Tame The Boom, Grow Your Cab: The Wonders of Polyfill

Too much boomy bass from your cab? First, try a riser. It may be the floor you’re hearing.

If that doesn’t cure the problem, you may want to know more about cabinet stuffing, which you’ve likely heard about in various forums, before turning your 2×12 into a giant plush toy.

First, some names.

  • Batting
  • Dacron
  • Polyfill
  • Padding
  • Hollofil
  • Fiberfill
Kelly LeBrock, but also polyfill.
Polyester fiber does not make for compelling illustration, so we will have this image of approx. 10% polyfill, 90% Kelly LeBrock in Weird Science.

For our purposes, these are all one and the same: light fluffy material made from polyester fibers, usually sold in sheets. You can use the loose fiberfill type, which looks like a cloud trapped in a bag, but it’s harder to work with as we’re not looking to stuff the cab, as you might a hi-fi speaker or subwoofer, so much as line it.

Polyfill vs. Poly-fil®

Don’t make my mistake and assume these are interchangeable. Polyfill is a general term for the compressible polyester fiber stuff we’re talking about; Poly-fil is a brand name used by Fairfield Processing Corporation on several products, including some that perfectly suit this project, and some that do not.

At Joann Fabrics I found what looked like exactly the right material, whitish polyester fiber padding, and it said Poly-fil on the box, which is what I thought I was looking for, having woven the two terms together. I had them cut a few yards, and nearly lost my eyebrows when asked for $45 at the checkout. Apparently I had picked up NU-Foam, a product of the Poly-fil brand indistinguishable to the layperson from plain polyfill, but varying in some revolutionary way probably important to people who shop at Joann Fabrics. After a short cry, having spent good video game money on advanced quilting supplies, I learned that they accept returns on cut-to-order fabric as long as you’ve not done any more cutting.

I got the right stuff for around $12. I overbought to experiment; you could line a cab for $8. It is available from upholstery suppliers, craft stores like Joann, and sometimes Wal-mart has it.

The Other Wrong Stuff

Do not use fiberglass, like home insulation, even though it looks kind of like what you want. It gets in your eyes, your lungs, your skin. If you don’t care about yourself, care about your speakers: this once popular cabinet-filling material breaks down over time and has been discovered invading the voice coils of speakers. It does not improve them.

Jute padding, or carpet underlay, isn’t hazardous or damaging, and works, just not as well as polyfill/Dacron etc. As I understand it, the closed-cell nature of this type of foam means that while it will absorb some of the boom from standing wave problems, it doesn’t have the surface area to perform polyfill’s neatest trick: making your cab sound larger. Oh yes. More on that shortly.

Stuff the Stigma

In some online communities a stigma surrounds lining a guitar cab, as if you are living on the fringes of acceptable society and performing unnatural procedures on unwitting wooden boxes. “It kills the cab,” is the sort of thing they say. Actually, it’s not just the purview of experimental hobbyists; a surprising number of very respected builders do it to get their cabinets sounding so good, most notably Bogner in its oversize 2×12, perhaps the most widely praised 2×12 there is. In addition, Cameron, Diezel, Rivera, Fuchs, Zinky and some upstart called Fender are among those with fluffy white secrets in their cabs.

I Feel Better About My Polyfill. Now Where Do I Put It?

Measuring the inside panels of my cab — sides, top/bottom, back — I cut two matching rectangles of each size from the roll of batting and experimented placing them in the cab in every configuration I could think of. I encourage you to experiment, too: it’s surprising how different amounts on different panels alter the sound. However, I’ll let you know what worked for me.

Most people attach the padding with light spray-on adhesive, or staples. I used brass thumbtacks; I like that I can remove them with little hassle or trace. At the experimental stage, save for the top and back panels, you may not need to attach them at all, as the stuff mostly stays where you put it.

The Embiggening

Avatar cab plus 30%
A frequently circulated number is that polyester batting can make your cab behave as though 30% larger, which Photoshop says is this much.

Here is where I encountered the strange but beneficial side-effect of cab stuffing. The more you put in, the bigger the cab sounds. Paradoxically, in this case filling a space makes the speakers react as though you’ve increased the box’s volume (as in cubic space, not loudness.) Though I started the whole business to tame a boomy bass problem, this stole my attention for a while. Suddenly finding that big woody thunk and chunk under your palm, as though dealing with a much larger cabinet, was arresting. I gather it’s something to do with all the added surface area increasing the time it takes a wave to travel to the cabinet wall and bounce back, which our ears take as a cue the enclosure is bigger.

Hungry for change, it’s easy to overdo things. I had batting on all but one side panel at this stage; top, bottom, left side, back. It sounded wonderfully deep and large, rich mature tones the cab couldn’t make before. That big Les Paul kind of heft. But when I came back days later and found my timing was horrible (worse than usual!) when playing reasonably fast, I realized I couldn’t hear my pick attack. Focussed too much on this addictive new depth while lining the cab, I had stopped hearing the whole sound and murdered my high end.

This led me to respect the oft-quoted rule: don’t cover parallel surfaces.

So, if you put batting on the top, don’t put it on the bottom; if you put it on the left, don’t put it on the right, and so on. Having a real wood surface on every opposing side seems to let the highs breathe. In all configurations, I found this a rule worth sticking to.

This doesn’t prevent adding as much fill as you like. You get better results, say, layering two pieces on the left side panel rather than one left and one right. The same extra sense of size, without the throttled high end.

I had cracked The Embiggening. Treat just one surface of each pair of opposing surfaces. My cab sounded much better.

Sadly I still had the boom problem.

Taming Boomy Bass

Top, bottom, sides, even a vertical hanging sheet of polyfill between the two speakers, none of it had as much effect as I’d hoped on that obnoxious bass hump around B4 and C5, I assume the product of some standing wave issue. What addressed it most directly was doubling up two layers on the back panel. This had the most dramatic effect, in fact, of any of the changes, and my first instinct was to undo it because right away it transformed the sound from mellow to in-your-face.

All that seemed to linger in the back of the box came rushing to the front; mids suddenly sounded very forward, present, the clearest and tightest tone of the dozens of options I tried. It sounded fantastic for riffs and rhythms, crisply gripping every note, but overpowered the big, round, woody chunk I’d uncovered in The Embiggening. My phantom 4×12 was gone. The speakers almost sounded like they were on planks instead of in a box.

Yet it had evened out the bass boom quite nicely.

The solution I settled on was doubling up the other covered panels as well, for yet more apparent volume, which recovered enough of the big box sound to make the best combination of both.

The totals of my roughly one-inch thick batting inside the cab were: two layers on one side, two layers on the bottom, two on the back. Clear and tight but fat and big. Glorious. Done.

6 Replies to “Tame The Boom, Grow Your Cab: The Wonders of Polyfill”

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  2. So, all this batting is only 1″ thick per layer? I had envisioned like 3-4″ until I ran into that comment at the end, “The totals of my roughly one-inch thick batting inside the cab were: two layers on one side, two layers on the bottom, two on the back.”

  3. Just what I was looking for, have the exact same issue with my 2×12 and will definetly try this out.

  4. […] on several forums regarding whether adding batting improved tone or not and I came across this page Tame The Boom, Grow Your Cab: The Wonders of Polyfill | Pick Roar and it made sense to try it out. As for the results my full stack sounds great now. Very 3-D. […]

  5. Brian Patrick Lee says: Reply

    Thanks for experimenting and sharing your results. What is the brand, wall thickness, material, and dimensions of your 2×12? What type of speakers? What specific type of polyfill did you use? I imagine that the density and composition of the polyfill would affect the results.

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