For my subwoofer I selected an NHT1259 woofer. I've
used it in another project and I was very happy with it.
Select a subwoofer based on a few parameters. The
most important are FS (resonant frequency) XMAX
(maximum throw) and QTS (total Q). I want the resonance
as low as possible, the maximum throw as high as possible and total
Q appropriate for the box style I have in mind.
In the case of the NHT, FS is 16.5 Hz which
will produce an awesome low end. XMAX is 13mm which is
a lot, and implies plenty of sound output, and QTS is
.445 which is appropriate for a sealed box (my preference for a
You can use a vented box for a subwoofer. Venting
produces higher efficiency and a deeper low end but it uses a much
larger box and a vented woofer can have serious problems with excursion
if fed frequencies below the resonance of the system (excursion
grows dramatically below system resonance). A sealed box uses a
smaller box and has no excursion issues but it will not go quite
as low and it does have lower efficiency. The NHT has plenty of
sound capability thanks to the really long throw so I'm not worried
about efficiency that much.
Here's a picture of the woofer. Click it for a larger
view. It's a 12" diameter woofer with a very big magnet.