This article is from the rec.audio.* FAQ, by with numerous contributions by Bob Neidorff others.
The following discussion will focus on practical facts on speaker
cabinet stuffing and on sealed systems. Theory is limited help
in selecting speaker stuffing. Vented system do share a few of
these same issues and will also be mentioned, but the goals and
physics of stuffing a vented box are different than those of a
NHT speakers use polyester fill. Some use a Danish polyester that
mimics the properties of fiberglas very closely. Excluding this
special poly, there are two kinds of polyester available: pillow
stuffing, and audio-spec polyester.
Forget common pillow fill. It's cheap and easy to get. If you
use enough, it will damp the midrange, and that's a lot better
than an empty box but it has little effect on lower frequencies.
"Mountain Mist Polyester Fiberfill" from Stearns Technical
Textiles is a common, inexpensive material that is said to
perform as well as audio-spec polyester. Stearns also sells
"Fiberloft Premium Grade Polyester" to some speaker makers.
Mountain Mist is a coarser fiber than Fiberloft, but both are
the same composition. We have no information on differences in
acoustic properties between Fiberloft and Mountain Mist, but
Fiberloft makes softer pillows and costs more. Both are
available from these chain cloth stores:
House of Fabrics
Jo Ann Fabrics
For more information, contact:
Stearns Technical Textiles
100 Williams Street
Cincinnati OH 45215
513-948-5252 or 800-345-7150
For lining the walls of a vented enclosure to reduce internal
reflections, or filling a transmission line to absorb the back
wave, highly absorptive wool or fiberglas are ideal. However,
these materials do not provide the desired results in a sealed
system. They will provide more reflection absorption than
polyester, but the latter is quite good in this regard in the
critical midrange. In a sealed system you don't want
absorption at lower frequencies anyway; you want damping and
isothermal conversion. (Author's note: I have tried "all-out"
efforts using fiberglas lining and polyester fill to achieve
the best of both worlds. I found little practical benefit over
Most professional designers agree that practical experience,
combined with trial and error is the best way to get optimum
stuffing material, quantity, and method for a given design.
This is why good designers routinely experiment with fill in
the development of a new system. If you are designing a system
that differs substantially in shape or volume or source
impedance (passive crossover) from one of known reference, you
will need to experiment to get best performance.
Adjusting the filling is the last step in getting bass right,
and is used mostly to fine-tune the system Qtc and resonance.
As increasing amounts of polyester are added to a sealed box,
the resonance and Q gradually go down. This can be shown
mathematically to be due in roughly equal parts to the effects
of simple resistive damping and isothermal conversion. At some
point, a minimum is reached, and further material reverses the
trend by taking up volume. An experienced designer can find the
optimum amount of fill in a few trials by monitoring the
impedance versus frequency curve as stuffing is added or
Filling also has the important effect of reducing internal
reflections, to reduce standing waves and comb filtering.
However, the amount of filling has comparatively little effect