Make a small box act like a larger one with polyester fiberfill
The word "FIBER" is turning up in a lot of hip conversations these days – you know, the
ones that take place in art galleries, bistros, and install bays. In the galleries, they're
talking about the fiber-optic conduits through which compressed digital audio and video
will travel when the Intergalactic Superhighway concludes the long and winding road to
our homes. In the bistros, they're talking about the colon-scrubbing glory of fiber-rich
delicacies like oatmeal quesadillas and bran flan. But, to us – the few, the proud, the
mighty Box Builders – fiber means dacron-polyester fiberfill, that magic and powerful
ingredient that helps deliver maximum bass from a tiny space.
(lb/ft³ ) System Resonance
It's no secret that you can use fiber- fill to make low-end magic; clever installers have
been using it for years Take two boxes of the same size and type, insert the same
woofer into each one, and stuff one with some fiber-fill – the one with the stuffing should
kick out lower bass.
In simple terms, it works like this: The fiberfill fools the woofer into thinking that it's in a
larger box (one with more air, or internal volume, in it). than it really is. And, in general,
the larger the box, the lower the bass you can get out of it.
Fiberfill stuffing is a popular alternative for people who can't or don't want to allot a lot of
space for a subwoofer box. A compound or Isobarik configuration, which pairs two
woofers in one box, is another popular option, though it has some considerable
downsides: For one thing, you have to buy two woofers. There is also a theoretical
sensitivity loss (on the order of 6 dB) because you end up with twice the cone mass,
though you can cut your losses – losing only a few dB SPL – by running a pair of the
drivers in parallel.
(lb/ft³ ) System Resonance
The particulars of fiber stuffing are pretty interesting: The air inside your enclosure
actually heats up as your woofer moves, making the air stiffer. (I'm absolutely serious.)
When the enclosure is stuffed with fiber, the fibers wiggle, dissipating some of the heat
and making the system work as though the box were larger. Theoretically, your
woofer/box bass system can act like a system that's a maximum of 40 percent larger
when you've latched onto the right stuffing recipe – in other words, if you have an
enclosure that offers 1 cubic foot (1 ft³ ) of internal volume, in a perfect world a good
stuffing job will make it perform like an enclosure that offers 1.4 cubic feet of internal
There are three types of stuffing that are commonly used for this purpose: fiberglass
insulation, long-fiber wool, and polyester fiberfill. Fiberfill is the best choice because it
doesn't come loose and fly around and irritate your skin or lungs like fiberglass, it works
as well as either of the others, it's a lot cheaper than wool, and moths hate it. I recently
bought five 20-ounce bags of it at $1.99 a pop (a total of 6.26 pounds for $9.95) at
Minnesota Fabrics; that turns out to be about $1.60 a pound. You should be able to find
some at any fabric store or in the bedding section at friendly stores like K-Mart or Home
To evaluate the effectiveness of box stuffing, I used an MLSSA analyzer to measure the
impedance of three enclosures – 5.l-cubic-foot sealed, 1.4-cubic-foot sealed, and
1.4-cubic-foot ported (the port measured 3 inches in diameter and 6 inches in length) –
with various densities of stuffing. For the sealed boxes, I was able to determine the
effective box size – as enhanced by the stuffing – using the system's resonant-frequency
and Qes values. For the ported box, I compared the tuned frequency of the empty
enclosure to the tuned frequency of the stuffed enclosure, using the Speak for Windows
computer program; this enabled me to find the effective box size that fit the actual
resonant frequency I'd measured.
(lb/ft³ ) System Resonance
In each case, the news was good – make that very good. With all three boxes, I enjoyed
roughly 25 to 35 percent of "space gain" by using stuffing at a rate of 1 to 1.75 pounds
per cubic foot of internal volume.
When making system performance predictions, be aware that the Qes figure – and,
therefore, the Qts figure – of the sealed boxes dropped. And with the ported box, the
peak of the impedance curve on the lower side of the tuned frequency became heavily
damped below the box's point of resonance. I also found that there is such a thing as too
much of a good thing: System resonance (Fsb) rises again, beginning with densities of
around 1.5 pounds of stuffing per cubic foot of box volume; this happens because the
fibers are jammed so tightly together that they stop wiggling and, consequently, stop
I also found that stuffing gets less effective as box size increases. The morale: The
bigger your box is, the harder it is to fool your woofer.
A few rules of thumb: Stuff small enclosures – those with up to about 3 cubic feet of
internal volume or less – with 1.5 pounds of fiberfill for each cubic foot of internal volume
and you should get about a 30-percent increase in box volume without seriously affecting
other performance variables. For larger enclosures, add stuffing at a rate of
approximately 1 pound per cubic foot and you should get a virtual-space boost of about
25 percent. One thing's certain: You'll impress the heck out of your friends at the art
gallery and bistro.
CAR STEREO REVIEW®
Poly-fill isn't necessary in all applications. If you built the box big engough, you shouldn't need it. You should get plenty of low-end extension with out fill. I only use it when I don't have the room for a larger box.