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I've read so much about polyfill, and its contradictive. Some say add it to any box, ported or sealed and it fools the sub into an "increase" in cuft. Others say it doesn't do anything, or it makes the box actually smaller. I see people adding 1lb polyfill per 1cuft. Is this something everyone is doing? I've never used the stuff besides what came in prefab boxes, so I couldn't tell a difference either way. Most of what I've read pertains to the box being too small, either due to limited space, or measuring/building f**k ups. So what if the box is the correct size? Do you still want to add some?

I've seen where people not concerned with space issues purposely build the box smaller, and add polyfill to make up for it. I know from personal experience that if you build a sealed box too big for a lower xmax sub, the sub will bottom out way too easy. Yea it plays deep, but it's still limited. So what happens of you build a box small and fill to simulate the box being bigger? Do you still get the "spring" the small sealed box has but with the ability to play deeper, or does the sub handle the same? I've been practicing my box fan skills, building boxes for friends, people from work, etc. I'd like to start charging for the labor as well lol, but I still have alot to learn. Polyfill seems to be an argumentive subject, and its just a matter of opinion. Is there any real proof to back it up?
 

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My only proof is my ears. I run a sealed enclosure 1.44ft*³, which is larger by .44 than the "spec" size for my sub...and I add 1.5lbs of polyfil. The sub is slightly smoother, cleaner, and seems to have less "ring" when driven a bit too hard.

I have listened both ways, and my ears say "add polyfil" every time I heard it without it.
 

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Fill is good, as it help to kill the back wave of the sub. Without some sort of fill, the back wave will reflect off the back of the box and fight the sub. I say the correct size and 35% fill will sound best.
 

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polyfill will have little effect in a sub enclosure. "Fill" in general works based on thermodynamics. Speaker is basically a piston. When it moves, it creates heat. Adding fill helps to dissipate the heat as the small fibers wiggles and move through the air movement.
Thus the theory is that if enough heat is being dissipated by the fill, the speaker is "tricked" into behaving as if it were in a larger enclosure bc more heat can be produced.

The issue with polyfill in general is that it is not dense enough to have any real effect, from a thermodynamic or an absorption stand point.
using a denser material like Fiberglass insulation (yellow or white), Rock Wool, Roxul etc...

from vance Dickinson LoudSpeaker Design Cookbook

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.
 

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When I was younger I just took the heat dissipation or thermodynamic benefits of polyfill as gospel. I always wanted to try to control the air temps inside the box in other ways beyond polyfill but never did.

The reason I question this theory is polyfill is a good insulator thus a poor conductor, so not much air cooling happening with conduction. Now these fibers move around from air movement and molecular vibration. Could they be dissipating the heat by convection? I'm not buying it!!

Here's what I think is happening: Poly is great for trapping air and preventing it's movement just like pretty much any good insulator does. When the air molecules are excited, these thousand of minute fibers resonate and in doing so absorb this excitation and transfer sonic energy into mechanical energy. Remember that sound travels at almost 800mph, so think how fast the sound is moving inside a box. So it acts like a dampner doesn't it? You darn right it does, no question. Now these fibers are excited but granted they have a greater, however slight, property to return to rest than air, this is how it dampens the sound waves. Ok now think of air as a dampner, cause it is. The further sound travels through air the more decay created. Air can be a sound barrier in and of itself, just like it can be a good insulator. This happens once you control it's movement, then these properties become apparent. If sound travels through a box, the amount of excitable air has an effect on the sound characterisitics within the box. The less the volume of air, the less molecules to excite, so less dampening, however slight. It takes energy to move air molecules, the more of them the more energy, so more air has a dampening effect, that's a given. So if we look at 2 boxes with nothing in them, one is slighly bigger than the other and we introduce a frequency within the box, the larger box will provide a slightly greater sound decay than the smaller box just based on the number of molecules available to excite.

Now compare this with what the poly is doing. It is mimicking the sound decay of a box with more molecules exposed to frequency vibration. What is happening is the same principal as sound deadening. Polyfill is decreasing reverberation time, decay time whatever you want to call it. Whether this is a stadium, large room, or a box, same principal applies. To deaden it, decay time is shortened. a box with poly matches or mimicks the decay time of a larger box.

That's what I think, I just can't buy the thermodynamics of it. If dissipation of heat was that noticeable, I think we would notice a major sound change as the air inside the box heats up during a normal listening session, or maybe during peak transients and sound changes with lulls as the air cools. I've never heard this happening. Also sealed boxes would be more prone to a heating affect more so than ported due to the trapping of heated air versus the venting of it. When you place poly in a box, it is immediately noticeable from the 1st note whether internal air temp is 70 or 110. When someone talks about thermodynamics of poly, it's always obscure reasoning with no sustance. If it's true, I need some sound explanation of the scientific process, not that it dissipates heat thus fooling a woofer. Just a good explanation of how it dissipates heat would be a good start. Cause if anything, it's trapping air, not moving it. Also poly is not a conductor, but we could mimick poly inside a box with a different material that is a good conductor if that was the case. Anyway just my opinions and thoughts on the subject.
 

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When someone talks about thermodynamics of poly, it's always obscure reasoning with no sustance. If it's true, I need some sound explanation of the scientific process, not that it dissipates heat thus fooling a woofer. Just a good explanation of how it dissipates heat would be a good start. Cause if anything, it's trapping air, not moving it. Also poly is not a conductor, but we could mimick poly inside a box with a different material that is a good conductor if that was the case. Anyway just my opinions and thoughts on the subject.
I agree, the thermo and heat transfer being put forth is not making much sense.

If someone were worried about heat behind their woofer in a sealed box, they would build the box out of a good conductor and put an A/C vent blowing on it. The sealed box portion behind the woofer is basically an oven (heat being provided by that giant resister we call a subwoofer), putting an insulator inside of an insulator is not going to do much for you by way of cooling.
 

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polyfill will have little effect in a sub enclosure. "Fill" in general works based on thermodynamics. Speaker is basically a piston. When it moves, it creates heat. Adding fill helps to dissipate the heat as the small fibers wiggles and move through the air movement.
Thus the theory is that if enough heat is being dissipated by the fill, the speaker is "tricked" into behaving as if it were in a larger enclosure bc more heat can be produced.

The issue with polyfill in general is that it is not dense enough to have any real effect, from a thermodynamic or an absorption stand point.
using a denser material like Fiberglass insulation (yellow or white), Rock Wool, Roxul etc...

from vance Dickinson LoudSpeaker Design Cookbook

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.
Thanks for the informative response as I was having doubts it would help in a sealed sub enclosure.
 

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I have it on pretty good authority to stuff a box with as much as it would hold. Haven't tried it, this way yet, but I trust him.
 

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I have it on pretty good authority to stuff a box with as much as it would hold. Haven't tried it, this way yet, but I trust him.
Polyfill is like salt, it takes a certain amount for optimum benefit. Adding too much will reverse the effect and it is definitely audible. Give it a try!!
 

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I would highly suggest measuring the response before and after to make sure its not a placebo effect that your hearing. When I tested my home bookshelfs, neither polyfil, nor acoustuff, nor black hole tile made any measureable difference in response, nor could you hear a difference. Ill be testing black hole tile again, but I dont believe for a second it is effective at absorbing anything. Black hole 5 might be better at absorbing, but not in the sub bass frequencies.
 

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I use poly for large sealed subwoofers. I'm not sure there is anything to gain on a bookshelf speaker? The results of poly I think we're posted earlier in the thread of a change in frequency response on a 10" driver essentially performing as if it were in a larger box. Applying more than optimum just reversed the effect.
 

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I tested after seeing multiple posts about polyfil and like materials helping to smooth midbass and midrange frequencies, and the results I got were pretty much negligible, which means none of them actually are absorbing anything in the midbass frequencies, and not enough up high to make a difference. That said, i havent tested with subwoofers, I just think one should measure if possible when doing it to get the best results.
 

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While the measurements provided are very good, the explanations they give for the results are just plain wrong. For instance, they talk about polyfil absorbing the 170hz peak, well 170hz is a 79.4' wave. Your not absorbing that in a box, period. Something else is definitely happening, but your not absorbing the wave. There wouldn't even be any standing waves at 170hz.
 

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While the measurements provided are very good, the explanations they give for the results are just plain wrong. For instance, they talk about polyfil absorbing the 170hz peak, well 170hz is a 79.4' wave. Your not absorbing that in a box, period. Something else is definitely happening, but your not absorbing the wave. There wouldn't even be any standing waves at 170hz.
Contact Josh. He is always (at least in my experience) willing to talk and answer questions.

He goes by Ricci on AVS. Great guy with some awesome horn designs for home theater use.
 
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