The OEM sub-woof in the TTR is a joke; everybody knows that.
I replaced mine some years ago with a Fosgate Punch 8", driven by a 5-chan amp.
The sub section is a 100W amp.
It sounded pretty good, but the "enclosure" was crap.
The metal frame vibrated, and even with loads of Dynamat, it still buzzed.
A radical change was due.
I modeled this after a post on the UK forum some 2 years ago, where someone put in 2 subs behind the seats in a TTR. The posting explained some radical internal cuts to get the results.
I have had my rear panels off so many times, I figured, what the hell.
First step, remove the seats.
I wanted to clean the interior anyway. What dirt! First time I had both seats out.
Remove the center frame where the OEM sub is. About 8 bolts,nuts to get it out.
Sorry, no good pic of it, but here's a mod to it.
Cut the back off, so it's just a front frame.
The part on the right of the cut here is the wrap-around back frame the the OEM sub sits in.
Remove this bracket, cut off the studs.
You now have an opening like this.
Build a box.
Measure the opening in various places, construct a box. I used 10-ply 3/4" plywood, not MDF, or any pressed board crap. I'm not building a show-winner, I just want good sound. I don't need any calculus to figure sq. footage either.
This ain't rocket science, and I don't need a heavy box. The plywood box is very strong, tight and more important, it fits the opening. You'd be surprised how much room is back here.
The box is nearly 17" long, 12" deep, and at least 5" high all the way back.
Some "custom" cutting is needed to match the curves of the rear frame under the top area. It took about an hour to build the box, and fit it well.
Paint the box, pack the sides and back with fiberglass insulation, install the subs.
These are 6.5" sub-woofs, not expensive-You don't need JBL's or others.
These have big voice coils, and lots of cone movement.
Use the front openings along side the woofs as openings for air movement.
There's your "tuned ports".
Like I said, it ain't rocket science to do this.
I used a rear side panel from a coupe, cut out the speaker grill, and matched it to the opening in the roadsters rear panel.
It's not a perfect fit, and one corner has a curve to it, but see, it's behind the seat, and it won't really show.
I can bend the upper left curve down when I glue the grill into place.
This shows the initial placement.
I've been driving around for 3 days now and the sound is exceptionally good to spectacular.
Much better bass, not boomy, not harsh, smooth, and it has a punch with some good source material.
Sounds better with the iPod-music than FM.
Panels go back on this weekend.
NOTE: I know some of you audio-people will not approve of my methodology.
This is not a true "tuned" system, nor is it designed for show-quality sound.
it's a daily driver, augmented audio system in a daily-driven car.
I have 4 MBQuarts, 2-6.5, with 1" tweets in the doors, and 2-4" coax in the rears.
Overall sound quality is miles above the OEM system.
I like it.
I just wanted to show you what can be done.