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.
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.
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.
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.
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.
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.........
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.........
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.
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.
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
cotton in sub box
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.
Imo.
_-_-bear
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.
Imo.
_-_-bear
I will say up front that I am no expert on speakers or acoustics, so I may well be wrong about how I think it works.
If the effect of stuffing was thermal, the popular view, then the best stuffing material would be the one with the highest thermal transfer rate. That should be something (of the commonly used stuffings) like fiber glass, not wool. If it is thermal, then stuffing the box with #400 steel wool would work better than wool or poly fill.
My thinking is that the stuffing reduces the density of the air in the sealed box by displacing some of the air. Less dense air is more compressible, making the box look larger to the driver. The figure of 10% might be the point at which the volume of the stuffing begins to dominate this effect. Just my thoughts. I am not in a position to prove or disprove either method of action.
hold on here. one second... so. the goal with box stuffing is to make the air appear less dense to the driver... well.. why not just create a vacuum inside the box and replace the air with helium? could something like that benefit a subwoofer? couldn't that make like a 1 cubic foot enclosure look like it's bigger? like instead 1 cube, 2 cubes? I've also read that the speed of sounds travels like 3 times faster through helium or something like that. *mindblown* ..... so?
To make the box seem larger you need to use denser gas -- as done by Michael Dayton-wright with SF4 (?)
dave
when it comes to box stuffing material type can be controversial.
but as to proper amount i like David B. Weems philosophy and test.
whether it's sealed or bass full reflex full range or sub stuffing is applied for two main reasons adequate excursion control (return from Xmax) and damping of higher frequency harmonic distortion created by the box.
the test is simple a 1.5 V battery and a switch.
you simply listen for the sound at make and break of the switch as in make should sound like a defined thud/click and then at break if it sounds like boom or a woosh that would indicate under damped.
i have used this method time and again on speakers of all kinds especially on car subs that are notoriously underdamped (saving many an amp from an untimely death)
but as to proper amount i like David B. Weems philosophy and test.
whether it's sealed or bass full reflex full range or sub stuffing is applied for two main reasons adequate excursion control (return from Xmax) and damping of higher frequency harmonic distortion created by the box.
the test is simple a 1.5 V battery and a switch.
you simply listen for the sound at make and break of the switch as in make should sound like a defined thud/click and then at break if it sounds like boom or a woosh that would indicate under damped.
i have used this method time and again on speakers of all kinds especially on car subs that are notoriously underdamped (saving many an amp from an untimely death)
I have a bookshelf speaker that has a 12 inch x 12 inch x about 1 inch (uncompressed) square of Dacron. I am going to experiment with a certain brand of acoustic foam that has a nice absorption curve and was just going to play around with a couple of different pieces inside. Does looking at RAL absorption curves mean anything? Also, I was told that Polyfill was more to change the Q and acoustic foam was to be placed along the cabinet wall to dampen the cabinet. The Dacron piece that's inside my speakers inside aren't lining the inside of the cabinet, they are just stuffed inside. My bookshelf's aren't transmission line speaker, they are just small bookshelf speakers that aren't ported.
This thread is old but just for the benefit of whoever stumbles upon
. http://www.nousaine.com/pdfs/Box Stuffing.pdf
. http://www.nousaine.com/pdfs/Box Stuffing.pdf
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