What type of fiberglass insulation for stuffing?

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Hey guys,

I'm finishing up the first of two sealed enclosures for 15" subs and the plans call for 32 ounces of stuffing in the box. I've paid for Acousta-Stuf to use in my sealed subs before now but I want to go with cheap fiberglass for these.

So does it matter much which type I grab from Home Depot?

Will this stuff do the job?

Also, the plans call for 32 ounces of "poly fill". So I plan to use 32 ounces of fiberglass. Any issue with that?

The drivers mount down firing in these enclosures so I'm gonna hot glue some grill cloth below the stuffing to keep fibers contained.

Thanks!

-Chris
 
Fiberglass is just nasty stuff. I really wouldn't want to use it when polyester filling is both cheap and easy to find.

That said, it would probably work alright.

Thank you, but from what I've read, and been told by one engineer at a car audio speaker manufacturer, the filling you can get at craft stores or Walmart is not very effective.

Fiberglass insulation is supposed to be very good.

-Chris
 
Fibre glass is ideal in a closed enclosure. Polyester or better still cotton is good otherwise.

From a very dim memory, R19 'pink' is ~1.5x more dense, so if I figured right, use ~32*0.833 = ~26.67 oz, though ideally you normally only want just enough to keep it from being 'boomy'/'loose'/'resonant' sounding, so always just did it by ear.

FWIW, historically, cheesecloth was just loosely draped over the driver and stapled to the baffle:
Altec Heathkit legato 2-15 inch horn & driver

GM

Thanks guys. These are pretty good sized enclosures at 20" X 20" X 20" internal, so 32 ounces each probably isn't as much as it would seem. Probably means I don't have to get carried away with exactly the right number of ounces either, I guess.

I'll just go with what I find locally at a good price. I've already got a good bit of grill cloth from Parts Express.

-Chris
 
Hi Chris,

The premium 100% poly-fill work good for me. I can't post the impedance sweep or frequency response graphs at the moment, but if I remember correctly the Fc lowered from 55hz to 50hz. And the frequency response looked much smoother than without the stuffing. The subwoofer box with three 10s is about 2.6cuft sealed and I used 40 oz of the poly-fill that I picked up from a local hobby store. I have used the fiberglass R19 on several enclosures in the past. After using the poly I probably won't use fiberglass again.

Regards,
Matt
 
Hi Chris,

The premium 100% poly-fill work good for me. I can't post the impedance sweep or frequency response graphs at the moment, but if I remember correctly the Fc lowered from 55hz to 50hz. And the frequency response looked much smoother than without the stuffing. The subwoofer box with three 10s is about 2.6cuft sealed and I used 40 oz of the poly-fill that I picked up from a local hobby store. I have used the fiberglass R19 on several enclosures in the past. After using the poly I probably won't use fiberglass again.

Regards,
Matt

Hey Matt,

I'll probably stick with fiberglass or Acousta-Stuf myself. It's not that the craft store stuff doesn't work at all but I have read a few articles where different materials were tested and fiberglass or Acousta-Stuf, or wool, came out pretty heavily on top.

A couple years ago I was working with an engineer at Image Dynamics to try to get the best out of one of their subs in a sealed box about 20 percent smaller than they recommended. He was very adamant about using real Acousta-Stuf or fiberglass to make the box appear as large as possible to the driver.

But, it really wasn't my intention to debate different materials with this thread or start any kind of arguments. If you're satisfied with the poly-fill, I'm certainly not looking to change your mind.

-Chris
 
fiberglass insulation dust/fragments can damage a driver.
if it's the only thing available find pillow cases with a high thread count (tight weave) to minimize the possibility of particles attacking the surround or worse yet getting sucked into the voice coil gap through a vented pole piece.
 
No argument or debate here, just my two cents. I have also read how fiberglass works better than other types of materials. I had built 6 or 7 sealed enclosures with one roll of the pink stuff so it is very cost effective. At the time the boxes where built I didn't have a good way to test what I had done. I had tried many different stuffing densities to find what sounded the best to me. Now you got me thinking, maybe I will try the pink stuff on a sealed rear chamber Bandpass that I am working on.

Regards,
Matt
 
Fiberglass insulation is supposed to be very good.

No 'supposed to' about it or it wouldn't be the default sound insulation for rooms, buildings, heat/AC ducts, etc.,.

Obviously, some care is required WRT health and why ~sticky cheesecloth is used for driver protection if packing the cab; and when lining a cab, one reason why best not to do it near a vent if not covered with grill cloth or similar.

Regardless, as noted, one can use more polyfil and up to a point get the desired result, so just depends on the app as to whether it's suitable or not: Sub Box Polyester Fiberfill

As you can see, there's a point where anymore just starts reducing the apparent net Vb whereas if you use FG you'll find that much higher densities can be used before it happens; or for lining, much less is required and why 1" acoustic FG [aka OC 703 'duct board' nowadays] on just the top, one side and back is sufficient in most sealed, BR alignments and 2" on really large cabs.

GM
 
Got my stuffing in tonight. I had a sheet of the quilt padding stuff so I hot glued a layer to the inside to help hold the fiberglass back. I still might do the cheesecloth thing right behind the driver though.

Just need to cut the driver hole all the way through and flush route it smooth then I can attach the bottom panel.


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TempestBox2_zpsg9x0hnrt.jpg



TempestBox3_zpsshwulpon.jpg
 
I'll throw in an angle you may not be considering.

The absorber used, actually any absorber used that uses a fibrous tangle, is useful to roughly 1/4 the frequency wavelength. The 1/4 is in relation to the thickest part presented into the area that you want to absorb the sound radiation in. So for your box in the picture I'd ball park it at around 6 inches thick. Even doubling that number because the enclosure edges are so close.

Your desired pass band or operating frequency in a subwoofer stops at around 200 hertz?

What's the wavelength of 200 hertz?

Some simple math will make you sad.

Speed of sound is 343 meters per second. Yes it varies slightly at higher elevations, and temperatures. But the variation from 343 is not to much in a home environment.

Take that speed 343/200(frequency of wavelength you are calculating) and you get:

1.715 meters ( remember the 343 is meters ).

If you want good old feet and inches take the 1.715 and multiply that by 3.208 ( Yes I looked it up. I do it a different way normally! ) 5.501 feet. So pretty much 66 inches.

A quarter wavelength thickness of fibrous material will start to be an effective absorber.

66/4= 16.5 inches. So you may be able to absorb around 200 hertz and higher with the tangle of insulation that you have installed.

It will do nothing more for absorbing sound.

It will to a limited degree do something for changing air ingress and egress. But I have not read any studies on that since the 80's. And I have discussions with trusted colleagues that tel me that the studies that purported to show a change in speed of sound in different fibers were a little off the mark of rigorous testing.

So all that being said.

Does it make any difference in a subwoofer to add stuffing or not?

I have done double blind tests and could not measure a difference when the enclosure was sufficiently stiffly designed and constructed.


A little math tells you that the effective absorber of sound idea that has been purveyed for decades is not applicable for subwoofers.

And testing from yours truly has shown me for low frequency enclosures it makes no difference.

Mid and high frequency enclosures or full range enclosures are a whole other animal.
 
I don't find any of that hard to believe and I've read similar blog/forum posts, articles or whatever before. Your math seems to make sense. But in this very thread we have someone claiming to have measured a fairly significant drop in resonant frequency after stuffing a subwoofer box.

When you say you have done double blind tests and could not measure a difference, what do you mean by that? Seems like a difficult AB test to conduct with just listening since small changes in location can have a significant impact on in-room frequency response at your listening position. And if you're measuring there would be no need for blindness.

I haven't tried any measurements with and without stuffing or even tried listening to my subwoofers before and after. Honestly, I use the stuffing as much because that's just what you do with sealed enclosures as anything else.

What are your thoughts on stuffing material absorbing heat that would otherwise cause the air in the sealed box to expand and change where the driver rests compared to equal pressure inside and out? That seems to make sense to me, assuming enough heat is actually generated to cause significant air expansion.

-Chris
 
I don't find any of that hard to believe and I've read similar blog/forum posts, articles or whatever before. Your math seems to make sense. But in this very thread we have someone claiming to have measured a fairly significant drop in resonant frequency after stuffing a subwoofer box.

When you say you have done double blind tests and could not measure a difference, what do you mean by that? Seems like a difficult AB test to conduct with just listening since small changes in location can have a significant impact on in-room frequency response at your listening position. And if you're measuring there would be no need for blindness.

I haven't tried any measurements with and without stuffing or even tried listening to my subwoofers before and after. Honestly, I use the stuffing as much because that's just what you do with sealed enclosures as anything else.

What are your thoughts on stuffing material absorbing heat that would otherwise cause the air in the sealed box to expand and change where the driver rests compared to equal pressure inside and out? That seems to make sense to me, assuming enough heat is actually generated to cause significant air expansion.

-Chris


I worded my comments carefully.

Sound absorbing is not happening with stuffing an enclosure. Not in the frequency span of interest in a subwoofer at any rate.

Lowering Fs may well be a function of stuffing versus the port. Sometimes literally lengthening the effective port. It depends on how it is stuffed.

I have read papers that also worked on closed boxes, and stuffed transmission lines.

George Augspurger's paper on stuffing effects on transmission lines is one of the best available. But that is not so easy to translate and use in direct comparison with what is normally performed in DIY subwoofer construction.

He relied to a degree on the Robert Robinson paper That is a little bit to large to post. If it is of interest, I can send it. But it is a PHd thesis. So a great read. LOL

Testing.

I work a good deal of my time as a engineering consultant for audio products and transducers.

So a test that would allow some direct comparisons is one that is performed outside on a flat area with at least 30 feet of clear unobstructed space in a circle around the test subject. A sixty foot diameter. Mic placed at 2 meters. And signals played through the cabinets. Pure tones and gated noise signals.

Pure tones are useful but not completely revealing by themselves. In comparison to FFT's of noise signals then when properly compared differences can be displayed.

Now directly to this question:

"What are your thoughts on stuffing material absorbing heat that would otherwise cause the air in the sealed box to expand and change where the driver rests compared to equal pressure inside and out? That seems to make sense to me, assuming enough heat is actually generated to cause significant air expansion."

I post a few files that can give you a better understanding of what really happens.

These are hand picked from my store of enclosure related information. I have much more math intensive articles that make my head hurt.

My calculus days are very long gone I'm afraid. My hat goes off to the folks who are mathematically unchallenged.
 

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I'll throw in an angle you may not be considering.

The absorber used, actually any absorber used that uses a fibrous tangle, is useful to roughly 1/4 the frequency wavelength. The 1/4 is in relation to the thickest part presented into the area that you want to absorb the sound radiation in. So for your box in the picture I'd ball park it at around 6 inches thick. Even doubling that number because the enclosure edges are so close.

Your desired pass band or operating frequency in a subwoofer stops at around 200 hertz?

What's the wavelength of 200 hertz?

Some simple math will make you sad.

Speed of sound is 343 meters per second. Yes it varies slightly at higher elevations, and temperatures. But the variation from 343 is not to much in a home environment.

Take that speed 343/200(frequency of wavelength you are calculating) and you get:

1.715 meters ( remember the 343 is meters ).

If you want good old feet and inches take the 1.715 and multiply that by 3.208 ( Yes I looked it up. I do it a different way normally! ) 5.501 feet. So pretty much 66 inches.

A quarter wavelength thickness of fibrous material will start to be an effective absorber.

66/4= 16.5 inches. So you may be able to absorb around 200 hertz and higher with the tangle of insulation that you have installed.

It will do nothing more for absorbing sound.

It will to a limited degree do something for changing air ingress and egress. But I have not read any studies on that since the 80's. And I have discussions with trusted colleagues that tel me that the studies that purported to show a change in speed of sound in different fibers were a little off the mark of rigorous testing.

So all that being said.

Does it make any difference in a subwoofer to add stuffing or not?

I have done double blind tests and could not measure a difference when the enclosure was sufficiently stiffly designed and constructed.


A little math tells you that the effective absorber of sound idea that has been purveyed for decades is not applicable for subwoofers.

And testing from yours truly has shown me for low frequency enclosures it makes no difference.

Mid and high frequency enclosures or full range enclosures are a whole other animal.

Check out the data-bass article on stuffing, especially the measurements. Data-Bass

In the sealed box example the stuffing does exactly what you would expect - it lowers the impedance peak dramatically, lowers the box q, smooths out the ripple at high frequencies and gives a bit more low bass (a direct result of the frquency shift of lowered q). The entire passband is affected down to the lower limit of the graph, 8 hz.

There are literally thousands of measurements on the internet that will show the same thing.

And in the other enclosure type measurements stuffing is also doing exactly what you would expect too.

Simple wavelength math isn't any good at predicting effects of stuffing in a box. A small amount of stuffing in or near a port can kill off the box's resonant output pretty effectively, but if you move that small amount of stuffing to the opposite side of the box well away from the port it does something very different - it attacks the higher frequencies and mostly leaves the port output alone.

Stuffing is very effective in subwoofers because of the laws of physics.
 
Fiberglass works great, so long as it's not ported as it'll become airborne. Wear gloves.

Sealed boxes, even ones that came from reputable shops that built in-house, always used the pink stuff back in the day.

Other option, which I used before was I bought a few drop ceiling panels. They had a yellow fiberglass, roughly 3/4" thick, then the vinyl sheeting on one side.

Took razor knife, cut them to exact squares to fit in box (fold it to get throw hole) then cram them into the corners. Can use a little bit of spray adhesive if need be. I liked this as it was a cleaner appearance inside and stuff isn't touching the actual basket, and could be applied on underside of hole. Was really cheap. Think 2 panels did both compartments.

Was never able to do any type of comparison between that and standard insulation which is obviously not as dense, but figured I'd throw it out there.
 
Got the first one running late Saturday. Sounds surprisingly good for being crossed over way too high at 100 Hz. If my multi-channel DAC ever ships from Hong Kong I'll get it crossed over around 60 Hz.

I've still got to get JRiver MC, REW and my USB mic playing nice together on this PC so I can take some in room measurements. But I'm already happy enough with the sound that I'm definitely going to build the second one just like it.

It's supposed to be down firing but I couldn't wait to hear it so I've got it on the side. That's why you see wires coming out the "front" in the pic below.

-Chris


SealedTempestAlmostDone_zpso3rcjabz.jpg
 
You can also check out 2' x 4' Corning 700 series ( several densities ) rigid fiberglass that comes in thickness up to 2". Some "high end" mfgs make expensive sound absorbing panels from this material. You can neatly cut this product on a table saw to fit your application. Just make sue you use proper eye, skin and breathing protection. Good luck.
 
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