How much polyfil/damping material?

[Bent]

how much fill is recomended per ft^3 of enclosure space in a sealed sub?

[ThomasW]

0.5-1.5 lb/cu ft....

https://www.htguide.com/forum/showthread.php?t=17794

[Curt C]

If the sub enclosure was designed for fill, than add sufficient fill to obtain the target Qtc, as measured by direct or indirect impedance measurements. How much is dependant on the design and what the designer intended. In lieu of this, merely stuff it until it sounds best to you. Thomas's link is good information.

OTOH: With all deference to Thomas and Ken Kantor, and in my opinion, there are many good reasons to not stuff a sub enclosure. The driver pass band is generally below the standing wave frequencies that would be generated for typical subwoofer enclosure dimensions. Panel flex can be better controlled by well-braced, ridged construction. IME, heavily stuffed enclosures can sound lifeless, will exhibit higher f3’s, and can lose a couple of dB of sensitivity compared to a properly designed unstuffed enclosure. Unfortunately, it will also be a larger enclosure. Consequently, much depends on the driver parameters and space constraints. -No one wants a 30 cu. ft. sub enclosure in the corner… :rofl:

My responses apply to home subs. Car subs are usually designed to maximize other constraints, space and power handling chief among them.

C

[Bent]

Ive built this:



for my two 15" RL-p's (D4),
Incomplete build photo here:



It's 44.5" tall, 22" wide, 22" deep, I'm guessing just under 8ft^3 internal after taking into account bracing and glue drips.

I'm wondering how much I should stuff it, give-or-take, I'd prefer not to have to pull the drivers out any more than needed.

It's used in a large room, no doors that can close, with a BFD 1124p

Oh, BTW, I lost interest in car subs when I was 18. (22 years ago).

[looneybomber]

how much fill is recomended per ft^3 of enclosure space in a sealed sub?

Depends on your enclosure size. Small sealed boxes can benefit from up to 1.75lb/ft. Large boxes are less responsive to fill and may only benefit up to .75-1lb/ft.

http://www.diysubwoofers.org/talkshop/messages/41415.htm

[Bent]

Do I just "stuff" it in?
or do I suspend it somehow in the enclosure?

Does contct with the driver(s) become an issue?

[ThomasW]

Fluff it up and throw it in the box..

[Curt C]

Oh, BTW, I lost interest in car subs when I was 18. (22 years ago).

Hah! You're still a cub! unk:

I modeled those subs in WinISD using the specs listed on the Sound Splinter site. Sealed in 8 ft^3 nets a Qtc of .685 -unstuffed, and about .6 fully stuffed. While you certainly can stuff them, they won't need it in that volume. Their modeled f3/f10's are virtually identical at 49Hz/28Hz and 50Hz/28Hz respectively.

Now, just to muddy the waters a bit: Punch a couple of holes in that enclosure and add a pair of 4" x 12" ports. The result will be an alignment tuned to 24 Hz that emulates a sealed transfer function down to f10, and an f3/f10 of 39Hz/21Hz. -Significantly better extention for HT effects, and very acceptable group delay numbers to boot.

FWIW: For monsters like these, I'd suggest 1.5" enclosure walls, and even thicker on the front baffle.

And for comparison: My last sub was a 5.25" in an 8 liter bandpass configuration for an aerospace application. (Space is at a premium on aircraft.) No, it didn't get very loud, but it had the same f3 as yours...

C

[mikela]

sealed Statements

Curt,

I was thinking of lining the sealed statements with 1.25 Sonic Barrier on the back, top, and sides as well as the immediate side of the midrange TL that faces the woofer. Nothing beyond the woofer cavity. Is this OK? I was thinking of mounting the crossover on standoffs on one of the sides above the Sonic Barrier or maybe forgoing the SB on the TL surface and mounting the crossove there. I have also considered using 703 or perhaps polyfill.

Mike

[Bent]

Curt, the advice is appreciated, but I have built a ported sub already - a copy of Thomas and Jon's AS-15 Klone. It works astoundingly well, but my workmanship is a little lacking.
(on the subwoofer, not the boy...)



I wanted to try a larger sealed sub - plus work a bit on my craftsmanship as well as eek out the best performance I can do.

[ThomasW]

Curt and I will agree to disagree on whether or not stuffing is needed/wanted....

My experience is that stuffing has improved the performance of any sub I've put it in....and since it's so cheap why not give it a try?

[Paul Ebert]

IME, heavily stuffed enclosures can sound lifeless, will exhibit higher f3’s, and can lose a couple of dB of sensitivity compared to a properly designed unstuffed enclosure.C

So, what constitutes a properly designed unstuffed enclosure? I could probably hazard a guess, but I'd like to hear what you have to say on the subject, Curt.

Thanks.

[looneybomber]

I don't recommend bare enclosures. At least line the walls with something to help dampen mechanical noises from coming through the cone.

[subynube]

Well, I guess while we are on the topic, is it recommended to line all enclosures with proper damping material? Or is it OK to go with unlined walls? In this thread ThomasW recommended that all sub enclosures should be stuffed with polyfil. How about damping material?

[ThomasW]

In this thread ThomasW recommended that all sub enclosures should be stuffed with polyfil.

Sorry for the confusion. I've never used polyfil for damping in a sub nor do I recommend it. Fiberglass or wool are far better damping materials.

http://www.bobgolds.com/AbsorptionCoefficients.htm

[kingpin]

Sorry for the confusion. I've never used polyfil for damping in a sub nor do I recommend it. Fiberglass or wool are far better damping materials.

http://www.bobgolds.com/AbsorptionCoefficients.htm

So in a ported sub do you line the walls with the full thickness of the fibreglass bats. In my case 2.5" thick or is that too thick. I remember reading a couple threads where it was recommended to the op to peel some of the fibreglass off as it was too thick. I even wrapped the outside of the ports in mine. lol

Mike

Ben. Sorry for all the hijacks in your thread.

[Bent]

no problem with hijacks at all.
I prefer to think I've sparked some good conversation - carry on as if you were normal.

[kingpin]

no problem with hijacks at all.
I prefer to think I've sparked some good conversation - carry on as if you were normal.

Hahaha.
Wait a second. Was that a dig. :E :rofl:

[ThomasW]

So in a ported sub do you line the walls with the full thickness of the fibreglass bats. In my case 2.5" thick or is that too thick. I remember reading a couple threads where it was recommended to the op to peel some of the fibreglass off as it was too thick. I even wrapped the outside of the ports in mine.

The thickness of the damping depends in the size of the box and what you're trying to accomplish with it. In the example you cite he was way beyond the standard definition of "heavy fill", that's why I told him to remove some material.

[subynube]

Yes Bent,
Im also sorry for the hijack. Thanks for allowing this!
My last question then is how much damping is recommended to line walls for sub enclsosures? Is there a formula of damping per cubic feet? Is it different for ported versus sealed? And finally, is it recommended to use damping material as well as polyfil?

Thanks all. This sure did get off track, but at least it is all similar topic.

[ThomasW]

Is it different for ported versus sealed?

Ported boxes are lined with damping material, sealed boxes are stuffed

And finally, is it recommended to use damping material as well as polyfil?

Polyfil is a damping material.

FWIW, these questions have been answered numerous times. That's why using the forums search function is a good idea.

[Paul Ebert]

I took a look at the data on the link that Thomas provided. I noticed that the coefficients were quite a bit higher for when the material was 16" away from the wall, especially in the lower frequencies. Has anyone experimented with leaving some space between the material and the cabinet sides?

[ThomasW]

I took a look at the data on the link that Thomas provided. I noticed that the coefficients were quite a bit higher for when the material was 16" away from the wall, especially in the lower frequencies. Has anyone experimented with leaving some space between the material and the cabinet sides?

That's SOP for making really low frequency bass traps. It's not going to do anything special inside the enclosure of a subwoofer

[Curt C]

Curt and I will agree to disagree on whether or not stuffing is needed/wanted....

My experience is that stuffing has improved the performance of any sub I've put it in....and since it's so cheap why not give it a try?

Actually, I will defer to our illustrious moderator on the stuffing issue. Certainly tossing some in can’t hurt, and if it provides audible benefits, it’s all upside.

I will throw out some statements for discussion, and will enjoy everyone’s opinion, as adding stuffing can have additional effects one might not expect.

The obvious reason to add stuffing is to increase the acoustic compliance of the enclosure. This is often used to lower the system Q to obtain some target Qtc and transfer function in a smaller enclosure. This effect of stuffing is well documented and easily measured with an impedance sweep. There is a practical limit to this as at some point the stuffing increases the system moving mass and the damping losses. This can result in significant losses in efficiency, and represents an upper limit to how much we can realistically change Qtc.

Another benefit of stuffing is to partially absorb the acoustic energy from the back of the driver cone. This may be true in many cases, but keep in mind the absorption coefficient of most material used for stuffing declines with frequency. It’s generally well documented that the absorption coefficients at typical woofer and sub frequencies are very low compared to midrange and lower treble frequencies. That is, they offer little absorption of acoustic energy at those frequencies.

Consider a typical transmission line will have acoustic output at its terminus rivaling a traditional vented enclosure port. Yet this acoustic energy may go through several feet of stuffing in a transmission line. This example offers empirical evidence that traditional stuffing materials fair poorly at absorption of low frequencies.

A third benefit often attributed with stuffing is damping the enclosure walls. That is, making them less prone to resonate and ‘leak’ acoustic energy through them. In my opinion, lining a sub or woofer enclosure walls with stuffing to attenuate the acoustic energy before it gets to them seems to have little merit, given the absorption coefficient example above. It would seem more advantageous to line the enclosure with a heavy damping material such as loaded vinyl, layers of roofing felt, or a constrained layer construction. Of course thick walls and multiple braces will help as well. If, through bracing, the enclosure walls are broken up into smaller panels whose resonance is above the pass band of the driver, they cannot resonate.

Edit: Added plot of stuffing absorption vs. frequency.

C

[Dennis H]

If you want to eliminate box resonances (internal standing waves, not wall vibrations), it works out that the insulation on the walls needs to be some fraction of the box length in that direction. Longer dimensions mean a lower resonant frequency and that needs thicker insulation to damp it. Zaph had something about it on his blog where he needed a pretty good plug of insulation at the bottom to damp the vertical modes of a tower. Here's a pic from SL where he shows the nearfield response of a sub driver in a 21.5" long box with no damping (he doesn't believe in it but that's another thread -- I think he's wrong.) To damp that, you'd need enough wall lining on the back to absorb a 300+ Hz wave. If it were a 43" tower, the resonance would be an octave lower and you'd need more stuffing at the top/bottom to damp the resonance.

[ThomasW]

Zaph had something about it on his blog where he needed a pretty good plug of insulation at the bottom to damp the vertical modes of a tower.

It's the March 27th entry in his blog

http://www.zaphaudio.com/blog.html

[Curt C]

It's been my experience that stuffing ameliorates standing waves best when placed at a point in the enclosure where the wave velocity is the highest. For the fundamentals, this point is equidistant from the enclosure boundaries. IOW, in the center of the box. Note in the attached pic that the velocity of the fundamental standing wave is '0' at the enclosure walls.

For all practical purposes, sub enclosures cannot support standing waves due to the wavelength of the frequencies involved.

C

[Bent]

Well I'm close to buying a bunch of polyfill pillows...

[Amphiprion]

PSB used to put the stuffing in the center of their enclosures for the reason stated by Curt C, IIRC.

[itssmoke]

wow !! you guys know your stuff , and with subjective oponions. trial and error = personal taste. i always thought you added filling(stuffing) to trick the driver into thinking it was in a larger enclosure. the driver can then reach its potential. you ad damping(side wall lining ?) to eliminate internal standing waves. im suprised to hear that a sub woofer cant have standing waves. i understand not wanting to resolve yourself to trial and error(installing... removing) the drivers ,, but sometimes thats ok. if when installed the sub goes whoompf. then it needs stuffing. if not ...but... theres still something not quite right then it needs lining. ive always liked sealed boxes. i also agree i have never heard a n enclosure that was hurt by adding lining. tune a port right is ok but sometimes you can notice the ports rushing sound. i will also add that mobile audio has come a long way. some manufacturers produce very musical drivers that will work nicely in home applications. anyway, i waded in because speakers are so subjective , variables everywhere! sometimes on paper just wont play certain rooms. gl best regards