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Old 19th April 2008, 05:17 AM   #1
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Question Sub Enclosure plus stuffing = Q?

Hi y'all:

The subwoofer enclosure for my respective sub requires 1.20 cuft to attain a 0.707 alignment (total including driver displacement and necessary air spring). But, I'm wondering what the effect of polyfill will have on its 'apparent' volume as there is no mention of polyfill stuffing in the manual.

I've read that 1.0lb of polyfill per 1cuft increases the enclosure's 'apparent' volume by 25%. That could cause an obvious problem by lessening the enclosure too much toward a bessel alignment (i.e. the polyfill would increase the 'apparent volume' too much). In addition, I would perhaps therefore be able to construct a 'smaller' enclosure using that same polyfill density and save trunk space!

Any expert advice out there?

Thanks!
Joe
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Old 19th April 2008, 04:38 PM   #2
jol50 is offline jol50  United States
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Default my .01 ramblings

Can't remember but there is bass box software on the net someplace that will account for the polyfill in its response curve, winISD does not. Find that and try it out. I have moved to more real subwoofer in my setups, meaning they only play under 50Hz at most so the stuffing seems to have limited effect unless the box is tuned high and is overrunning the crossover. You can lose db with a lot of stuffing, but I still use it or try it because I try everything time permitting. I ran into that, a small box would be down in db at low Hz and the stuffing made it worse....or it seemed worse, anyway I hate small boxes and hardly use them (under recommended) as I tend to like 30Hz minimum.

IMO it does not 'increase' displacement as much as some say, more like 10%. With small boxes in my use it seems to cut the upper bass more than help to lower bass. Given any cut to db with a small sub setup I tend to try working with crossover before I stuff unless I have adequate output or it is really tuned high. Offhand I have used gross box volume (not subtract the driver volume) instead and stuffed to offset that, it seemed to come out ok for 1cf or better min recommended size sealed boxes. That just by ear. From what I read the fibers slow the waves down in the box, but this takes energy away to do it so not good if you don't have enough boom to start with. The stuffing must be loose. Personally I always make the largest box I can, larger than minimum, because I like deeper bass. Right now I have an IB setup so I don't have to worry about it going low enough. It seems to me you will lose more db going smaller box than you could ever recover with stuffing...me I like to stay more efficient and use less power to run it.
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Old 19th April 2008, 11:53 PM   #3
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IIRC, stuffing the enclosure in higher densities affects the thermal properties. At some point (~1lb/ cu ft for polyfill) the system becomes approximately isothermal as the filling absorbs and releases heat. This results in an apparent increase in enclosure volume because pressure is related to temperature and volume.

In stuffing automotive subwoofers to this density, I've drawn similar conclusions to jol50. Efficiency *seems* less, although that may simply be the effect of lowering the Qtc. I've never done any measurements, only listening tests.
The exception I've found is when the stuffing is used in conjunction with a variovent (flow resistance/aperiodic damping.) Those have worked well for me.

If you're using only a mild to moderate amount of filling, the enclosure volume will actually be reduced by the volume of filling used, and this is all moot.

I typically lined sub boxes with 1" polyfill sheet or similar linings and left it at that. IMHO, if you're not going up above sub-bass frequencies it's just not effective to do much more.
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Old 20th April 2008, 12:44 AM   #4
jol50 is offline jol50  United States
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When I stuff them I usually get a bag and put 2/3-3/4 of the box full leaving some air for the sub motor to breath, put it in there loose. Sometimes a little lining or 1/4 box does help, seems to cut box resonance or something just cleans it up a little. I have a single mtx 10 that I have never stuffed, it sounds pretty good so I never did. That is an mtx box also. Have a sony P5 12 that was too high and kind of a one note wonder, I stuffed that thing full close to the motor. It helped I guess, but still it is not what I like. It hits hard on bass tracks but drops off high and low, could turn the xover up to 200Hz and it sounded the same as 70. I ran a test cd on it and it hits at 45-60 the most and drops either way of that. Those were sealed in recommended box. Also have a 400w alphasonic I test with that is in 1cf+ ported, but inverted so close to recommended ported. I filled that box with stuffing trying to enlarge it and it works quite well testing but never used it in a car. It gets stronger at 50 and more, but still has some down to 30 before it unloads. Strong for a single 10, I'd like to try it in a car. Took a spin tonight and cranked up my 4 12s IB and they lay little single 10/12 boxes I've had to shame.

I would add, to me it is something that does not do much on a sub unless your box is making noise or resonance. I use it as a last resort usually, when I have to use that small a box or it did not perform like I had planned. It only makes a detail change IMO, and is not that much different from what you had so I would be very careful about counting on it. I never plan on it more than driver volume assuming I have enough output or can push enough power into that driver to work for me. I always figure a larger box would be much better. I also had a kicker truck box a while back, it was ported and stuffed so hard the blown sub made soot marks in the stuffing with its vents when it blew. It came to me that way.
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Old 21st April 2008, 02:42 AM   #5
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Thank you all for your thoughts and expertise.

The sub will be a Polk SR124 SVC in a sealed enclosure powered by either a JL 500/1v2, 1000/1v2 (if I can afford it as I like generous no-clip headroom), or their brand-new HD750/1 (just noticed that one on their website tonight).

The Polk SR124 is 4ohm and rated at 700Wrms/1400Wpeak. The total box volume needs to be 1.20 cuft. There is a misprinted whitepaper out there (by Polk believe it or not) that states 1.02 cuft and that is wrong. Yep, 1.20 cuft includes the driver displacement.

I don't know where I read this but someone claimed that 1 lb of poly per 1 cuft will make the box appear 25% larger. So, my main concern was if I build it to be 1.20 cuft (as correct), would my addition of poly screw the alignment up thus creating an apparent box around 1.50 cuft?! And, therefore if so, should I build the box to be around 0.95 cuft then?! I want that Q of 0.707 for certain. Plus, it would be nice to decrease the box's volume while still getting a Q of 0.707 (grin).

I only want to build one box, heh. Thanks again for all your advice.
Joe
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Old 21st April 2008, 03:36 AM   #6
jol50 is offline jol50  United States
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I once knew the details of it but it has been a while so someone else here will hopefully add a comment. I see what you are saying, but in my experience it does not work that way. It depends on a couple things, like how low do you need the sub to go? Second how much db do you need? I'm going to guess that if you don't care about the lowest notes and have ample output you could miss some of, then it may work for you to go a little smaller but think 25% is really pushing it. Going smaller is always a losing game with a sub. What I do is play around with the software, I put in known subs I can compare to not just look at one curve.
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Old 23rd April 2008, 03:02 AM   #7
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Hi Jol50!

Thanks for your questions (i.e. interest in my situation here).

I want the sub to go as low as possible (without being a Bessel alignment though). I realize that I may be limiting that 'super low' ability by selecting a Polk SR124 driver but that's the driver I really want to try first (to match my Polk SR6500's up front). And, the Q of 0.707 should provide a very nice slope. Cabin gain should make-up for the decrease in dB as the frequency gets lower.

Around 117-120 dB would be nice but not completely necessary (that's computed from the efficiency, Wrms, Wpeak, distance and corner loading. First I seek perfect accuracy and flatness, then loudness if it happens (and it usually does with the technology out there today, heh).

You mentioned playing around with the software and I've tried some 'freeware' sealed enclosure apps but all seem to tell me to build a 2.4+ cuft sealed enclosure when Polk recommends a total sealed enclosure volume of only 1.20 cuft. What gives? And, yes, I made it clear to Polk techs that I want a Q of 0.707 too.

Could you let me know of the 'definitive' enclosure designer program that is out there? If it only covered sealed designs, that would be completely fine with me.

Or, I could provide you with all the SR124's specs and you could see if you come up with 2.4+ cuft too? Or, maybe figure out how Polk figured-out only 1.20 cuft?

Thanks!!
Joe
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Old 23rd April 2008, 04:38 AM   #8
jol50 is offline jol50  United States
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If I get time I will check that driver out, I just use winISD most of the time because I am not that picky. When I get it in the car I will adjust as needed to my tastes. If I do stuff for someone else I make it the way they want.

What happens with sealed is the box size makes limited difference when large, as you go smaller you lose the lowest bass more...and then all the sudden it changes a lot. That is where the factory often tells you to build at since going larger makes less difference than going smaller. However the factory knows you want the smallest box...they know their sub will handle more power in a smaller box....so why tell you to go larger? I've never made a box smaller than minimum factory spec (or a hair under and stuffed) and could tolerate very few of those. With EQ you can force it to make more bass, I just don't like to do that much if possible. I'm old school and don't like small boxes for the most part, then again I like lots of 30 maybe 20hz you don't get with those.

The software does not really tell you what it sounds like in the car of course, unless you can test it and adjust, but if you have another sub you can tell how the new sub will be different and that is how I use it. Each car is different, so I toss a spare sub I have in and see what I like, then build what I think I need....one with more 30Hz usually in my case. I've even slapped together temp boxes to test with if I had to, before I built a nice one. Now I keep a couple prebuilt ones stored away; I have a 10 sealed, ported, and BP and a 12 sealed and ported sitting here as I got rid of the dual boxes. I ran 10 and 12s in my car then figured the box I needed would be way too big. So I ended up making an IB baffle instead. It works great but now I want to change that too and make it lighter.

The issue is the car; you can have a sub box that sounds one way and it changes when you put it in another car. You might move it and eq it a little to tune it, but the box is what really changes how it works. I'll try to look at that, that is half the box they are telling you. In my experience I almost always error larger than factory to get the deep bass I like, but again that is me. With a home speaker you can make a flat curve in the software, build it, and it sounds kid of that way but that has never worked for me with a car sub.
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Old 23rd April 2008, 05:33 AM   #9
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I hear ya most definitely on the bigger box is better. And, yep, I do want 'full low bass extension' as much as possible to use the driver to its fullest.

I truly don't want to end-up with a box, as perhaps recommended by the Polk specs, that has a huge peak around 40-50Hz (which I find annoying and would require major attenuation). While I haven't used my FFT spectrum analyzer software (self-written C++ coding) on Kraftwerk's Autobahn or the Fifth Element's soundtrack (faves of mine), both of which have complex super low bass, I'm assuming that an incorrectly designed box would sound just horrible.

I cannot imagine that Polk would recommend a 'too small' box especially when their SR124's are directed at 'audiophile listeners.'

Would purchasing LinearX or another product help me out?
Thanks! Joe
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Old 26th April 2008, 11:08 PM   #10
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jdgonko:
Quote:
You mentioned playing around with the software and I've tried some 'freeware' sealed enclosure apps but all seem to tell me to build a 2.4+ cuft sealed enclosure when Polk recommends a total sealed enclosure volume of only 1.20 cuft. What gives? And, yes, I made it clear to Polk techs that I want a Q of 0.707 too.
For estimating sealed boxes, the formula to calculate enclosure size for desired Qtc is simple. Since youíve written FFT software(!!!), you should be able to manipulate the formula in your sleep. Formulae for the F3 and response curves are more in depth, but a spreadsheet can make quick work of them if you want automation or graphs.

Box volume for a desired Qtc (total closed box Q):
Vb = Vas/((Qtc/Qts)^2-1)

System resonant frequency (Fc) increases proportionally with Qtc:
Fc/Fs = Qtc/Qts

-3dB down point is a function of Qtc and Fc:
F3 (Hz) = Fc*[(1/Qtc^2-2+([1/Qtc^2-2]^2+4)^0.5)/2]^0.5

Using the Parameters from Polkís site:

Vas = 53 liters
Qts = 0.55
Fs = 30Hz

So, to run the estimateÖ
Targeting a Qtc of 0.707:
Vb would be 2.49 Cubic feet
Fc and F3 ~ 40Hz

Reversing the formula to extract the Qtc in Polkís recommended box volume of 1.2 cubic feet:
Qtc = 0.86

While this ainít 0.707, itís not out of line for good sound quality. Especially in a car, where fractions of a db in variation can be inaudible or completely overwhelmed by the environmental acoustics.

Fc = 48Hz, with a rise of only 0.50 db or so in that area.

F3 = 41Hz

Given the fact that in this 50% smaller box the F3 is essentially the same, the ripple due to the higher Qtc is negligible, and the wrench that the carís acoustics are going to throw into the results, Polkís engineers probably estimated this volume as the point of diminishing returns.

Which is pretty much what jol50 said.

Quote:
And, yep, I do want 'full low bass extension' as much as possible to use the driver to its fullest.
As far as minumum F3 goes, by combining these two formulae

Fc/Fs = Qtc/Qts (rearranged as Fc = Qtc*Fs/Qts)
F3 (Hz) = Fc*[(1/Qtc^2-2+([1/Qtc^2-2]^2+4)^0.5)/2]^0.5

and using your Polk's Fs/Qts of 56.6, here is a chart of various Qtcís along with their corresponding Fc and F3. Notice the F3 is within 10% for Qtcís anywhere between 0.55-1.0:

Qtc -- F3/Fc -- F3 -- Vb (cu ft)

0.55 -- 1.359 -- 42.3 -- 25.26
0.6 -- 1.209 -- 41.1 -- 6.90
0.65 -- 1.095 -- 40.3 -- 3.85
0.7 -- 1.010 -- 40.0 -- 2.61
0.707 -- 1.000 -- 40.0 -- 2.49
0.75 -- 0.94 -- 40.2 -- 1.94
0.8 -- 0.897 -- 40.6 -- 1.52
0.85 -- 0.8592 -- 41.3 -- 1.24
0.9 -- 0.8294 -- 42.3 -- 1.03
0.95 -- 0.8055 -- 43.3 -- 0.88
1 -- 0.7861 -- 44.5 -- 0.76
1.05 -- 0.7701 -- 45.8 -- 0.66
1.1 -- 0.7567 -- 47.1 -- 0.59
1.2 -- 0.7358 -- 50.0 -- 0.47

Transient response is important for detailed low frequency performance, and is better in the lower Qtc region. Since you're starting with a Qts of 0.55 and that can't go anywhere but up when enclosed, box volumes for lower Qtc targets will be large in relation to the Vas. Series impedance (speaker cables, connectors, amplifier's output impedance) will all combine to raise the speakers Qts as well.

Also keep in mind that production tolerance from one woofer to another isn't that tight (10% variance ain't uncommon in identical woofers). So if you're truly striving to stick a particular alignment as close as possible, you'll have to measure the actual parameters of your own speaker and tune the box to compensate for losses after it's assembled.

(edit)Sorry for the poorly formatted chart, hopefully you can make it out!
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