A technical understanding of Polyfill

[DevilDriver 28]

Have you ever completed an enclosure that was a touch too small? Your low end sounds cramped but you don't want to or can't build a brand new enclosure. You throw in some polyfill and are blown away by the improvements. Then you go online to talk about it and everything gets confusing. You don't know why it works or what it really did, but you know that it sounds better. Maybe it's time you learned more about it!

Firstly, there are a few primary types of fill that are used. This includes polyester fiberfill, fiberglass insulation, and long-fiber wool. Of these three, polyester fiberfill is perhaps the best option, and also the origination of the term "polyfill". These products can easily be found in several stores, including Wal-Mart, Home Depot, or a local crafts and fabrics store, and all are extremely affordable (typically less than $2/pound). It is also very easy to apply to the inside of your enclosure: simply staple or glue it to the inside of your enclosure. How does it work?

Stuffing a box with polyfill makes it seem larger and it all relates to thermodynamics. When polyfill is added to an enclosure, it changes the behaviour of the airspring in the enclosure from "adiabatic" to "isothermal". The term "adiabatic" implies that there is no heat transfer occurring. An isothermal process occurs once the polyfill has been added. As the air passes through the polyfill, the fibers wiggle and cause some of the energy created by the airspring to be dissipated as heat. This heats the surrounding air molecules warmer, causing the air to become less dense. Being that sound passes easier through a denser medium, the speaker interacts with your enclosure as if it is larger than it actually is. The effective increase in enclosure size can be as much as 40%!

This has some very obvious benefits that are inherent of a larger enclosure. Firstly, it becomes more efficient (a larger enclosure is always more efficient than a smaller one for any given driver). Second, the f3 (or the frequency at which SPL is down by 3dB) will be lower, providing a little bigger bottom end. While these are both great advantages, they decrease the effective damping of the speaker as well, meaning the speaker can be more likely to bottom out or over-excurt itself. Naturally, this is speaker, frequency, and power dependent. If used in a ported enclosure, you will also see the Fb (or the resonant frequency of your port) drop lower.

There are some additional worthy considerations. Adding polyfill to an enclosure can be a great choice. However, too much polyfill can be a bad thing. At a certain point, the stuffing becomes too dense and the fibers no longer wiggle. At this point, not only have you taken away the size benefit of adding polyfill, you have actually decreased the effective volume as the polyfill is now taking up room inside your enclosure. It is also worth mentioning that polyfill is not as effective in a large enclosure. Let's combine these two thoughts into two simple rules:

1. If the enclosure is less than 2.5 - 3.0 cubic feet in size, you should use no more than one and a half pound of polyfill per cubic foot available in your enclosure.

2. If the enclosure is greater than 2.5 - 3.0 cubic feet in size, you should use no more than one pound of polyfill per cubic foot available in your enclosure.

Specific examples of polyfill's effects on various enclosure sizes (with varying amounts of polyfill in each size) can be found in The Loudspeaker Cookbook by Vance Dickason or in an article written by Tom Nousaine for the March/April 1995 edition of "Car Stereo Review".

There is one last point that you will hear from time to time regarding polyfill: that polyfill stops standing waves in an enclosure. When referencing an enclosure for a subwoofer playing a fundamental frequency that falls in the typical range, this is simply false. A standing wave in this range of frequencies would be several feet long and, thus, unlikely to occur. However, higher order harmonic distortion is possible, and can potentially colour music. Being that these higher order harmonics will be progressively shorter (in terms of wavelength), polyfill can be effective for this purpose. However, audibility, particularly at high SPL, can be quite minimal. Using polyfill in an effort to absorb standing waves or various distortion is most effective in large enclosures for your midrange and is not particularly effective for a subwoofer.

Hopefully you now have a greater understanding of what polyfill does and doesn't do, while also enjoying the opportunity to absorb some scientific content as well. If you're still undecided, be wild and adventurous: put some polyfill in your enclosure right this minute!

Edited February 26, 2007 by DevilDriver

[filtor1 13]

Another great thread! Thank you again for the information.

[CBFryman 2]

Great write up. The only issue (not really a big deal at all) is the air density thing. Fixed volume and fixed mass of air. The only thing that can be raised by heat is pressure.

[Aaron Clinton 2,372]

Nice job neil!

[amatt85 0]

i realize this is an old post, but....since polyfill creates an "illusion" of a bigger enclosure for the sub, would it change the tuning frequency of the enclosure as well. if ur port is say, 20" long without polyfill, then u add polyfill to make it seem bigger, would u then need to lengthen ur port to compensate for a seemingly larger enclosure?....

[DevilDriver 28]

i realize this is an old post, but....since polyfill creates an "illusion" of a bigger enclosure for the sub, would it change the tuning frequency of the enclosure as well. if ur port is say, 20" long without polyfill, then u add polyfill to make it seem bigger, would u then need to lengthen ur port to compensate for a seemingly larger enclosure?....

Actually, as the enclosure size increases, the length of the port may decrease (if we assume that our port cross-sectional area and Fb stay the same). But yes, you've got the general idea.

[menikmati 0]

Sorry to bring this back up, but I just want to stay on the safe side. I'm running two subs in 1.1 sealed each, separate chambers. I stuffed each chamber with approx 14oz of polyfill, but it seems like a lot. Is just stuffing the box okay? It filled up most of the chamber. I keep hearing that the subwoofer can be damaged if polyfill is just stuffed in, is there a different way I should have put in the polyfill? Thank you.

[mrray13 30]

you want the polyfill in loosely, not packed. if you pack it in, it becomes more of a "solid" object that will actually take away airspace. you want it to remain fluffy to do what it does.

wheeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee

[menikmati 0]

There's no way a pound can fit in a cube loosely, guess I'll just use less, thanks.

[MikeS 0]

Excellent topic.

From what I understand, I think Polyfil works in a resistive manner, which goes along with what was explained.

I use Dacron/Polyfil quite often in bass reflex cabinets. From my experiences, if you stuff too much in a cabinet/enclosure, it dampens way too much. You'll get to the point where the bass pretty much dissapears. IIRC, it seemed to limit cone movement as well. I dunno.....I ain't too sure....

My experiences have been by trail and error. Add some fill, listen, add some more, take some out.

It works well with a speaker/cabinet that is boomy, or peaky at certain frequencies. It dampens the boom and tightens up the bass a bit from what I have noticed.

It is at the sake of amplitude and sensitivity.

And peaks can be adjusted by tuning the vented alignment which ever way necessary.

I have tried poly in a 6 foot enclosure, and it is not as effective, like mentioned.

One way to get rid of boomy sound in a enclosure using polyfil is using a AA battery. Hook the woofer up to the battery, and listen to the DC when connected. If it's a big thump, add polyfil. Add fill until the thump sounds more like a mild thud.

I never tried this, I just went with the trail and error approach.

I wonder if that is some general rule of thumb, calling for a pound and a half of polyfil for 2.5-3.0 ft enclosures, and a pound for larger sized enclosures.

What I'd like to know is how one could determine the added volume for the amount polyfil that it "fakes" in the enclosure. So one could have a way (formula) to come up with the numbers for the added volume to add into a box plot program....like WinISD or something.

Mike

Edited May 31, 2008 by MikeS

[Dr Durden 0]

What I'd like to know is how one could determine the added volume for the amount polyfil that it "fakes" in the enclosure. So one could have a way (formula) to come up with the numbers for the added volume to add into a box plot program....like WinISD or something.

Mike

http://web.archive.org/web/20041027051204/...rces/fiberfill/

[MikeS 0]

What I'd like to know is how one could determine the added volume for the amount polyfil that it "fakes" in the enclosure. So one could have a way (formula) to come up with the numbers for the added volume to add into a box plot program....like WinISD or something.

Mike

http://web.archive.org/web/20041027051204/...rces/fiberfill/

Cool, thanks. It'll help.

Mike

[bose301s 1]

Great write up. The only issue (not really a big deal at all) is the air density thing. Fixed volume and fixed mass of air. The only thing that can be raised by heat is pressure.

Exactly, I was going to point that out.

Anyone remember PV=nRT from chemistry? Well pressure * volume = # of moles of gas * a constant * temperature. In a sealed enclosure Volume, n and R are all held constant meaning P~T, they are in a positive linear relationship, meaning that as one goes up the other does. This can be shown in tires, in winter the pressure goes down and in summer it goes up for the same reason, V, n and R are all constants. This assuming STP though, but that is generally the case.

[bigharv 0]

sweet article

[porkchop 304]

hey fellas,

if we have a box that is 6 cubes common chamber, but with a center port somewhat dividing that area, would it be considered 3 cubes per side.... if so should we go with .50 lbs. per cube or would it be safe to give it a full pound per cube?

thanks for the input.

if all else fails, i will take the op's advice and go with the smaller and work up from there.

chop

[mjmarovi 8]

hey Chop, you should try some polyfill in your WMD2 setup. that will give you an idea of what just a larger box will do.

[porkchop 304]

lol, i just text my bud that same thing,,,,, i am going to tomorrow morning. i already have some hear, and it is doing exactly what you are suggesting.

[porkchop 304]

larger box .... lower tunning.

[Dammed 12]

how big does a enclosure have to be for the polyfill to have close to none effect?

[///M5 2,833]

how big does a enclosure have to be for the polyfill to have close to none effect?

If the right amount is used it will have an effect in any size enclosure.

[Dammed 12]

what's the right amount for a 10,25 cu ft enclosure?

[Duran 387]

what's the right amount for a 10,25 cu ft enclosure?

A general rule is one lb./Ft³

[Dammed 12]

so about 10lb, thx.