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If it were a measurement anomaly, I'd expect to see differences across the entire sweep, but all 3 curves appear to track identically above 45Hz. I know OP said he had EQ disengaged, but that low end bump does kinda resemble what happens when Audyssey DynamicEQ is added to the mix. However, you (usually, I'm sure someone allows it) can't engage DynamicEQ until MutliEQ itself is activated.

As a side note. I just did a quick doodle with WinISD comparing the same driver in multiple similar boxes. Increasing the box size and lowering tune slightly rather than playing with Ql to simulate the stuffing effect. A .25ft^3 / 5Hz change predicts increasing output advantage to the larger/lower box below Fb of the smaller/higher box. At Fb-larger, the advantage is nearly 3dB and continues to increase with decreasing frequency. However, there's no free lunch...above Fb-smaller, the larger box has reduced output until both alignments converge around 100Hz. Results could vary with driver and box size/tuning. For this thought experiment, I'm using an old Adire Audio Shiva in 3/25 and 3.25/20 with 200 watts applied. I think the OP's Klipsch is on the order of 2ft^2 give or take and probably has a much higher tune along with at least some modest signal shaping built into the amp.

Chris, the concern of ported boxes with fiberglass becoming particle cannons has also been looked at. I've not seen it personally and I think it was generally dismissed as a non-issue. FWIW, SVS, at least when they originally launched their cylinders in late '90s/early 2000s, used fiberglass batts. Tom may have even participated in some of those "health" discussions. Now, if you tease the batts enough to have loose particles, maybe...

As another point of trivia. Reverse engineering of the SVS cylinders and running the numbers for the enclosure volume and port size strongly suggests that the stuffing was used to help achieve the final Fb. IOW, the 20-39, at least in some iterations, had a predicted Fb of say 22-23Hz, but as built, would measure with the spec'd 20Hz. Tom may have even confirmed this at some point, but I could be dreaming that part.

I would expect your PR box to follow the rules of physics as we currently know them, as long as there's a suitably clear airway between the driver and PR.

-Brent
 

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Interesting results. FWIW, I did a similar experiment a few years ago with my Mini Marty's. I built two of them and lined them with denim insulation. I later built two more and didn't insulate them. I ran apples to apples sweeps comparing lined ported subs to unlined ported subs. Singles and pairs, exact same locations and distances. The result was zero difference by ear or by REW. Not a fraction of a decibel. I'm not doubting your objective findings at all, just relaying my own data.

I'm copying and pasting a post I made in a similar discussion here a few years back. I respect the guys that advocate lining ported subs, I just cant reproduce any data that shows it does anything:

"This topic has always confounded me. Go on Facebook and check out pages of respected builders like JTR and PSA and you’ll see them lining their vented subs. I changed an amp on one of my Hsu subs and it was lined. And guys on here like Bill Fitzmaurice says do line them. On the other hand, other guys who definitely know a thing or two about subs say don’t bother. Check out the GSG site. Those guys are pros and they say not to line them. On my four mini marty’s I lined two and didn’t line the other two. By ear and by REW I can’t detect a lick of difference."

And, for completeness sake, a few other members here have also done similar experiments:

Full Marty Wall Lining Comparison | AVS Forum

I did some testing of lining the walls of my sub with foam | AVS Forum
 
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It does look suspiciously exceptional.

Not that I would make accusations, but if I were to call it reliable, like anything in science, I would want to see verification.

If it is measured in-room, things in the room can change response. One apartment I lived in benefitted greatly from a Sony Trinitron box filled with laundry, which served as a bass trap. I've also had REW and the UMIK behave weirdly if I unplugged the UMIK while REW was running, then plugged it back in.

I don't know what's happening, but it would be interesting to do measurements outdoors in a flat area, verifying identical physical locations and distances, etc...
 

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this is a new one- box size relative to driver Vd, seem to matter
I haven't thought of that relationship, but it makes sense.

I think the OP's Klipsch is on the order of 2ft^2 give or take and probably has a much higher tune along with at least some modest signal shaping built into the amp.
That's the key here, imo--small commercial subs that are higher tuned will realize a larger effect than a Marty sized enclosure.
 

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lawdog, it gets touched on in the second thread you linked, but a lot of people seem to be comparing apples and oranges when they talk about lining/stuffing subs. All of the research Chris and I have been referencing uses fibrous materials like fiberglass insulation, wool, Dacron, polyfill, etc. OTOH, a lot of the talk I see, and not just in those threads, is about using foam sheets for lining. Even all foams are not created equal. As mentioned in that thread, I would expect most foams to simply act as a solid object and reduce the net enclosure volume to some degree since I assume most don't have the wiggly fibers credited with the stuffing volume effect.

Where I see foam usage as common and beneficial is in lining full range speakers. Just like when used to treat room acoustics, the "solid" nature of the foam allows it to absorb/dampen sound waves and reduce their interaction with the backside of the drivers (in the case of lining speakers).

I would also make a distinction between "stuffing" and "lining". Nousaine was careful to document the pounds per cubic foot used in his experiment and noted he reached a point of diminishing returns where the stuffing started to reduce the apparent size of the enclosure. To me, "lining" an enclosure doesn't suggest enough material being used to really make a difference...see comments about full range speakers above. Of course, if you "line" with batts of something like R-30, that could be a different story, depending on mass to volume ratio.

I know you mentioned using denim insulation and I would expect it to behave similar to other fibrous materials. I'm seeing the internal volume of a Mini-Marty quoted as 7.5ft^3. Definitely on the large side by most standards. So, given that various experiments tell us that larger enclosures already show less effect from stuffing, it's possible between that and your use of the word "lining", there just wasn't enough material to make a noticeable difference in your measurements. It could also be that denim insulation simply doesn't behave like fiberglass/Dacron/wool, at least at subwoofer frequencies.

One other point that I think bears addressing in general is I don't think anyone is saying one "needs" to stuff a subwoofer. If the net volume of an enclosure is already sufficient to meet the desired performance goals, there's no advantage. However, as is the case with inexpensive subwoofers such as the OP's Klipsch, sometimes a sub cabinet has to be small to satisfy shipping/packaging/stocking/aesthetic/materials cost. Hoffman tells us low/loud/large...pick 2. In such cases, using stuffing can allows us to get back some performance lost due to compromises in cabinet volume.

-Brent
 

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Even all foams are not created equal. As mentioned in that thread, I would expect most foams to simply act as a solid object and reduce the net enclosure volume to some degree since I assume most don't have the wiggly fibers credited with the stuffing volume effect.
the "solid" nature of the foam allows it to absorb/dampen sound waves and reduce their interaction with the backside of the drivers (in the case of lining speakers).
Stuffing has two functions and they both work because it's not solid.

1) Increase effective volume.

By absorbing the heat of compression and reducing the temperature/pressure rise which fights the driver.

In engineeringese this is called adiabatic-isothermal conversion; this is never fully achieved in our case because there's not enough time per cycle.

Adiabatic means no energy transfer (from the compressed/heated air to something else); isothermal means the transfer of heat from the air to something else prevents the air's temperature from rising; isothermal means same temperature.

Fibers and the cellular nature of foam allow greater surface area of material in contact with the air, improving heat transfer.

Aluminum wool would be an excellent stuffing material because it has a high specific heat (a given heat input causes relatively lower temp rise) and high thermal conductivity, so the heat absorbed will quickly transfer into the depth of the material; the thinner the fiber, the less this matters.

Closed-cell foam should work fine; even though the air can't move between cells, it's in contact with the material.

Since the air can't try to move through the fibers, I'd think Ql won't be lowered as much, which reduces efficiency loss.

2) Absorb acoustic energy.

The spaces between the fibers/cells allows the sound to transmit into the material and transfer its vibrational energy to the fibers, which dissipate it by friction with each other, or in the case of foam, by its own internal material damping.

I'd think closed-cell wouldn't be as good as open-cell because the sound waves hit a continuous surface and will reflect some.
 

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Interesting.

49.8 > 45.8 Hz equates to an 18% effective volume increase.

But the lower Q will sap some output.

Did you get data for SPL vs. freq vs. stuffing qty?
 

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I did not because the whole intent with the JBL's is to use SO MANY of them that SPL is meaningless as you just turn the knob up a notch, heavy eq is mandatory with them. Plus DATS can be a little finicky so I wanted to not move the sub box at all during the removal/installation of poly. I have gotten rid of most of the JBL's, but as an example I still have 4 of them as my "computer subwoofer" under the desk, they act as a foot rest. 4 twelves is not a typical computer sub for the most part it meets my needs.

I did notice the JBL's were less happy at high volume levels with the heaviest stuffing, seemed like regardless of how it measured, that we had crossed the line and the fiber was taking up some volume and I could hear distortion at higher SPL, it reminded me of a speaker in too small of a box, maybe we just ran out of motor but it is what it is. This is why my generic recommendation is 1-1.25lbs per cuft, which is a lot more stuffing than most people think of using.
 

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not saying it won’t work…. No offense intended…

there are just way cheaper and easier ways to deal with treating an enclosure…. Structural resonances are easy to deal with via braces when building (since this is the DIY section…)

sound resonances are easily dealt with by using $2 wal mart pillows, or more expensive ecocore….

I don’t know how much open cell foam you need for any given frequency of treatment…. None of the testing tables I use measure it, only more traditional box stuffing materials and acoustic panels materials
No offense taken. Sometimes I go a little overboard with explaining what I've done. 😉
 
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