The V.B.S.S. DIY subwoofer design thread - Page 58 - AVS Forum | Home Theater Discussions And Reviews
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post #1711 of 1808 Old 12-09-2019, 01:01 PM - Thread Starter
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Quote:
Originally Posted by AngryJeremy View Post
I'm having trouble wrapping my head around this. Wouldn't the output be the same for any given cone area and excursion? One box volume/tune may take more power to get to a certain excursion, but DSP can correct that if its within the power ratings of the sub and amp. Are you saying with the 15Hz tune, there isn't enough power handling to reach the same excursion levels as you can reach with the 31Hz tune?
Since the tuning frequency is changing so is the range of contribution from the port which is the part your are overlooking. At the 31Hz tune a large portion of the output comes from the port so it requires less power and far less excursion to reach any giving output level in that range then the 15hz tune where the port contribution comes in much lower (and thus requires more power and cone excursion for equal output).

Here are graphs of the SPL and excursion for each tune at roughly max power without DSP. You can see that in the 31Hz tune output is maximized and excursion minimized in that range do to the port.
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post #1712 of 1808 Old 12-09-2019, 01:13 PM
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Quote:
Originally Posted by a77cj7 View Post
In a sealed box, output is limited by either input power or driver excursion, dependent on frequency.

A ported box is somewhat more complex, as you both minimize excursion at port tuning as well as increase output.

So comparing a 15hz tune to a 20hz, you will lose output near 20hz that you can’t gain back with dsp.

A rule of thumb for port tuning is you start to get very limited returns by tuning lower than 2/3 of fs. Since the pa460 has a fs of 28hz, lower than 20hz tuning greatly hurts output.

Modeling these situations should give you a nice visual representation of this. The max spl feature demonstrates the maximum possible output for driver xmax and rated power. You can use dsp to shape response under this curve, but you can’t exceed it.

Chris
What DSP setting (15, 20, 31) would you suggested I should use if I have 2 X V.B.S.S in HT with approx. Volume of ~1600 ft3?
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post #1713 of 1808 Old 12-09-2019, 01:25 PM
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Quote:
Originally Posted by a77cj7 View Post
In a sealed box, output is limited by either input power or driver excursion, dependent on frequency.

A ported box is somewhat more complex, as you both minimize excursion at port tuning as well as increase output.

So comparing a 15hz tune to a 20hz, you will lose output near 20hz that you can’t gain back with dsp.

A rule of thumb for port tuning is you start to get very limited returns by tuning lower than 2/3 of fs. Since the pa460 has a fs of 28hz, lower than 20hz tuning greatly hurts output.

Modeling these situations should give you a nice visual representation of this. The max spl feature demonstrates the maximum possible output for driver xmax and rated power. You can use dsp to shape response under this curve, but you can’t exceed it.

Chris
That is a huge help once again, Chris. I've been using Unibox because WinISD won't install on my work laptop. Without the ability to limit output by both power and excursion simultaneously, it's a crap shoot trying to model the hundreds of scenarios and checking to see where excursion limits are hit. I haven't figured out how to create and import an FRD file in Unibox.

I was leaning towards an 18Hz tune based on the curves I've modeled, so your advice to stay above 2/3 Fs confirms that's the right ballpark for what I'm trying to accomplish.
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post #1714 of 1808 Old 12-09-2019, 01:26 PM - Thread Starter
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The DSP setting used should reflect the tuning of the enclosure you build. The idea behind the original cabinet design is that you could easily swap between all three turnings with little work.

That said for HT use I recommend either the 20 or 15hz. 20Hz will give a little more output at the expense of some extension while the 15hz tune is the other way around.
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post #1715 of 1808 Old 12-09-2019, 01:46 PM
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Quote:
Originally Posted by mtg90 View Post
The DSP setting used should reflect the tuning of the enclosure you build. The idea behind the original cabinet design is that you could easily swap between all three turnings with little work.

That said for HT use I recommend either the 20 or 15hz. 20Hz will give a little more output at the expense of some extension while the 15hz tune is the other way around.
So, if I have one nearfield and one in behind AT screen I would be ok with 15Hz setting in that small volume room or I should wait till I get a microphone and do REW measurement and see? Any rough advise to give me an idea.
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post #1716 of 1808 Old 12-09-2019, 01:57 PM - Thread Starter
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Yes 15Hz should be ok. Have you built them yet?
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post #1717 of 1808 Old 12-09-2019, 02:05 PM
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Quote:
Originally Posted by olegg View Post
So, if I have one nearfield and one in behind AT screen I would be ok with 15Hz setting in that small volume room or I should wait till I get a microphone and do REW measurement and see? Any rough advise to give me an idea.

As Matt stated, the dsp is dependent on the tuning the box is built at. They are not interchangeable.
What tune did you build the boxes at?

Chris
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post #1718 of 1808 Old 12-09-2019, 02:07 PM
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Quote:
Originally Posted by AngryJeremy View Post
That is a huge help once again, Chris. I've been using Unibox because WinISD won't install on my work laptop. Without the ability to limit output by both power and excursion simultaneously, it's a crap shoot trying to model the hundreds of scenarios and checking to see where excursion limits are hit. I haven't figured out how to create and import an FRD file in Unibox.



I was leaning towards an 18Hz tune based on the curves I've modeled, so your advice to stay above 2/3 Fs confirms that's the right ballpark for what I'm trying to accomplish.

I’d probably just build the 20hz design for simplicity and so you can run the existing eq file. Output should be very similar to 18hz tune.

I’m sure 18hz will work as well, just have to change the box design a bit and refine the eq on your own.

Chris
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post #1719 of 1808 Old 12-09-2019, 04:47 PM
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Originally Posted by mtg90 View Post
Yes 15Hz should be ok. Have you built them yet?
They are all done, I am going to use them with behringer N3000 & Marantz sr7013 ...
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post #1720 of 1808 Old 12-09-2019, 04:52 PM
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Originally Posted by a77cj7 View Post
As Matt stated, the dsp is dependent on the tuning the box is built at. They are not interchangeable.
What tune did you build the boxes at?

Chris
Oh wow, you got me there. I think I need a bit of information before I answer this question. I just built them according to specifications on the first page. Tuning the box?
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post #1721 of 1808 Old 12-09-2019, 04:55 PM
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Originally Posted by olegg View Post
Oh wow, you got me there. I think I need a bit of information before I answer this question. I just built them according to specifications on the first page. Tuning the box?


Well, the first post has several port options. Did you use precision ports or a slot?
How long did you make them?

Chris
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post #1722 of 1808 Old 12-09-2019, 05:08 PM
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I built them with precision ports. The nearfield one took me approximately a few months to build but only because it's down firing nice looking end table with polished slab on top and SVS SoundPath Subwoofer Isolation feets.
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post #1723 of 1808 Old 12-09-2019, 05:26 PM
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Quote:
Originally Posted by olegg View Post
I built them with precision ports. The nearfield one took me approximately a few months to build but only because it's down firing nice looking end table with polished slab on top and SVS SoundPath Subwoofer Isolation feets.


Did you use the precision ports full length, 17” long?

Chris
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post #1724 of 1808 Old 12-09-2019, 05:31 PM
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Yes, 4 parts, glued together for each of two ports on a single sub. Oh I see what you mean, that is totally escaped me. I think I was too eager to put them together.
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post #1725 of 1808 Old 12-09-2019, 05:33 PM
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Originally Posted by olegg View Post
Yes, 4 parts, glued together for each of two ports on a single sub. Oh I see what you mean, that is totally escaped me. I think I was too eager to put them together.


Using them full length like that results in a 20hz tune. Use the 20hz eq file.

Plugging one of the ports results in a 15hz tune, use the 15hz file.

I’d suggest using the 20hz setup.

Chris
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post #1726 of 1808 Old 12-09-2019, 08:26 PM
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Originally Posted by olegg View Post
I built them with precision ports. The nearfield one took me approximately a few months to build but only because it's down firing nice looking end table with polished slab on top and SVS SoundPath Subwoofer Isolation feets.
You should post up some pics of the completed sub. Sound like it would look pretty sharp.

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post #1727 of 1808 Old 12-10-2019, 12:21 PM
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When modeling the adjustable slot port option, I'm getting something closer to a 14 Hz tune with half of the port plugged. I could easily shift the center brace to one side, creating a smaller plugged port, to get closer to the 15 Hz tune. Has anyone else seen this, and is it a concern?

20 Hz version | 6.0 ft3 | 21.25"x1.5"x29.5" long port | Unibox predicted Fb of 20.4 Hz

15 Hz version | 6.2 ft3 | 10.625"x1.5"x29.5" long port | Unibox predicted Fb of 14.3 Hz


*I'm measuring slot length from the center of the channel instead of the short edge
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post #1728 of 1808 Old 12-10-2019, 02:17 PM - Thread Starter
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Why is the model of that 15Hz version a larger volume then the 20hz version?

Either way the difference in tuning frequency isn't enough to worry about.
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post #1729 of 1808 Old 12-10-2019, 02:58 PM
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Why is the model of that 15Hz version a larger volume then the 20hz version?

Either way the difference in tuning frequency isn't enough to worry about.
The plugged port (plugged at outlet) adds volume to the enclosure. If it were plugged at the port inlet (at the back of the box), then it would be the same volume as the 20 Hz version.
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post #1730 of 1808 Old 12-10-2019, 03:36 PM - Thread Starter
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I'm not 100% positive it behaves entirely the same as adding that volume of air to the box. But it really doesn't make much difference either way.
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post #1731 of 1808 Old 12-17-2019, 07:33 AM
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I want to build this, but I am limited to a 26 x 22 x 25 size (LxW xH) cabinet, is this doable or should I look into another design?
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post #1732 of 1808 Old 12-17-2019, 07:50 AM
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I want to build this, but I am limited to a 26 x 22 x 25 size (LxW xH) cabinet, is this doable or should I look into another design?
Different sized box.... different outcome... you have to model it. You can't change the box size and expect the same results.


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Sub List: 1- Red Five Mini Devastator 18" w/460HO, 1- Red Five Devastator LFE 21", 1- BOSS platform [The Hideaway Theater], 1- 21" "Marty" tuned to 20hz, 1- 18" "Marty" tuned to 17Hz
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I want to build this, but I am limited to a 26 x 22 x 25 size (LxW xH) cabinet, is this doable or should I look into another design?
I come up with more than 6.75 cft internal with .75 stock. With driver, bracing and ports this should work just fine as the initial design has about 6.25 cft internal. The only modification would be to add a piece of wood the same distance behind the ports as they would be with the original dimensions to keep the port tuning. 26" deep is about 5.5" deeper.
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post #1734 of 1808 Old 12-17-2019, 08:39 AM
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Different sized box.... different outcome... you have to model it. You can't change the box size and expect the same results.


Juju
I used speakervboxlite and from the image below, nto sure if all my parameters are correct, this would be a very good for Sub / MBM module
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post #1735 of 1808 Old 12-17-2019, 11:09 AM
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I used speakervboxlite and from the image below, nto sure if all my parameters are correct, this would be a very good for Sub / MBM module
That could work... please post the response with an SPL Db scale on the left to get a better idea of the response.


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post #1736 of 1808 Old 12-18-2019, 11:14 AM
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Will the original DSP file filters still apply to this slim build? I assume so...but just checking.

@LTD02
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--------------------------------------------------
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Quote:
Originally Posted by olegg View Post
I built them with precision ports. The nearfield one took me approximately a few months to build but only because it's down firing nice looking end table with polished slab on top and SVS SoundPath Subwoofer Isolation feets.
How tall are the isolation feet? There is some debate on this topic, but in general the consensus is you'll want at least 3-4" of clearance between the sub and the floor for proper performance. Not sure of the physics involved, I believe there's compression of the air under the box if you don't have enough clearance.

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post #1738 of 1808 Old 12-18-2019, 06:34 PM - Thread Starter
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Originally Posted by DavidinGA View Post
Will the original DSP file filters still apply to this slim build? I assume so...but just checking.

@LTD02
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That box is a bit smaller then the normal. The 20hz DSP files will still work assuming the shallow box is tuned around 20hz but there may be a little more rolloff of the bottom end due to the smaller volume.
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post #1739 of 1808 Old 12-18-2019, 07:16 PM
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Quote:
Originally Posted by mtg90 View Post
That box is a bit smaller then the normal. The 20hz DSP files will still work assuming the shallow box is tuned around 20hz but there may be a little more rolloff of the bottom end due to the smaller volume.
Hmm...early LTD02 said this slim build would actually measure a bit bigger net vs the original design.....????



Post 1682

the cab in the that you linked is not 7 cubic feet internal. it is closer to 6.3 or so, which is about 5.8 c.f. net after bracing and driver.

this one is about 6.4 cubic feet, or 5.9 c.f. net after bracing and driver.

it will have roughly 20hz tuning.



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post #1740 of 1808 Old 12-18-2019, 09:09 PM - Thread Starter
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Sorry I must have fudged a number when adding it up. Will still work fine with the DSP files.
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