As I stated, no scientific studies. Just from reading subjective impressions from people over many years.
I don't care how much stuffing it takes, so long as the end result turns out the best sound. However, I would rather not pay ten times as much for a type of stuffing that may not sound better than another at one-tenth the price. But as far as scientific data go, I posted a link earlier in this thread about the absorption coefficients for various materials, and cotton seamed to do as well or better than other materials over a wider frequency range. Being not too significantly more in cost than fiberglass, but significantly less than wool, it seems a good value.
There is no best material for all applications.
1. Large closed sub bass box. Aim Increase the accoustuc volume of the box, perhaps add som resistive losses to lower Q. Higher frequency effects irrelevant.
2. Closed midrange box or behind tweeter dome. High frequency absoption and reflection is the only rellevant thing.
3. Corner "reflectors" in a quarter wave pipe, should absorb short wave and reflect the long fundamental wave.
4. damping material bound to the box walls that serve both to absorb some internal reflection as well as damp vibrations in the walls.
Number 1 is easy to measure, just have driver in a box well below Vas then add various stuffings (why do I start to think about turkeys?) and measure apparent Fr and Qm.
I might do just that....
1. Large closed sub bass box. Aim Increase the accoustuc volume of the box, perhaps add som resistive losses to lower Q. Higher frequency effects irrelevant.
2. Closed midrange box or behind tweeter dome. High frequency absoption and reflection is the only rellevant thing.
3. Corner "reflectors" in a quarter wave pipe, should absorb short wave and reflect the long fundamental wave.
4. damping material bound to the box walls that serve both to absorb some internal reflection as well as damp vibrations in the walls.
Number 1 is easy to measure, just have driver in a box well below Vas then add various stuffings (why do I start to think about turkeys?) and measure apparent Fr and Qm.
I might do just that....
As with many stuffing threads on the net, this one is no different. It's become a dual discussion about two different response characteristics.
The OP wanted to find which material affected Fs and Q. These are arguably the most critical closed box performance parameters. Just pay a visit to classicspeakerpages.net discussion forum, the web's largest home of vintage acoustic suspension speakers and enter the search word stuffing. No mention has been made in any of his posts regarding absorbtion coeficients or the material's damping performance. These parameters are certainly affected by stuffing type and amount, but again, their importance is of secondary concern at best.
The OP wanted to find which material affected Fs and Q. These are arguably the most critical closed box performance parameters. Just pay a visit to classicspeakerpages.net discussion forum, the web's largest home of vintage acoustic suspension speakers and enter the search word stuffing. No mention has been made in any of his posts regarding absorbtion coeficients or the material's damping performance. These parameters are certainly affected by stuffing type and amount, but again, their importance is of secondary concern at best.
Please do. Further validation of prior studies is always welcome. Just be sure to take careful measurements. The WT2 instrument the OP used is a great tool. I know, I use the same one.
DrBoar: I disagree with your point number 2. I have an alpair6 i would like to use as mid tweeter in a sealed box.
In this case, with the high Qts markI driver it would be useful to have a reduction in box volume or Q, in combination with good rear wave absorption.
For that both wool and foam look good.
I would line the walls with wool felt, and stuff with a mix of cubed foam and longhair wool. Maybe a bit of cotton. Whichever is cheapest.
In this case, with the high Qts markI driver it would be useful to have a reduction in box volume or Q, in combination with good rear wave absorption.
For that both wool and foam look good.
I would line the walls with wool felt, and stuff with a mix of cubed foam and longhair wool. Maybe a bit of cotton. Whichever is cheapest.
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id imagine rockwool would be similar to FG and Earth wool or whatever it was. Isnt it just mineral wool, mineral glass or something. I think the lateral bonding stratified structure helps make it lossy IMO. its a quality that i reckon BAF fluff was trying to emulate, but ends up to fluffy to work anything like as well. I think the same sort of bonds make felted fibres good for denser materials to line with.
Just a crackpot theory of mine though, perhaps.
Just a crackpot theory of mine though, perhaps.
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that'd have been mr crabbe? - driver/passenger weight distribution in one of those can have quite an impact on cornering.
Rockwool does work very well. It's a very cheap waste product with lots of glass shot. It's density is higher than R-13 OCFG simply due to it's structure. Acoustic Research used it for years as stuffing material in the early days (late 50's early 60's) due to scarcity of regular FG at the time. When I restore one of those old timers, I remove the rockwool and replace it with OCFG.The sulfur content of rockwool has been blamed for corroding the rheostats in the vintage AR's rendering the mids and tweeters silent.
Some early Dynaco A-25's also had rockwool pads as stuffing.
tvrgeek,
Just look in an old house for some insulation, was pretty common on the East Coast in the past. But seriously, if the outcome of this material was acid erosion why would we even consider this material no matter its properties? Is there a new manufacturing method that does away with the sulfur content mentioned earlier?
Just look in an old house for some insulation, was pretty common on the East Coast in the past. But seriously, if the outcome of this material was acid erosion why would we even consider this material no matter its properties? Is there a new manufacturing method that does away with the sulfur content mentioned earlier?
My understanding is that rockwool and fiberglass are more or less interchangable.
Lowering resonance as in increasing apparent volume is a usefull benefit of stuffing, but dealing with internal resonance is, in my opinion, the more critical criterion. The most revealing test is to drill a hole in a cabinet corner and insert a small microphone. With no stuffing you will see all the standing waves. You can add a thin lining or whatever thickness of your material of choice and see how well damped the resonances become.
In my experience the FG and rock wool materials are best in this test and the polyfil and BAF (pillow) materials the poorest. I'm not sure what end of the spectrum that wool and cotton come in at.
To expand on a previous comment we know that stuffing will increase compliance or make the apparent volume look larger. It will also lower Qm or damp resonance, as you will see in the impedance curve. It has also been shown to add some effective mass to the driver, so the total effect is complex.
For lowering resonance there is an optimum stuffing density. Stuff more material in than that and resonance starts to rise again.
If only compliance is changed then the fs/Qt ratio will be constant. In that case Q is only changing because the box looks bigger. If Q is lower than the resonance change implies, then mechanical damping is occuring.
Finally, if you really want to lower resonance then look into the KEF activated charcoal approach. I measured one of their systems using it and it really worked, although it is hard to cram enough stuff into the cabinet to make a big net effect.
David S
Lowering resonance as in increasing apparent volume is a usefull benefit of stuffing, but dealing with internal resonance is, in my opinion, the more critical criterion. The most revealing test is to drill a hole in a cabinet corner and insert a small microphone. With no stuffing you will see all the standing waves. You can add a thin lining or whatever thickness of your material of choice and see how well damped the resonances become.
In my experience the FG and rock wool materials are best in this test and the polyfil and BAF (pillow) materials the poorest. I'm not sure what end of the spectrum that wool and cotton come in at.
To expand on a previous comment we know that stuffing will increase compliance or make the apparent volume look larger. It will also lower Qm or damp resonance, as you will see in the impedance curve. It has also been shown to add some effective mass to the driver, so the total effect is complex.
For lowering resonance there is an optimum stuffing density. Stuff more material in than that and resonance starts to rise again.
If only compliance is changed then the fs/Qt ratio will be constant. In that case Q is only changing because the box looks bigger. If Q is lower than the resonance change implies, then mechanical damping is occuring.
Finally, if you really want to lower resonance then look into the KEF activated charcoal approach. I measured one of their systems using it and it really worked, although it is hard to cram enough stuff into the cabinet to make a big net effect.
David S
Ha-ha! Yes, of course. I didn't mean you, just generally in the thread. Thanks for the nice test. Real DIY.
This test was not to see which did a better job damping reflections. Never said it was, and I would agree, that is probably more important as we all would be selecting a driver/box that is at least very close, right?
I was only testing for the effect of different stuffing on Fs and Qts. Nothing else. Expensive boutique materials that claim to be the cat's meow are on the market, so what is the truth? Actually, it is good to see they work fine. That is not always true with anything audiophile.
To be practical, about the only shift in Fs is about enough to compensate for the reduction in volume from the driver itself. There is no magic other than a correct size box. Maybe, like on my desktop speakers where a Hz or two could help, it is worth pushing the limit. ( 3 inch Fountek, no sub).
Now, here is what is important. The really full stuffing did not HURT the box performance, so that will be very good news for reflection suppression no matter which material you use. Don't worry too much about over stuffing.
So high priced acoustastuff, high price wool, high price cotton, cheap foam, or dirt cheap FG? They work about the same. So moving to what has been mentioned, the effectiveness of reflection suppression is a good next test. What I found in other tests, the cheap pillow poly does a poor job of that too.
Yes, only the wool was able to go over the hump so to speak. As I said, the ABSOLUTE weights may be off because I was using a POS scale. The RELATIVE weights will be valid. I have the stuffing in bags so I can see if I can find a better scale.
I also won't argue at all with "the best stuffing is the one that sounds the best". Would different fibers react diffidently if the box had a Q of about .75 to start with? I don't see a reason so, but if you believe it would, please do the test.
Tvrgeek,
Thank you for all your hard work. My interest is in building a desktop speaker that is on the larger size but much smaller than a standard monitor enclosure. The suggestion of the gas filled enclosure is very interesting in this regard. Being able to create the conditions of a much larger enclosure would allow a much lower fs than a small box can allow. It is not a problem to go from a vented enclosure to a sealed box, the tuning would be much easier with the gas condition than having to do it all with electronic equalization. Thank you again for the great thread this was.
Steven
Thank you for all your hard work. My interest is in building a desktop speaker that is on the larger size but much smaller than a standard monitor enclosure. The suggestion of the gas filled enclosure is very interesting in this regard. Being able to create the conditions of a much larger enclosure would allow a much lower fs than a small box can allow. It is not a problem to go from a vented enclosure to a sealed box, the tuning would be much easier with the gas condition than having to do it all with electronic equalization. Thank you again for the great thread this was.
Steven
How about just Argon? Really, the problem with really small desktops is a small driver just can't push much air. A little electronic eq. can give far more extension to a too small box, but you are still limited to the cone size and displacement. I would not be in line to play with exotic gas when an op-amp will do fine. If you want a smaller box, buy a driver with a bigger motor or do an isobaric. You just are not going to cheat physics. ( remember, an LT is for a too big box to get usable output, not force a small box, A small box needs a peaking filter and a notch filter.
A lot of good discussion. It seems that damping reflections is where to go next. As I am still waiting on my coils form PE, I may have a little time. Or I might just stick with wool. ( My forearms itch)
PS: Treat wool for moths? Only if you intend on trapping moths inside a sealed box. I'll give that if you are talking about a ported box.
A lot of good discussion. It seems that damping reflections is where to go next. As I am still waiting on my coils form PE, I may have a little time. Or I might just stick with wool. ( My forearms itch)
PS: Treat wool for moths? Only if you intend on trapping moths inside a sealed box. I'll give that if you are talking about a ported box.
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