Cabinet Stuffing polyfill/foam

Discussion in 'Amps and Cabs [BG]' started by basss, Feb 11, 2005.

  1. basss

    basss Supporting Member

    Aug 27, 2001
    I have read that stuffing a cab with acoustic foam will effectively increase the volume of that the driver "sees" by 30%. I know that this can increase low end extension but what effect does it have on the mids & highs?

    I will probably use Acousta-Stuf Polyfill from Parts Express. What is the best way to stuff a cab with this material? Do I need to keep it away from the back of the driver and port? If so what is the best way to do this?
  2. IvanMike

    IvanMike Player Characters fear me... Staff Member Supporting Member

    Nov 10, 2002
    Middletown CT, USA
    i always thought the foam/whatever was to tame a lot of upper mid type frequencies............never heard that about it adding "apparent" cabinet volume and/or increasing low end response :confused:
  3. The Clap

    The Clap

    Jan 5, 2004
    Scottsdale, AZ
    Stuffing is done more predominantly with closed box designs, and there is an important difference between lining the walls with fiberglass, using stuffing throughout, foam, etc. The uses and pitfalls of stuffing a ported cabinet should be looked into before you try anything, because the 30% apparent volume quote is generally applied to closed boxes. As an example, stuffing that is not well tied down near a port opening can do more harm than good by impeding airflow if it gets sucked into the port opening.
  4. ESP-LTD


    Sep 9, 2001
    As has been said, there is a difference between stuffing a box with polyfill to gain apparent volume and lining it to cut reflections.

    It's not unusual for ported cabs to be lined with something to absorb reflected highs that come off the back of the drivers. The idea is that you don't want them reflecting off the wall and then into the back of the driver and thru the cone out of phase with the initial signal.

    I've never made any lab measurements before/after to compare different products. If you use fiberglass insulation I'd avoid paper-backed because it can rattle.
  5. basss

    basss Supporting Member

    Aug 27, 2001
    I think someone here (bgavin?) mentioned that box stuffing to get the volume gain works for ported boxes also. If I were to try this I'm wondering if anyone has ideas as to how to keep the stuff away from the speaker and port.
  6. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    The advantages of stuffing to alter the Q of a box and gain added extension are greatly over exaggerated. In many cases stuffing or lining of the box will actually hurt bass response. It works best with a driver specifically engineered for this practice, and pro-sound drivers aren't.

    Lining or stuffing is an absolute requirement to absorb midrange reflections in the box that otherwise reflect back to the cone and cause all sorts of phase related response peaks and dips. If your cab is fully lined with an inch or so of polyfill or fiberglass or foam you're probably OK. If not, make it so. Acousta stuff is OK when stuffing is required, as in TLs, but inexpensive polyester furniture upholstery batting stapled into the box to eliminate reflective surfaces is sufficient for your needs.
  7. basss

    basss Supporting Member

    Aug 27, 2001
    Does a more dense material like egg crate foam actually decrease box volume?
  8. I'm interested in something...

    I've got a Hartke 4x10 (Transporter) cab run off of a Hartke 2000 amp. To my ears (which aren't the best, but others have said the same about it) it's really "boomy". I just stuffed some of that cheap polyfill from wal-mart in there to see if it helps. But now that I'm reading this, I'm thinking it might not.

    Anyway, we play tomorrow night, so I'll post after that the results.
  9. With sealed boxes, stuffing with fiber fill at a ratio of 1 pound per cubic foot, results in a measureable apparent increase in cabinet volume of about 30%. This is somewhat driver and cabinet volume dependent. YMMV.

    This is measured by calculating the effective volume using an empty sealed box at its Fb, and the measuring again with the full stuffing. The techique changes the internal operation from adiabatic to isothermal. In short, the speed of sound changes in the stuffing material, making the box appear "larger" to the driver.

    In theory, there is no reason an air chamber in a vented cab cannot do the same as a sealed cab. The material would have to be restrained with netting, etc, to keep it away from the vents. Again, in theory, the volume filled with stuffing should react as if it were a larger volume.

    In practice, I don't stuff my sealed boxes, nor my vented ones. I find the size reduction is not worth the hassle.
  10. thejohnkim


    Sep 30, 2003
    from my experience with my own DIY home theater subwoofers, it would be wise to stay away from the driver when you stuff. dont stuff the ports, but you're free to put a little stuffing on the wood baffle that the port is mounted on, just make sure its all fixed well with something like that 3m spray adhesive from parts express so bits dont come shooting out while you play. i would also keep it as close to between 1" - 1.5" (at teh very most) all around as possible. if you're highs are being handled by a tweeter or horn then it probably wont ahve a noticable effect. i dont know about the mids, because as i said, i've only stuffed subs that are crossed over below 80hz
  11. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    As far as true subs go stuffing or lining can do more harm than good. Damping reflections is only required if the longest internal dimension of the box approaches 1/4 wavelength. At 100 Hz that's about 2.8 feet. Even then long wavelengths tend to go right through damping material as if it wasn't even there. All things considered damping materials are really only required when the passband of the speaker runs above 200 Hz, minimum. That's where wavelengths get small enough to be problematic, and damping materials have enough effect to make using them worthwhile.

    The notion that stuffing significantly alters the speed of sound through the medium was a pet theory of Bailey when he experimented with transmission lines back in the 60's and 70's. That tack has been pretty much abandoned, as the actual alteration in the speed of sound has been shown to be insignificant, although the necessity for lining/stuffing TLs to control midrange reflective modes in the pipe is valid.
  12. basss

    basss Supporting Member

    Aug 27, 2001
    Thanks all. I ended up using 1 1/2" eggcrate foam as a lining rather than stuffing the whole thing. Turned out great.
  13. Petebass


    Dec 22, 2002
    QLD Australia
    I've only ever tried completly stuffing a cab twice. The first time on a sealed 10" car audio sub, worked a treat. The second time in a ported 1x12 enclosure I'm currently grappling with - the speaker hated it.

    I suspect the car audio audio speaker didn't mind the stuffing because the rear of the magnet had no visible venting grate. The 12 does, so the stuffing stiffled it's performance.

    If stuffing sealed cabs seems to work better then on ported cabs, it's probably just because sealed cabs are more forgiving for cab misalignents anyway.
  14. Well here's the results I noticed from packing polyfill in my hartke cab. BTW, I packed one bag into each side/half of the cab, so two bags.

    Low end has a definitely more noticeable presence. Resonation (boomy sound) is gone. Cab sounds tight and clean. Definite dark tone, but that's what I like. I was able to turn up my treble a little more without it being harsh and resonating so I got a little more clarity.

    Overall, I like it for now. I'll give it a few more gigs before I really make up my mind though.
  15. BassGreaser


    Aug 22, 2002
    Austin, TX
    will stuffing working on my late 70's bassman sealed box 2x15?
  16. The problem with subjective tests (read: use your ears) is the tests are biased in favor of what the user wants to hear. Worse, unless the user can do fast switch A/B blind comparison of the the cabs with/without stuffing, the ear has a short memory.

    So, "it's all good." If you want to stuff, have fun. If not, have fun.