[woody]

Das Problem is bekannt und Wiki weiß was dazu:

Zitat: In a closed-
box, the air inside the box acts as a spring,
returning the cone to the 'zero' position in
the absence of a signal. A significant increase
in the effective volume of a sealed-box
loudspeaker can be achieved by a filling of
fibrous material, typically fiberglass, bonded
acetate fiber (BAF) or long-fiber wool. The
effective volume increase can be as much as
40% and is due primarily to a reduction in
the speed of sound, and not to the popular
misconception of a change in operating
conditions from adiabatic to isothermal . The
enclosure or driver must have a small leak so
internal and external pressures can equalise
over time, to compensate for barometric
pressure or altitude; the porous nature of
paper cones, or an imperfectly sealed
enclosure, is normally sufficient to provide
this slow pressure equalisation.

http://en.wikipedia.org/wiki/Loudspeaker_enclosure