Enclosure Stuffing

[gedlee]

Could you please give me your opinion on this:
Isolant laine de coton - Isoa

Is it similar to the shreadded coton stuff you were talking about in your previous post?

(sorry, the linked page is in french...)

My opinion is that "Yes, it is in French!" Beyond that I can't tell much. Cotton is a very good fibre for acoustic damping in general.

"I thought stuffing was for mechanically damping the driver " Mechanical damping is a very small effect since virtually all LF drivers will have a dominate electrical damping that swamps out the mechanical. Hence the "main" effect of this damping in a small closed box will be the apparent volume increase and the damping of the standing waves. The mechanical effect on the driver at resonance will generally be negligable.

 

[planet10]

Could you please give me your opinion on this:
Isolant laine de coton - Isoa

Is it similar to the shreadded coton stuff you were talking about in your previous post?

That is very similar to the link to the blue stuff i posted. Either they bleach the cotton or don't use blue jeans (likely the former given how white it is).

This stuff has excellent properties.

dave

 

[badman]

I've worked with a small amount of it thus far, and it is the best absorber I've ever used. Like a black hole.

 

[gedlee]

I've worked with a small amount of it thus far, and it is the best absorber I've ever used. Like a black hole.

Thats why the automotive guys have been using it for years, even decades. They didn't bleach it and it was all kinds of colors.

 

[wintermute]

Generally noise barriers do not have the same properties that one would want in a box filling material. They would tend to be closed cell, which would decrease the box virtual volume not increase it. One wants an open structure with many small fibers, like FG, but foam does work too. FG is hard to beat for the money, its just such a mess to work with.

Hi Ged, yes that is why At this stage I need to do more experimenting with how much I can add, as I have not determined what effect it has on box volume. I can't remember if I allowed for it in my original calculations of volume or not.... It is an open cell foam I believe but I will do some impedance measurements after adding more and see what the effect is. As I'm only going to run the speakers down to about 200Hz I'm not worried about the bass extention that much

It was never the intention to increase the box volume, only to damp the reflections (which I think is what the OP was touting stuffing as the answer to..

Tony.

 

[tresch]

Similar stuff, different application: Natural Cotton Fiber Insulation, Natural Cellulose Insulation

dave

Where in the US would one order this stuff? Is it very cheap? How does cost compare to standard fiberglass insulation? If it absorbs sound as well or better than fiberglass, this would be a great material for some DIY sound absorption panels for room treatment

 

[planet10]

They list sources on their website. We got ours from the Canadian distributor in ABbotsford BC. IIRC something like $75 for a "bale" of 3.5". They also have a denser ~1/2" thick version that is more in line for our uses... it is about a half-dollar per ft.sq. (minimum 400 ft.sq). I got a smaller quantity off Bob at CSS (or higher cost). He supplies as little as 4 ft sq for a reasonable mark-up.

http://www.diyaudio.com/forums/css/150054-new-product-acoustic-insulation.html

dave

 

[rcw]

The test cell that I outlined is intended to test the sound absorption of a material.
There are many materials that do not transmit sound well, that's because the sound reflects off them, sound absorbent materials do transmit sound, but they absorb some of it as they do, and this is the property we need for box stuffing.

The type of foam Rod mentioned in the article, (used by a speaker designer aquaintance of his), does not transmit sound well because it reflects a lot of it. Thinking that this is what is needed for box lining the speaker "designer" in question assumed this was what was needed, and was absolutely convinced of it despite the speaker system in question having very obvious resonances coming through the cone.
rcw.

 

[CLS]

I'm surprised it seemed no one mentioned about the midrange.

In a commonly seen 2-way, midbass driver covers a very broad range, maybe as broad as 40-3kHz. And since the cone is largely sound-transparent (or at least in some frenquency range), whatever trapped in the box would probably escape through the cone (if in a sealed box).

In my own (limited) experiences, the mid to midbass range (say, centered around 200Hz) is the most troublsome. The absorbability drops, the cone can't effectively separate it, and the strong standing wave frenqucies are nearby.... How can this be dealt with stuffing? I'd prefer box with non-parallel walls for this reason, if OB is not an option, but this is beyond this topic.

From here, I'm also curious about the midrange performance of those 'pipe-style' speakers, i.e., TL, MLTL, MLQWT, BIB... etc. Classic TL for example, all frequencies above tuning point should be 'transmitted' down the pipe and the mid to mid-high should be absorbed along the way.... But in reality, the back of the driver is probably facing a very near rear wall. Doesn't the reflection penetrate the cone right away? How effective the stuffing is working here?

 

[gedlee]

Doesn't the reflection penetrate the cone right away?

Cones are not as acoustically transparent as you might expect. I've tried to measure the effect and was unable to do so, basically because it was low enough that it got lost in other secondary effects. A dominate effect it is not.

 

[wintermute]

I'll attach here a picture of the test results of a number of tests I did on various damping materials. This was originally posted in this thread

Note that the measurements are nearfield, but regardless you can see a dramatic difference between the result with no damping to the box with just a single sheet of the material I linked to earlier in the thread.

Here is the explanation from the other thread as to what the measurements are:

attached are my initial tests on bare, eggcrate and "soundsorber barrier" the soundsorber is the dark blue trace the bare cabinet is the light blue, and the eggcrate foam is the yellow trace. In this test the packing foam was slightly worse than the egg-crate foam, though it is a different cabinet to the one when I tested before.... as you can see the soundsorber is much more effective than the egg crate foam!

Note that soundsorber barrier is the previous name of the material I linked to. Also this was only with one piece of damping material on the rear wall of the cabinet. all other walls were bare.

I also did tests on the acoustic tiles mentioned by RCW and from memory they were the worst of all of the things I tried. I'll see if I have any results for plain old polyester fill.

Tony.

 

[badman]

Cones are not as acoustically transparent as you might expect. I've tried to measure the effect and was unable to do so, basically because it was low enough that it got lost in other secondary effects. A dominate effect it is not.

And yet, people seem highly sensitised to it, which seems to be the case with many low-level artifacts.

 

[Key]

Has anyone messed around with metal felt or some kind of metal stuffing?

 

[CLS]

I think the 'transparency' of cones should vary a lot by their sizes, shapes, thickness's, materials... etc. Thick heavy cone of a sub woofer and a thin light cone for a high efficiency wideranger should be very different in this regard. Of course this is only a 'feel', objective tests would be nice.

Hi, Tony, what do those different colors stand for?

----------------

For those widerangers usually used in pipe-type cabinets, their cones tend to be very light and thin. I once tried covering my ears with such drivers, and sounds seemed easily penetrating through....

 

[Borat]

Borat, perhaps first learn about the properties of the various stuffing materials. Here is a good resource.
http://www.bobgolds.com/AbsorptionCoefficients.htm

I seen those tables two forums ago. What is your point ?

As for massive stuffing I wonder what do you mean by limits ?

Trust me if you plug the whole cabinet with a slab of auralex you will have NO resonances.

 

[Borat]

Borat,

I may be one of the fringe "TL people". I believe a bigger case is a fair trade-off for fidelity...

I say that "proper" cabinet geometry can greatly decrease the need for stuffing.

I have run hours of sim's using MLK's worksheets with the intent of minimizing the need for stuffing. My weapon of choice is the MLQW alignment, which looks to the world to be a bass reflex...

The stuffing is ~1/4 pound of polyfil. The lining is 5/8" spun fiberglass over 1/4" ceramic tile backer board (the idea provided by Bob Brines)

The design is successful through choosing the combination of dimensions and positions for the case elements to minimize resonances. It was chosen this year as "Best in Show" @ diyAtlanta.

For a subwoofer the same approach is successful.

This reminds me of how I was watching a YouTube video about new BMW hybrid and somebody made the comment "BMW is a good car why gay it up with a hybrid" and another person replied "because they can"

the fact that u can make a TL work is great. But it would work even better if it was just a stuffed box only instead of 1/4 pound use 4 pounds per cubic foot.

 

[Borat]

I did a substantial study of box stuffing some 20+ years ago that was an AES publication. AT very LF the stuffing does little to absorb the sound waves, but does add an apparent volume increase that is a very good thing in a closed box. The material matters a lot and "stuffing" has an optimum. We found that "lightly filled" was best and that once you got to the point where you had to start compressing the material the effectiveness begain to decline. We found the best material to be shreaded cotton clothing (used in automotive for noise control). I now find that blocks of open cell polyurethane works very well, but that stuf is very expensive. All round fiberglas was a good choice from a cost and availability point of view, but I hate working with that stuff. As is well known, I am a big fan of simple closed boxes mostly filled with foam. These are easy to design and make, very repeatable, no noise leaks (like ported), pure monopoles, a lot of advantages. LF extension is of course NOT one of them, but I always suggest subs.

Stuufing a ported enclosure does esult in a lot loss at the tuning frequency kind of defeating the point of using a port.

Great post. Yes the cost of foam is hideous. The best deal I seen is 60 bucks for a 2 cubic foot slab of auralex. I placed some on the sides of my box and stuffed the rest.

What percentage of box volume would you dedicate to foam in a cost no object sealed speaker?

 

[Borat]

Interesting thread.

I am building a sealed box with an 8" woofer to play up to 820Hz (24dB LR). I have behind the woofer thick acoustic foam. The rest of the box I plan to fill entirely with BAF and a bit with lambswool which I already have.

Is this a good idea? It seems it in theory, to reduce internal reflections and standing waves, but will I "suffocate" the sound? StigEric just mentioned that low bass will disturb the material and possibly be detrimental to the sound; my stuffing will be packed between shelf braces so shouldn't move much, should I be concerned? Perhaps it's more relevant with ported designs.

Put the foam on the wall opposite the woofer not directly behind it. Stuff the rest. My philosophy is 3 zones - no stuffing closest to driver, stuffing throughout most of the box and foam lining the walls ( thick foam )

 

[Borat]

Dr. EM. The most important function of closed box stuffing is to make the box volume behave like it's bigger than it really is. That will give more bass. An optimized amount of stuffing can effectively 'increase' the volume 15-20%.
I think your acoustic foam behind the woofer is okay. Just make sure you use some fibrous stuffing as well. The thinner the fiber you use the better.


At one point, US speaker maker Advent used five 1 foot square by 2 inch thick blocks of open cell foam as the stuffing in their New Large Advent model. It worked, but not as well as fiberglass. When I upgrade a NLA xover I usually replace the foam with FG. It lowers the box Fc up to 5 hz.

100% wrong. Thanks for spreading disinformation. Increased volume and lowered fs is a side effect and not the point of stuffing. You should have left the foam in place.

 

[Borat]

I only have annecdotal evidence (I didn't take measurements, actually I probably did but I couldn't lay my hands on them readily) But I found that adding stuffing seemed to make my prototype MTM sound somewhat muffled.. Not sure how to describe it really but I didn't like it. I think maybe it did help to reduce the back wall reflections, but at the cost of making the speakers sound terrible

what I am using is this stuff (32mm) on the back wall and the bottom of the cabinet. I've yet to experiment with adding more than that. It far exceeds anything else I tried for reducing back wall reflections. It doesn't completely eliminate them but it tames them pretty well certainly worked much better than stuffing without the detrimental effect on the sound.

Tony.

Yes I think if stuffing gets too close to driver or port it might produce audible artifacts. That's why to be safe I say leave some space around the driver free and don't use ports altogether.