Here you can see the internal bracing (3/4" Maple ply), foam sticking out of the variovent, and wires from the plate amp. Dadoes were cut to hold the internal braces.
Subwoofer 1
Intro: On a trip to a local audio dealer I noticed a "blown" 10" Peerless woofer from an NHT subwoofer sitting on a shelf. After negotiating the price down to "free", I took it home and began operating. The spider had come unglued from the former. I removed the dust cap and shimmed the voice coil with playing cards to center the former in t he gap. I then used a plastic syringe and epoxy to reattach the former to the voice coil. Everything worked fine and I got a decent woofer for free. The same model woofer can be found at Madisound for about $90.
Enclosure: For fun I though I'd build a sealed enclosure with a home-built variovent. The enclosure was made from 3/4 MDF with Red Birch veneer. The edges were chamfered and and covered with iron-on Red Birch strips. I also added a 150W plate amp with remote from Apex Jr. for about $99.
Here you can see the internal bracing (3/4" Maple ply), foam sticking out
of the variovent, and wires from the plate amp. Dadoes were cut to hold
the internal braces.
The variovent is nothing more than a 4" plastic (ABS) drain fixture stuffed
with a 2" thick piece of open cell foam. Seemed to work pretty good
on the first try so I didn't bother with further tuning. It was
flush-mounted and held in place with caulk.
The grill was cut from 3/4" pre-veneered MDF with a large oval in the
center. The grill was held with 1/8" window screen spline.
I cut the 1/8" groove around the edge with my router table. No glue
is used to hold the grill cloth in place and can easily be replaced if it gets
dirty or snagged.
I had to place the amp control panel sideways so it would fit on the front
baffle. The remote has no problem operating the unit through the grill
cloth.
The internal volume is somewhere around 1.5 cu. ft. The variovent probably
makes things somewhere around 2 cu. ft.
I also chamfered the grill to match to profile of the front baffle.
Actually I attached the grill to the baffle and cut the chamfer for both at the
same time. I finished this project with a water-based polyurethane.
It turned out ok but I'm pretty sure I wont use it again - dried WAY too fast
for me. Also I think the finish was a bit lacking in depth. Although, it is fairly durable.
Here are the impedance curves with and without the flow resistance vent.
With the flow resistance vent the impedance peak smoothes by about 30% and the
system Q drops a bit. This sub woofer actually sounds pretty darn
good. Basically I was able to get the sound of a sealed system in a
smaller enclosure. Although, the amp isn't the best ever made.
Subjectively, the low end extension is quite decent (depending room placement). I had trouble integrating it into my small listening room so I sold it to my brother who has a nice sized living room and a really great home theater system (now with a great sub woofer).