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View Full Version : pollyfill question?



05fronty
01-28-2007, 08:04 AM
i searched and can't find it so here goes. my subs need like 1 cube per sub but my box is like .75 per sub. will using pollyfill make the subs see more air space? how much would i need to do this? each sub has it own chamber(sealed). can i use the stuff from like a hobby store? thanks for any info

05fronty
01-28-2007, 09:20 AM
anyone i am trying to fix this today

Radridd
01-28-2007, 09:39 AM
What I have read suggests using 1.5 lb. per cube. I am running 1 lb. in my .65 cu sealed box.

05fronty
01-28-2007, 09:42 AM
What I have read suggests using 1.5 lb. per cube. I am running 1 lb. in my .65 cu sealed box.

so each sub should see about 1 cube each then? will this mess with the sq or spl by any? they are infinity reference 12's

Radridd
01-28-2007, 09:50 AM
Made mine sound better. I tried both with and without it. Not sure about SPL but definitely made bass tighter.

05fronty
01-28-2007, 09:51 AM
thanks

05fronty
01-28-2007, 09:51 AM
can i just use the stuff at wal-mart or any hobby store?

Radridd
01-28-2007, 09:55 AM
Yes, Got mine at walmart. Just stuff it loosely.

05fronty
01-28-2007, 10:00 AM
thanks

PV Audio
01-28-2007, 11:01 AM
Rip open a pillow and be done with it. :)

hoxie08
01-29-2007, 10:17 PM
How exactly does polyfill make your box seem bigger to the sub? Im just curious to the science behind all of this....

kballa422
01-30-2007, 05:55 PM
i think it does it by slowing down the speed of the air inside the box, so it tricks the sub into thinking it is in a larger enclosure. Not sure if that is right, but that's what I heard, it makes sense.

PV Audio
01-30-2007, 08:31 PM
Yep, and I SWORE I posted exactly that. Musta gotten deleted when my firefox got sniped by the SAS.

aaron7114
01-30-2007, 09:51 PM
just wondering what the pros and cons are of putting polyfill in a ported enclosure? How much per cu ft?

aaron7114
02-01-2007, 12:34 AM
bump...still curious on the polyfill in ported boxes.

innsanes
02-01-2007, 12:38 AM
it actually dissapates the heat created inside the box so the sub is operating more optimum as if it was a larger box.

PV Audio
02-01-2007, 01:33 AM
....What? I wasn't aware that speakers created a lot of heat, and I'm not too sure how pillow stuffing dissipates heat. You're going to have to find me some proof.

baseballer1100
02-01-2007, 01:34 AM
....What? I wasn't aware that speakers created a lot of heat, and I'm not too sure how pillow stuffing dissipates heat. You're going to have to find me some proof.

interesting.

Volenti
02-01-2007, 05:33 AM
....What? I wasn't aware that speakers created a lot of heat, and I'm not too sure how pillow stuffing dissipates heat. You're going to have to find me some proof.

It's not the heat from the sub's motor, it's the heat caused by the air compressing;

As the cone moves backwards it compresses the air, this increases the temprature of the air and this causes the air to expand, this resists the force of the cone motion. The reverse happens when the cone moves out, the air is rarifed, cools and shrinks, the atmospheric air pressure resists the cone motion in this case (trying to fill the partial vacume) A smaller enclosure will heat up and cool down faster than a larger enclosure simply because there's more air to heat/cool in a large encloaure than a smaller one.

Now when you add fill to an enclosure you increase the Specific heat (http://en.wikipedia.org/wiki/Specific_heat_capacity) of the air volume. This means when you compress (or rarify) the air it heats (or cools) less, thus from the point of view of the driver it's in a larger enclosure since the expansion and contraction characteristics of the enclosure now match those of a larger enclosure.

But wait there's more, The other very important thing that happens when you add fill to an enclosure is the addition of a whole lot of resistive losses, these losses are most prevalant at resonance, in a sealed enclosure with an ideal fill load the resonant frequency will drop a few hz (from the thermal effects) and the impedance rise at resonance is reduced by ~half. What this does to the amplitude response of the driver is to reduce the amplitude volume at resonance by ~3db (it's closely linked to the change in the impedance peak)

The combination of both the thermal effects and the resistive losses results in an enclosure environment that is more complex than a simple increase in apparent volume, indeed a filled enclosure and a larger empty enclosure that has the same resonant frequency will have significant audiable differences.

ffkk
02-01-2007, 07:16 AM
HI Volenti,

Whats your source/reference for your theory above?

I thought the reason was that the sound waves have to get past the polyfill to get to the enclosure walls, thereby taking the waves longer to get to the wall and bounce back. This makes the sub think its in a bigger box than it actually is.

Volenti
02-01-2007, 09:41 AM
HI Volenti,

Whats your source/reference for your theory above?

I thought the reason was that the sound waves have to get past the polyfill to get to the enclosure walls, thereby taking the waves longer to get to the wall and bounce back. This makes the sub think its in a bigger box than it actually is.

I don't a definitive reference for this (I would have quoted it, much easier :)) it's a compilation of my own experimental data and others from various web sites.

The impedance reduction and amplitude reduction at resonance I've proved exprimentally (though I wasn't the first to do this, it's easy to do with just some test tones, a resistor, and a multimeter)

I haven't looked at the time delay side side of the theory as of yet, I suspect it may only come into effect where the largest filled length of the enclosure becomes a signifigant fraction of the wavelength if interest. For example a 50hz tone has a 22' wavelength... unless the enclosure is very large or very long there's no room for a wave to form in the enclosure, it's simply changing pressure, wave delay would also imply that a long skinny enclosure (with a driver mounted at one end) would have a lower fs than a cube of the same volume, and this isn't the case.

ffkk
02-01-2007, 09:59 AM
Oh, so u r researching this stuff. u a sound engineer at a research institute or for a private audio company?