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post #1 of 11 Old 07-27-2011, 09:48 PM Thread Starter
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To polyfill or not?

Question for the DIY box builders/designers out there. If a sealed box is made to spec (optimum internal volume), is polyfill necessary? I know it's been used to "trick" subs to perform as if the box is larger than it is. But is polyfill needed for any other reason?

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post #2 of 11 Old 07-28-2011, 04:12 AM
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I believe it reduces box resonance also.


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post #3 of 11 Old 07-28-2011, 04:18 AM
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In my experiences I've sometimes noticed slightly deeper bass with a box that had a slightly greater volume than the sub called for (remember to factor in the space that the back of the sub takes up in the box too). Polyfill allows you to do that without increasing the box size.

Oh, and don't waste the $ on real Polyfill. Go to your local Walmart and get the big bags of polyester fiberfill for about $2.

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post #4 of 11 Old 07-29-2011, 05:00 PM Thread Starter
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I did a little experiment with the sub with and without polyfill. But some preliminary facts about my setup...

My Alpine sub requires 0.85-1.50 cuft net sealed, with a recommended volume of 1.17 cuft net. My current sealed box gives the sub 1.10 cuft net, virtually the recommended volume. I tested the bass output using some test tones and some bass heavy songs. This is what I found...

Low freq (30-50 Hz) output did not differ much between polyfill vs. no polyfill. Both setups played low frequencies very well. But at the higher bass frequencies (60-80Hz), the bass had more punch without polyfill in the box.

I don't have any scientific explanations for this, but from my experiment, I found that polyfill is not needed, in fact, hurts bass response, if the box is already built to specs. If the box was made much smaller, maybe polyfill could help, but I'm not sure. For now, I'm not using polyfill for my current setup. Hope that helps!
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post #5 of 11 Old 07-29-2011, 06:31 PM
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Quote:
Originally Posted by kalel34 View Post
I did a little experiment with the sub with and without polyfill. But some preliminary facts about my setup...

My Alpine sub requires 0.85-1.50 cuft net sealed, with a recommended volume of 1.17 cuft net. My current sealed box gives the sub 1.10 cuft net, virtually the recommended volume. I tested the bass output using some test tones and some bass heavy songs. This is what I found...

Low freq (30-50 Hz) output did not differ much between polyfill vs. no polyfill. Both setups played low frequencies very well. But at the higher bass frequencies (60-80Hz), the bass had more punch without polyfill in the box.

I don't have any scientific explanations for this, but from my experiment, I found that polyfill is not needed, in fact, hurts bass response, if the box is already built to specs. If the box was made much smaller, maybe polyfill could help, but I'm not sure. For now, I'm not using polyfill for my current setup. Hope that helps!
It will depend on the box, the sub, etc. It's interesting to hear your results, but there are way too many variables to say that it will always sound worse or always sound better.

I would recommend trying different amounts of polyfill, as packing it tighter or looser will likely affect the results.

Anyway, here's your explanation...

With a sealed box, generally the larger the box, the lower the sub will play. The compression of air inside the box creates a high-pass filter of sorts, and the smaller the box, the harder the sub has to work compress the air within to play those deep notes. It's not uncommon for a well-built infinite baffle set up (basically free-air with no enclosure at all) to play loud, clean, and flat down to 15Hz or lower.

Anyway, when you added the polyfill, in theory you tricked the sub into thinking it was playing in a larger enclosure than it actually was. The result is (theoretically) the ability to play a bit deeper at the cost of the upper sub-bass range. Sounds like the change in the upper range was more noticeable than the change in the lower range for you.

If you're interested in an experiment, try the opposite. Use no polyfill and put something in the box that will reduce the internal volume (like a brick, block of wood, etc). You'll probably notice even less deep bass but the upper range will be more pronounced, though not not necessarily in a good way.

In the mid 90s, the Kicker Solo Baric subs boasted their super-small sealed box requirements (I want to say .8 cu ft for the 12" but I could be wrong). In that size box, they could play insanely loud, but were 'buzzy' (all upper sub-bass) and didn't go deep at all. Once you bumped that box size up, they actually sounded quite good.

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post #6 of 11 Old 07-29-2011, 11:01 PM Thread Starter
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Quote:
Originally Posted by jperryss View Post
It will depend on the box, the sub, etc. It's interesting to hear your results, but there are way too many variables to say that it will always sound worse or always sound better.

I would recommend trying different amounts of polyfill, as packing it tighter or looser will likely affect the results.

Anyway, here's your explanation...

With a sealed box, generally the larger the box, the lower the sub will play. The compression of air inside the box creates a high-pass filter of sorts, and the smaller the box, the harder the sub has to work compress the air within to play those deep notes. It's not uncommon for a well-built infinite baffle set up (basically free-air with no enclosure at all) to play loud, clean, and flat down to 15Hz or lower.

Anyway, when you added the polyfill, in theory you tricked the sub into thinking it was playing in a larger enclosure than it actually was. The result is (theoretically) the ability to play a bit deeper at the cost of the upper sub-bass range. Sounds like the change in the upper range was more noticeable than the change in the lower range for you.

If you're interested in an experiment, try the opposite. Use no polyfill and put something in the box that will reduce the internal volume (like a brick, block of wood, etc). You'll probably notice even less deep bass but the upper range will be more pronounced, though not not necessarily in a good way.

In the mid 90s, the Kicker Solo Baric subs boasted their super-small sealed box requirements (I want to say .8 cu ft for the 12" but I could be wrong). In that size box, they could play insanely loud, but were 'buzzy' (all upper sub-bass) and didn't go deep at all. Once you bumped that box size up, they actually sounded quite good.
Thanks for the scientific explanations, man I'm generally familiar with sub behavior when you play around with the internal volume (up to a point). That's why I was happy to find a well-built, prefab sealed box with the correct volume. Maybe I didn't polyfill correctly, but it definitely has more punch without it.

I actually had the same sub in a much smaller box, about 0.88 cuft net sealed (my goal was to save trunk space). I was not happy with the sound from it all at, and I tried using polyfill as well. So I looked around and found a slightly bigger box. I suppose there's no beating having a sub matched to box specs.
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post #7 of 11 Old 03-05-2016, 03:25 AM
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thanks for sharing your results. I had a similar question for a box I built to the exact volume the speaker suggested, and it doesn't feel punchy with fill, I'll try removing it today!
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post #8 of 11 Old 03-05-2016, 05:14 AM
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I always add polyfil...has been proven to trick the woofer to think its in a slightly bigger box.

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post #9 of 11 Old 03-05-2016, 05:22 AM
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I didn't think to add polyfil until keep_hope_alive suggested I do. Big, big difference. Much deeper, louder bass. Very happy after I did so.

It's on page two of my thread about building a custom enclosure.

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post #10 of 11 Old 03-06-2016, 10:21 AM
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adding absorption in the box does certainly change how the box sounds, it also changes several system parameters.

polyfill is the least useful fill you can use because it's not naturally fibrous - the strands are solid and don't have air pockets and cavities found in other fibers. fiberglass is awesome, but itchy. i've moved to Bonded Logic's Ultratouch - denim-based.

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post #11 of 11 Old 02-06-2017, 06:37 PM
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I use fibreglass for a sealed enclosure - terrible idea for ported.

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