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
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
Greets!
No, the stuffing will actually lower Qtc due to making the box Vb appear acoustically larger: http://web.archive.org/web/20021007....cyberglobe.net/caraudio/resources/fiberfill/
GM
No, the stuffing will actually lower Qtc due to making the box Vb appear acoustically larger: http://web.archive.org/web/20021007....cyberglobe.net/caraudio/resources/fiberfill/
GM
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
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
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.
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.
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 $$$ interest?
Anyway, just scanned the Polk WPs on these and they do appear to be primo offerings, though a shame they're not more efficient for when you want to crank them.
What year/make/model vehicle you installing them in?
GM
You're welcome!
Yeah, box 'stretchers' tend to blow them apart at the seams. 😉
Since a car's cabin gain rises at ~12 dB/octave, a ~0.707 Qtc is typically the goal, so a smaller cab stuffed to achieve this is the norm AFAIK, though with sealed you can be technically pretty far off before you'll notice it's obviously over or under-damped. I prefer low Q vented to get increased efficiency, but they're much bigger in today's shrinking car world, so not an option for many.
My interest in car audio peaked when I installed what in today's parlance would be called a horn loaded phase delayed 4.1 channel system with separate 8 track tape deck in my late wife's Camaro SS back in '70, so not familiar with any current after market speaker systems, though the first gen. Infinity Kappas I installed in a car and truck many years ago were quite good compared to the Mazda and Chevy factory 'premium' units they replaced and better overall IMO than the more expensive MB Quarts' separates systems that was the rage back then. Still got the 8 track complete with Cadillac logo from when I upgraded to cassette. Hmm, wonder if the vintage car crowd has any $$$ 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
simon5 said:
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
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
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
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
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
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
Joe
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.
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.
Make sure you keep the fill away from the basket unless you want to alter the T/S parameters. Qms can be dramatically reduced if fill gets anywhere near the cone/basket and causes excess resistive damping.
BTW, I also mostly doubt the thermodynamic (adiabatic-isothermal) properties of fill.
BTW, I also mostly doubt the thermodynamic (adiabatic-isothermal) properties of fill.
Thanks again for the most informative suggestions!
I have yet to hear back from Polk Audio regarding the actual 'true' internal volume of a sealed enclosure for this subwoofer. As aforementioned, their website and whitepapers specify different values!!
I may just build the enclosure at the larger of the two volumes and have the transfer function of my vehicle kick-in the lower bass. I lean towards Butterworth/Bessel versus Enhanced-Q anyway (i.e. peakiness)
Joe
I have yet to hear back from Polk Audio regarding the actual 'true' internal volume of a sealed enclosure for this subwoofer. As aforementioned, their website and whitepapers specify different values!!
I may just build the enclosure at the larger of the two volumes and have the transfer function of my vehicle kick-in the lower bass. I lean towards Butterworth/Bessel versus Enhanced-Q anyway (i.e. peakiness)
Joe
I was noting that the discussion on "lowering the Qtc" was not framed as "lowering the Qtc/Fs" ratio. Putting fiberfill into a closed box essentially has the same effect as making the box larger. This will lower the in-box driver Q as well as the in-box Fs, but the ratio Qtc/Fs will not change. The advantage is that for a given alignment, we can make the box smaller than without stuffing. In contrast to fiberfill, fiberglass and wool has excellent damping properties and can add significant losses to the system, causing a decrease in the ratio Qtc/Fs by increasing the mechanical damping of the box-driver system. This may or may not be a desirable property depending on your application. In car audio, I would try fiberfill first to avoid unnecessary losses unless you know that your particular driver-box combination will produce an in-box Q unsuitable to your taste. Personally, it would not be afraid of experimenting with an in-box Q beyond the ubiquitous 0.707.
/Kranis
/Kranis
Kranis said:
Well...
In a closed box there are three parameters that are of interest for the behavouor around resonance; mass M, compliance C and resistive losses R. The mass is tha mass of the cone and co-oscillating air, the compliance comes from the driver and box compliances, and the resistance comes from the driver and also from the box and its stuffing.
From these three parameters one can calculate the resonance frequency fs, and the Q value Qtc of the system:
fs=1/(2*pi*sqrt(MC))
Qtc=1/R * sqrt(M/C)
Now, stuffing increases R and also C (makes C less stiff). The effect the stuffing has on R and C varies with the material. Polyfill affects R only a little, but C a lot. Fibreglass affects both.
Combining the two formulas above results in a fs/Qtc ratio as
fs/Qtc=R/M
Here it can be seen that this ratio is independent of C. So if the damping material affects only C the ratio is unaffected. However, all materials affect R, at least a little, and thus also the fs/Qtc ratio.
I hate to sound worrysome but fiberglass (i.e. fiberglass insulation that can be purchased for homes) isn't something I want floating around the air.
BUT, with a sealed enclosure, I do not think that would be a problem at all.
So, does using fiberglass cause a better distribution between R and C as aforementioned?
Thanks!
Joe
BUT, with a sealed enclosure, I do not think that would be a problem at all.
So, does using fiberglass cause a better distribution between R and C as aforementioned?
Thanks!
Joe
jdgonko said:
There is no such thing as "better" in this case. It all depends on what you want.
If you want isothermalization (change of C), any material is good. Mostly you do, since it results in an apparent box volume increase.
If you want to lower the Qtc of the system and reduce the effects of standing waves, you should select a material that also modifies R. Like fiberglass.
I recently tested polyeter foam* and it turned out to be a good alternative to fiberglass it you don't want it. It can be cut in pieces that fits nicely in the box and is far easier to handle.
*I am not entriely sure of its english name, but it is the stuff that mattresses are made of.
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