
If you're willing to buy an HS-24, you actually have the volume to build a pretty much perfect ported box for that driver that you could tune pretty much as low as you want, have crazy output across the board, and have enough port area that you can squeeze everything out that ridiculously great driver without the port being much of a limiting factor. IMO it would immediately be the best ported sub on the entire forum.

Here's an example, 50cf net with a 12"x12"x44" port tuned at 12Hz, with 2000W and a 10Hz BW4 HPF, which should yield a phase response that plays very nicely with your existing subs. Capable of 120dB+ natively from 11Hz up! That is an absolutely stellar native response! Even if you wanted to power it with a cheap NX3000 amp, you only lose ~2dB.



That is an incredible native response. To the OP I think you have to do it right

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post #12 of 17 Old Yesterday, 06:01 AM
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@matt-sf , I looked through your equipment list and didn't see a sub amp mentioned. The HS-24 can take 2000w, but knowing Nick, that is may be a conservative rating. If it were me, I'd call SI and speak with Nick and ask him if he can recommend a particular amp to drive that beast...
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You're going to want big boy power if an HS24 is in your future. FP14k, FP20k, SP-6000 would be good choices

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Originally Posted by matt-sf View Post
You had me at “best ported sun on the entire forum.” I’m definitely down with the HS-24.

What would be the trade-off in using a sealed design instead of ported???


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In this case, the biggest downside to me would be integration with your current subs. I know Dirac bass management sounds really good on paper, but there's only so much it will be able to do with drastically different phase responses, and that is probably not much at all. This is why I think you'll be really glad you went ported when it comes time to integrate them. If you instead go sealed, just consider yourself warned about how the integration will go.

BTW 50cf was just an example I threw out there based on the size you listed. There's not much difference between 40cf and 50cf.

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post #15 of 17 Old Yesterday, 11:16 AM
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Quote:
Originally Posted by matt-sf View Post
What would be the trade-off in using a sealed design instead of ported???
Ported is fine if you don't mind being -40db @ 6hz and prefer being 120db @ 15hz etc.

4 sealed HS-24's will out-do 1 ported HS-24 at nearly all frequencies (say 1-300hz etc), especially the mid-bass and the single digits.
You probably wouldn't even need your existing subwoofers at that point...
A single FP20k can easily power quad 24's for <$1000, although dual FP14k's would be slightly better.

Some prefer optimizing 15-30hz instead and saving money on drivers, and so ported works well for them.

Personally I wouldn't go ported, as I prefer mid-bass and single-digits to be as loud as possible; and that basically means stuffing as many sealed cones and amps into the room as-possible.
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post #16 of 17 Old Yesterday, 11:43 AM
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Originally Posted by BassThatHz View Post
Ported is fine if you don't mind being -40db @ 6hz
Sealed and ported are tied at 8.4Hz and there's only an ~8dB difference between the two at 6Hz in this case.



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Originally Posted by BassThatHz View Post
4 sealed HS-24's will out-do 1 ported HS-24 at nearly all frequencies (say 1-300hz etc), especially the mid-bass and the single digits.
You probably wouldn't even need your existing subwoofers at that point...
A single FP20k can easily power quad 24's for <$1000, although dual FP14k's would be slightly better.
Did you miss the part where he said he wanted to use one driver?

Of course 4x the drivers and 4x the power will out-do the single driver. Sometimes I wonder if you are a robot with canned responses.

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I too use under stair space for rear subs and equipment rack. I’m using 2 horizontal Martys tuned to 15 hz with hst18’s in them. These are vital to my setup, as it flattens response at my mlp with proper delay settings. I like the path you are on. I would go ported, it works for me.
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