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Discussion Starter · #1 · (Edited)
Title pretty much sums up where I'm at and going...

I've ordered my UXL-18 driver and in my garage is a big 24" diameter sonotube that's 6 feet tall....it looks bigger than I thought a 2' tube would look. I think the brain plays tricks on you when you look at it. Also it says that it will withstand 48 hours or rain, good to know if my basement gets flooded!

I'll need help since this is my first build but I've logged a LOT of hours reading and learning about DIY subwoofers and such...

Here's the 'big blue' sitting in my garage....





I'm going to place it here....it's close to back seating (about 3 feet from listening position). I've got a svs pb12 plus in the front, so this is the only location that I could put this in the room. Room is about 3200 ft3 with a pool table/bar area attached....




Ok...so onto the build. I guess my first thought is opinions on the actual size of this thing. I've got option to go all the way to 6' tall but want to get the best out of the UXL-18. The room is for HT use only, so I want get 'low'...14-15hz tuned is what I'm thinking.

I've modelled the following in WinISD.

11 ft3
15 hz tuned
1200 W
port 8" x 48"
is the 1st port resonance too low?
no HPF
all other variables (cone excursion and air velocity seem fine)....

Since I'm flexible with the actual volume, what would be the 'sweet spot' in getting the most out of the driver for HT?

Thoughts or opinions.....anything is good!

David
 

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Looks like a monster of a sub project! The first port resonance looks ot be around 140Hz, which is a little low. If you limit your Xover point to 70-80Hz or below, and it's 48dB/octave, I think you'll be okay...
 

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Awesome to see another UXL subwoofer build. I am thinking of getting a few of these now. So watching these builds is helping me be a patient man.
 

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Unfortunately this isn't going to work. Although your volume and tuning frequency match the driver for what you want, the port is too long for the required height of the tube. The endcap to endcap distance would be 51.5", the driver depth is 9.5" and you need at least 8" of distance between the back of the driver and the port opening. That leaves 34" of room for a 48" long port.
 

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Discussion Starter · #5 · (Edited)
You are correct Mike, I guess I need to work with WinISD and SonoSub at the same time. I think I have a solution though. I have a piece of sonotube that is 7.375" in diameter. If I use that then the port length is only 40.9". My air velocity is 34 m/s peak so I don't know if that will be a problem?

Also, my calculations are coming out a bit different from sonosub.exe so I need some help here. I'd like to add 6" of foam under the top plate and I'd like to add 2 layers of 3/4" wood within the tube for strength.

If someone can confirm my grade 7 math here....(volume=pi*r2*h)

Need to calculate gross volume (after determining net volume in winisd).

Gross Volume would be total volume of (ignoring any wood outside of actual tube)

bottom endcap (1.5" x 24")= 0.3927 ft3
top endcap (1.5" x 24") = 0.3927 ft3 - 0.0371 ft3 (port hole volume in brace) = 0.3556 ft3

port volume (40.91" x 7.5")= 1.0459 ft3
Driver = 0.22 ft3 (estimate)
modelling needed= 11 ft3

total volume without 6" top foam is 13.014 ft3 (sonosub shows gross 12.39?)

I'm assuming 6" top foam would add to volume of (6"x24" less port hole) = (1.57-0.148)= 1.42 ft3

total gross volume of tube would be 14.43 ft3 . Giving me a height of tube of 55.12"

port length inside tube would be 40.91 + 8" gap would be 48". Driver mounted to bottom plate would stick into tube 7.5" (9.5" less 2.25")....so 55.12"-7.5" = 47.87....should work. I could always add another 2" of foam to topplate to increase height so that I get closer to 10" between back of driver and port.

Sorry for the long math work, but just want to make sure the calculations are correct....I'm getting a difference from sonosub (without the extra 6" foam) and I want to ensure that my logic is correct...I'm sure that sonosub has been verified, so should I just take sonosub info and just add the height for the 6" top foam?

thanks...

update: working my #'s against sonosub again shows that we were both correct.... sonosub displays gross volume (less bracing), but does use them in calculating the height to cut sonotube...so we both came out to the same cut height values...whew....
 

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Discussion Starter · #7 · (Edited)
the foam I was referring to was the insulation that others have said is a good idea to put on the top endcap ....they suggest 6" of insulation to help....???

What I was trying to say is that instead of 3/4" inside the tube for the endcaps, that I was going to dbl up and make it 1.5" inside the tube on the endcaps (not putting any bracing other than the endcaps). I've edited the post to reflect endcaps instead of brace to clarify... If the port is 40" is it a good idea to do a brace for it around 25" down or so? Any ideas on the volume calc...is 14.42ft3 a good target? air velocity of 34 m/s is ok with a flared 7.375" port?

thanks
 

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The foam doesn't displace any volume since it is acoustically transparent. If you have 11 cu.ft. and add the foam you still have 11 cu.ft.

The endcaps should be double 3/4" as shown in the pic on sonosub.exe. This volume is accounted for in the program. You still only have 34" of room for a port.
 

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Discussion Starter · #9 ·
Thanks for the info, it's my first time at this........Just want to be 100% certain about this, the following is from another thread:

"I would however line the endcap with about 6 inches of OC703 each in order to minimize resonances."

This will NOT displace any volume in my calculations...correct?
 

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Discussion Starter · #11 · (Edited)
2nd attempt at modelling this: I guess I should have gone with 22" sonotube as this would have made the port length easier to deal with! Anyhow, with my 24" tube this is what I'm thinking....

12 ft3
15 Hz tuned
7.25" port x 35.7"
1200 W
HPF at 12 Hz

in sonosub I have the total thickness of top endcap(e) and bottom endcap(f) at 2.25"
Good to go hopefully! :nerd:
 

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Discussion Starter · #13 ·
Thanks for the help Mike...

I have an idea that may or may not work....I don't know if I'm modelling this correctly. But the idea is to have 4x4"x32" flared ports that will allow me to 'stuff' one and have ability to adjust tuning.

I'm I doing this correctly?

10.5 ft3
19.18 hz
4 ports 4" x 32"

with one port plugged I would have 3 ports at 4" x32" that would give me 16 hz tune. Is that how it works? I would have to change the DSP on amp to adjust for air velocity when lower tuned...

BTW: I purchased inuke 3000dsp....got the wood and just waiting for final design to start 'routing'...

David
 

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Yes and no.

Modeling software defaults to showing excursion use with a continously powered sine wave - that's rarely the reality. Music and movies have more burst like bass notes, not continuous sine waves, so the shorter duration my prevent the excursion use from ever actually getting as high as a model would show. Additionally, the xmech of the driver usually allows for additional travel at the expense of greater nonlinearities, and a driver's suspension tends to tighten up and become more resistant at the extremes. Lastly, in a ported system, when below tuning at high levels, when the port air mass isn't in sync with the driver, it's known that the port is working against the driver - but, it can simultaneously help by acting as a final layer of protection. At high enough levels to potentially hurt the driver, the port will be overwhelmed with air, resulting in compression, acting almost like a damper. So something that we try to avoid in the useful range - port compression - can actually be a bit of a safety net down lower.

You also need to consider how much single digit information there is on a track at a level greater than the highest >20hz peaks on the same track. There is definitely single digit information, but rarely is it the hottest stuff on a track. Typically people will set their volume to a level where the hotter stuff sounds comfortable, making the playback level of the single digit stuff still relativley safe based on the things I listed above.

That said, with a beefy 18" like this, I'd tune just a tad lower.
 

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:huh: How's that work?

Any ported box will unload below the tuning frequency. I realize it's tuned pretty low at 15Hz, but high output content below that can bottom the driver.
Steve explained it well, and there also the fact that many amps start to roll the response off below 15 hz. The Subwoofer Database if full of LLT's with no HPF's, I've yet to hear a complaint that a sub bottomed out. Yes it's possible to bottom a sub even with a 15 hz tuning if you push things to far on specific movies, only then should a HPF be considered. Until that happens there's no sense buying something you don't need.
 

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Thanks for the help Mike...

I have an idea that may or may not work....I don't know if I'm modelling this correctly. But the idea is to have 4x4"x32" flared ports that will allow me to 'stuff' one and have ability to adjust tuning.

I'm I doing this correctly?

10.5 ft3
19.18 hz
4 ports 4" x 32"

with one port plugged I would have 3 ports at 4" x32" that would give me 16 hz tune. Is that how it works? I would have to change the DSP on amp to adjust for air velocity when lower tuned...

BTW: I purchased inuke 3000dsp....got the wood and just waiting for final design to start 'routing'...

David
The ports would be 34" long for the tuning. The air speed is a bit high with 3 ports and 1200 watts. The only thing the DSP can do is apply a HPF at 20 hz.
 

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Steve explained it well, and there also the fact that many amps start to roll the response off below 15 hz.
That may be true for some amps, but it can be dangerous to count on that unless you measure your gear and know what it actually does. For example, when I was testing my QSC PLX2402 with my Audio-gd USB DAC at my office looking at the voltage output of the amplifier I found the -3dB point of the amp / DAC combo to be at ~2.4Hz. Counting on that to act as a viable HPF (if your design requires it) is going to leave you vulnerable to high output low frequency content.

Yes it's possible to bottom a sub even with a 15 hz tuning if you push things to far on specific movies, only then should a HPF be considered. Until that happens there's no sense buying something you don't need.
That's true, however, I guess I fall into the better safe than sorry camp.
 

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Hello Steve,

Very revealing and reassuring to read this!
I'm trying to build my first 'real' sub and I'm thinking of keeping the port velocity under 14m/s
Would the compression at high levels and low frequency hold truth, even in such a large port?


Yes and no.

Modeling software defaults to showing excursion use with a continously powered sine wave - that's rarely the reality. Music and movies have more burst like bass notes, not continuous sine waves, so the shorter duration my prevent the excursion use from ever actually getting as high as a model would show. Additionally, the xmech of the driver usually allows for additional travel at the expense of greater nonlinearities, and a driver's suspension tends to tighten up and become more resistant at the extremes. Lastly, in a ported system, when below tuning at high levels, when the port air mass isn't in sync with the driver, it's known that the port is working against the driver - but, it can simultaneously help by acting as a final layer of protection. At high enough levels to potentially hurt the driver, the port will be overwhelmed with air, resulting in compression, acting almost like a damper. So something that we try to avoid in the useful range - port compression - can actually be a bit of a safety net down lower.

You also need to consider how much single digit information there is on a track at a level greater than the highest >20hz peaks on the same track. There is definitely single digit information, but rarely is it the hottest stuff on a track. Typically people will set their volume to a level where the hotter stuff sounds comfortable, making the playback level of the single digit stuff still relativley safe based on the things I listed above.

That said, with a beefy 18" like this, I'd tune just a tad lower.
 

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stereodude said:
That may be true for some amps, but it can be dangerous to count on that unless you measure your gear and know what it actually does. For example, when I was testing my QSC PLX2402 with my Audio-gd USB DAC at my office looking at the voltage output of the amplifier I found the -3dB point of the amp / DAC combo to be at ~2.4Hz. Counting on that to act as a viable HPF (if your design requires it) is going to leave you vulnerable to high output low frequency content.
It's kind of funny, because in the early 2000s, when I really started getting into the idea of building a DIY sub, electronics in the chain would have relatively early rolloffs with -3db points at ~10hz. Whether it was because of us subwoofer fanatics or not (probably not), receivers, amplifiers, and even EQ units started staying flatter lower.

Remember when the Velodyne SMS first came out and it started rolling off at 30hz? Big backlash. How about that one device, I forget the name, that acted as a signal boost for people using a Crown K2 amp (or other pro amps with really low input sensitivity) - it started rolling off at 30hz as well. Then there were the early Behringer Feddback Destroyers, which I think Ilkka measured to start rolling off at 15hz. Anyway, things are a lot better now, so yeah, I wouldn't rely too much on electronics rolloff anymore unless your electronics chain is from the early 2000s or sooner.

That said, I listed several other reasons as to why you should be safe. Even with the torture tests Ilkka, Craig, Ricci, and others used to do, I don't recall any driver bottoming or damage, and those tests were way more demanding than actual material.


I'm trying to build my first 'real' sub and I'm thinking of keeping the port velocity under 14m/s
Would the compression at high levels and low frequency hold truth, even in such a large port?
You'd have to use a combination of a modeling program and Collo's "Port It" program to determine that. Regardless though, I wouldn't worry about not having the highpass with a LLT. My first one is over 7 years old and it's never been in harm's way.
 
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