I run 18" sub and bolting it between the back seat rear parsel shelf
was the only way I
could get enough area behind the speaker-but it is still kind of in an
enclosure(albeit
not a perfect one) -but have 6/9's in ther own rear cabnets to stop
the sub trying to
push them around and tuning purposes of course.The rear of the sub is
seperated from
the front by the rear firewall/pasel tray otherwise the sound waves
produced from the
front of the speaker would cancle out the waves being produced by the
rear and so on-
that's one of the reasons why a speaker sounds bad until it is in some
sort of cabnet
too stop this cancellation effect.speakers mounted behind the back
seat are normally
boomy and without sealing off the rear of any other speakers that
protude into the boot
they will be badly affected by the soound pressures created by the
sub. Only do this
if you know what your doing otherwise just put it in abox

cdd

I have seen a few "free air sub's" around but these are just normal
bass speakers
some subs if you like that are designed for a ported enclose and have
up to about
15-20mm more throw before the voice coil just ends up hitting the
magnet of leaves
the basket altogether
                                   cdd