How Polyfill actually works.

I was doing some research on polyfill and its use in sound situations. I found a really good explanation from a guy on a camry forum. Figured I'd post it here. It's really awesome to know polyfills benefits and how it actually works.



"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

$30 Loudspeaker Design Cookbook

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! "

 

I'm sold.

Thanks for the post.

I didn't realize you were suppose to staple and glue it in the box, I always thought you just stuffed it in there.

 

yeah thats one thing in question on that post. if you do just an image search on google it shows people just stuffing it in there.

 

All you had to do was ask. j/k, it's nice to see someone explaining things correctly instead of spreading B.S. all over!

 

I'm an engineer so that was a good read for me. I don't think many people even consider the idea of heat dissapation through polyfill

 

So, what do you do Marv? Do you staple it in?

 

I personally loosely stuff it for reasons stated in that article (stuffing too tight will keep the fibers from wiggling)

 

I've been trying to explain that for years yet I will still get arguments from people who believe it "slows down sound waves" and all kinds of stuff!

 

Haha. It just makes sense to me. I guess it depends on how thorough your science based education is. Sometimes you just gotta accept it when it doesn't make sense. I think thats called something like faith?

 

Well just made a quick trip to Walmart and picked some up - well Lowes first, then Target and then Walmart.

I try to stay out of Walmart, but the others didn't come through for me.

 

Once my new box comes in, that is where I will get mine (since there is nothing else in my little town without going into San Antonio).


...that is, if the manufacturer recommends it.

 

I personally get mine from Walmart.

 

Ever since I tore my first speaker box apart (many MANY years ago) I always had a fleeting "wonder what that foam stuff does" question go through my skull. Never really gave it too much thought. Pretty cool to finally know!

 

nice!

 

I actually seriously considered just purchasing a pillow when I was at Target - it said it was 100% polyester filling. But, I'm glad I waited and went to Walmart. If it is good enough for Marv, it is good enough for me.

 

I have no idea how long people have been using polyfill in enclosures, but I know its been quit some time.. So I wonder... as vehicles (and enclosures) get smaller, and more people utilize (and hopefully spend the time to read threads like this and learn how polyfill actually works), how long before we see companies in the industry start pushing their own "unique" super duper hybrid versions of polyfill?

"Maximize your available air-space"

"Guaranteed to raise the ambient temp of a 1 cubic foot enclosure by 3.78274 degrees* ".

"Our new blend of red & black fibers guarantees more efficient bass response"

"Each strand measured to .0000000000000000009" ** tolerance to ensure optimum results"


*accurate to within 50 degrees F.
** +/- .05"