Home Theater Subwoofer

My basement is divided in two parts - one half is the workshop and the other half is finished. In the finished half of the basement there is a wall which has for years been begging to be turned into a home theater screen.

This year I finally put it together...

I suspended a 1080p projector from the ceiling, projecting on a 120" painted screen (yeah, I'm cheap). I installed some dimmable wall sconces and recessed speakers in the walls and suspended ceiling. I added a set of front speakers to complete a full 7.1 surround sound system. The receiver and other components are concealed in a closet. The front speakers are from Pioneer's Andrew Jones series, and the in-wall/in-ceiling surrounds are all from Monoprice.

The Pioneer front speakers are inexpensive but fairly well-reviewed, and are regularly found for clearance prices. However, by far the weakest link in the set is the 8" subwoofer. It's not bad - probably fine for music, but it's just not up to the task of a DTS movie soundtrack.

So... I did some research and found that a home-built subwoofer might actually be a decent project for me.


The first thing I had to do was learn a little something about subwoofer design. I discovered the AVS Forum and Home Theater Shack forum, and I read. A lot. I learned about a program called WinISD that allows you to play around with certain design parameters such as the volume of the enclosure and the size of the port(s), to get the optimal configuration for a specific driver.

The driver I chose was the Dayton RSS390HO 15" subwoofer from Parts Express, which is reputed to be pretty good for home theater use. I chose a 500 W BASH amp from O-Audio, because I liked the selectable high-pass filter and the parametric EQ controls.

I had a piece of 4" S&D PVC pipe left over from my shop dust control project, and a sheet of 3/4" MDF which had already been cut down to 2'x4' pieces - a hand-me-down from a friend. I wanted to design a box to make the most of the materials on hand.

With all of this in mind, I played around with the parameters in WinISD to get the most (I think) performance out of that driver in a relatively small (about 4.5 cubic feet) ported box, tuned to 25 Hz with 4" ports.

WinISD: driver info and theoretical frequency response drop off, with filters.
Of course this does not take room effect into account.

WinISD: speaker cone excursion, with the high pass filter on the BASH amp set at 20Hz.

WinISD: port air velocity, with two 4" ports each about 21" long.
Not nearly enough to generate any noise from port air flow.

Next I had to figure out how to make the most of the pieces of MDF I had on hand, and fit the 4" ports inside the box without getting too close to the walls or the speaker, with adequate internal bracing. I played around in Sketchup until I had a design that I thought looked pretty good.

Including the parts for the inner bracing, I had very little MDF left over from the full sheet - basically just the circular and rectangular cutouts. Considering that a sheet of MDF weighs about 90 lbs and the speaker weighs about 30 lbs... gives a pretty good idea how massive this thing is.


The first parts I made were the front baffle and the inner bracing parts. I wanted to make sure I could fit the PVC pieces together to get exactly the port length I needed for proper tuning. I ended up widening the  two cutouts in the middle bulkhead to fit the elbow collars - I could tell it would make the final assembly easier.

After that I cut out the main speaker hole and rounded over the edges of the ports and all the inner bracing parts. Then I added the back, sides and top/bottom.

The roundovers were all done on the router table, with a 1" roundover bit from MLCS. The ends of the PVC pipes fit into rabbets in the MDF, to form a seamless transition with the roundovers - I actually put a piece of PVC in the rabbeted holes to serve as the bearing guide for the router bit. I cut out around the edges of the braces to try to minimize how much volume they displaced.

Assembly was glue-and-screw, with plain old wood glue. I used epoxy to glue the PVC pipes in their fitted recesses.


Once I had everything glued up, I painted the front, back, and bottom with hammered finish black spray paint.

I used threaded inserts in the MDF to install the driver and the plate amp, and I installed some spike feet on the bottom.

I stuffed it with a bunch of poly fill before attaching the plate amp. I then resawed some sapele to veneer the top and sides.

I think it turned out pretty well...


Frequency response, measured with Room EQ Wizard at the main listening position (i.e., my seat).
This is NOT calibrated for SPL levels, but shows relative peaks and dropoffs.

It's difficult to express how this thing sounds. The best way to describe it is that it's as much of a sensation as it is a sound - sometimes you can feel the sound more than you hear the sound. 

If you're seated in front of the subwoofer, you can feel the air movement - like a puff of wind in your face... a nice extra dimension for canon fire, etc.

I had to go around the room and add padding to the air conditioning vents, etc, so they wouldn't rattle. 

Occasionally there's still a little rattle in one of the suspended overhead fluorescent light fixtures. I'll figure out how to stop that one day.

I cut back some of the power from the amp, so it sounds good without getting out of hand... but still the dishes rattle in the china cabinet upstairs. 


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