Mucho Polyfill affect Sealed Sub Q? - diyAudio
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Old 14th November 2007, 03:03 AM   #1
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Question Mucho Polyfill affect Sealed Sub Q?

I'm looking at building a sealed enclosure for a Polk model SR124 sub and Polk recommends an internal volume of 1.20 cuft (this includes both the driver's rear consumption and then the needed air volume). According to Polk, this 1.20 cuft internal volume will produce a Q of 0.707 which is what I want. I don't want a 'peaky' response but rather a musical and accurate sub.

I've come up with a box having internal dimensions of 13.0" x 12.14" x 13.14" as that fits nicely where it will go. It's cuft is 1.20009.

I plan to stuff the box with a little more than one pound of polyfill as one pound per one cuft seems to be the rule of thumb.

Will I need to make the box's internal dimensions larger due to that polyfill stuffing amount? Or, in this sealed enclosure configuration, does the amount of polyfill not affect the resultant Q?

I can always make the box's depth deeper to compensate for the polyfill's modification of the Q (i.e. bring it back to 0.707) if need be.

Thanks a ton!
Joe
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Old 14th November 2007, 05:04 AM   #2
GM is offline GM  United States
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Greets!

No, the stuffing will actually lower Qtc due to making the box Vb appear acoustically larger: http://web.archive.org/web/200210070...ces/fiberfill/

GM
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Old 14th November 2007, 05:17 AM   #3
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Hi GM!

Thanks for the info there. I had read about polyfill causing a driver to 'see' a larger enclosure than truly exists but I wasn't 100% sure of that info's accuracy and I can always cut-down a too-large box rather than try and make it bigger later, ya know?

With those dimensions I gave above, and with the polyfill I intend to stuff, I will bring the enclosure from a 0.707 (Butterworth Q) closer to a Bessel Q, right? That would be OK I guess due to in-car cabin gain. I just don't want to go in the other direction called "Enhanced Q" which I've seen causes a peak usually around 40-50Hz.

Thanks again and please do offer any other suggestions! My goal is to measure twice and cut once.

BTW, I just noticed your 'loud is beautiful if its clean' signature. That is EXACTLY what I'm aiming for. The SR124 will complement the SR6500's in front which really do sound astounding.

Also, thanks for that link there! I'll read over the info tonight!!!

Joe
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Old 14th November 2007, 08:48 AM   #4
simon5 is offline simon5  Canada
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If you go past a certain threshold, adding polyfill will start bringing the Qtc back up where it was.

Too little is like too much...

1 lb per cu.ft is perfect on the other hand, you will achieve nice results and probably near Bessel alignment like you suggested.
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Old 16th November 2007, 10:21 AM   #5
GM is offline GM  United States
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Greets!

You're welcome!

Yeah, box 'stretchers' tend to blow them apart at the seams.

Since a car's cabin gain rises at ~12 dB/octave, a ~0.707 Qtc is typically the goal, so a smaller cab stuffed to achieve this is the norm AFAIK, though with sealed you can be technically pretty far off before you'll notice it's obviously over or under-damped. I prefer low Q vented to get increased efficiency, but they're much bigger in today's shrinking car world, so not an option for many.

My interest in car audio peaked when I installed what in today's parlance would be called a horn loaded phase delayed 4.1 channel system with separate 8 track tape deck in my late wife's Camaro SS back in '70, so not familiar with any current after market speaker systems, though the first gen. Infinity Kappas I installed in a car and truck many years ago were quite good compared to the Mazda and Chevy factory 'premium' units they replaced and better overall IMO than the more expensive MB Quarts' separates systems that was the rage back then. Still got the 8 track complete with Cadillac logo from when I upgraded to cassette. Hmm, wonder if the vintage car crowd has any $$$ interest?

Anyway, just scanned the Polk WPs on these and they do appear to be primo offerings, though a shame they're not more efficient for when you want to crank them.

What year/make/model vehicle you installing them in?

GM
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Old 16th November 2007, 10:31 AM   #6
GM is offline GM  United States
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Quote:
Originally posted by simon5
If you go past a certain threshold, adding polyfill will start bringing the Qtc back up where it was.
Greets!

Yeah, as noted in the article, polyfil has some interesting thermal properties compared to the fiberglass and cotton batting I used, which requires less for a given alignment and doesn't roll off in the other direction until a greater stuffing density is used and WRT vented alignments I never could cram enough in to find a point of raising its Q when trying to make a cab truly aperiodic.

GM
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Old 18th November 2007, 07:31 AM   #7
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Hi GM!

The sealed enclosure SR124 will be in a 1998 Jeep Grand Cherokee with nothing else in the trunk space (a full size spare tire is already locked-down atop the roof via the Yakima rack). I intend to have the SR124 firing toward the back gate, with the enclosure right behind the rear seat back, and have it heavily mounted/reinforced to eliminate any sort of physical movement. I'd hate to have a weighty object flying forward during an accident, ya know?

The Polk SR6500's already installed up front sound phenomenal! I used custom kick panels for those and damped those properly.
Joe
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Old 18th November 2007, 09:52 PM   #8
GM is offline GM  United States
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Greets!

Hmm, not familiar with its construction, but if you can, fill any door, etc., cavities back at least to the wheel wells with as much damping foam as practical and add sheet damping to the floor.

Good plan, I lost a friend when he didn't secure his toolbox in the back of his hatchback some years ago.........

GM
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Old 19th November 2007, 09:09 PM   #9
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Thanks for the advice there. And, sorry about your friend. I too often have tools in the trunk and should be more mindful about those.

Joe
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Old 25th November 2007, 01:23 AM   #10
Volenti is offline Volenti  Australia
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From the testing I've done the effect on Q comes more from resistive damping than from thermal effects of fill.

Case in point, it's possible to end up with a lower system Q than the driver it's self has in free air using polyfill.
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