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xiayu 4th April 2010 08:26 AM

Hi, and what does polyfill do?
So I'm doing a little research about what exactly polyfill does in a sealed sub enclosure, and to be honest I can't get a straight answer. I've got people telling me it's just a dampener or that it converts pressure into heat or that it converts heat into kinetic energy or that's it has magical properties that are contrary to other fibrous materials like for example cotton stuffing. I'm thinking that this is perhaps the right place to ask and that maybe everyone here agrees on what it does.

bunkie 4th April 2010 11:44 AM

The latest issue of AudioXpress magazine(Aptil 2010) has an article by Cornelius Morton about the effects of stuffing when used in a transmission-line speaker enclosure. In that application, one of the effects is that it slows the speed of sound which makes the effective length of the line longer.

But the primary effect of the stuffing is to lower the Q of the driver/transmission line system. It does this by increasing the frictional load of the air mass within the line.

Cal Weldon 4th April 2010 04:09 PM

Hi xaiyu,

Even here you find lots of opinions but safe to say that (polyfil and other similar fiberous material like fiberglass and rockwool) in a sealed enclosure it's use will be minimal unless you stuff it as full as you can and then it will act as though the cabinet is slightly larger as in 10% or so. Yes the stuffing converts the pressure into heat but not really enough to tell. The heat of the motor will warm up the box a lot more than any pressure wave will. Remember that below a certain frequency, it has little value at all so in a sub enclosure, depending on your XO point, it may have almost zero effect. As far as it slowing down the speed of sound, yes but it is so minimal so as not to be considered for all intents and purposes.

Oh, welcome to the forums.

Richard Ellis 4th April 2010 04:39 PM

Yes it converts pressure waves?? to heat & increases the volume of the enclosure the driver "sees". Materials is where you get controversial.. Myself I was shopping for material & found the 'approved' fibreglass. But when I put a mass of said fibreglass to the ear I noticed no difference in hearing thru it???
So I went to a fabric shop & got some cotton batting for a fraction of the price...putting it up to my ear...."What did you say??"" I can't hear you?".
Another effect of 'stuffing' is the damping of frequencies radiating into the enclosure. The idea being that you don't want internal pressure waves bouncing around & trying to set the diaphram in motion. Of course different materials will have different damping attributes.

__________________________________________________ __Rick.........

bunkie 4th April 2010 05:40 PM

It's true that the reduction in the speed of sound is minimal, somewhere about 2.5%. Interestingly, ambient air temperature changes of a few degrees C has about the same effect.

I suspect that in non-TL enclosures the speed of sound reduction makes no real difference. For tuned-port boxes, it's my understanding that it's the mass of the air in the enclosure that matters. But in a TL since the line is tuned to specific frequency, it does have an effect, however small. Of course, variations in Fs from one driver to another of the same model will have just as much, if not more of an effect.

Cal Weldon 4th April 2010 07:59 PM

Hello Peter,

Not disputing what you say but how did we get on to the topic of TL's when he was asking about sealed?

xiayu 4th April 2010 08:36 PM

Yeah, I'm only interested in how it works in sealed enclosures. Here's the thread that got me started on my search for the answer:

cotton in sub box

bear 4th April 2010 09:07 PM

There is no doubt that if properly used - not tightly or fully stuffed - that the ideal operation of "fill" for a speaker box is to increase the effective size 10% or less. Not all materials are equally effective. Long hair wool apparently is the best.

The other effect that fill of any sort has is to absorb any higher frequency energy - but in a subwoofer with a proper xover, that should be nil already.

It would be very difficult to "hear" the effect of fill in a subwoofer box, unless the box was sized and set up so that the Qts (system Q) was at some relatively critical point, and a little one way or the other effected a subtle tonality...

A tightly stuffed box will behave like a box with lower volume, fwiw.

Otoh, with a driver that goes up higher, the loss of bass might be offset by an improvment in the midbass or midrange... but that is largely irrelevant with a sealed box subwoofer.

The key with a sealed box subwoofer is to select the proper driver Fs, Qt and VAS, along with the same thing WRT the cabinet size and construction...

Stuffing is usually not magic.


bunkie 5th April 2010 12:27 AM

My bad. I was so excited about the article in AudioXpress that I got a little carried away.

But I would argue that with the right TL design, there's no need for a subwoofer ;--}

Carry on...

xiayu 5th April 2010 05:17 AM


Originally Posted by bear (
There is no doubt that if properly used - not tightly or fully stuffed - that the ideal operation of "fill" for a speaker box is to increase the effective size 10% or less.

Can you explain to me how it works?

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