Sunday, August 07, 2011

Custom Linkwitz Stealth Subs

The only space we have in our living room for a subwoofer is under our end tables. Most commercial subs won't fit under them, and if they were small enough, they'd have terrible response. So I decided to build custom subs with 12" drivers that just fit. But you can't simply stick a 12" speaker in a 1.7 cu foot box and expect it sound good. More on this later.

The best material I have access to is 3/4" MDF. A 4x8' peice is around $30 and is enough to do both subs with some left over. That sheet is heavy. VERY HEAVY.

Glued and screwed together with titebond II and drywall screws. This avoids the need for clamping.

A quick check to make sure it fits. It does! The boxes are very long to give as much internal volume as possible. The dimensions are 13" x 17" x 22".

After making the driver and terminal cup cutouts with a jigsaw, I sealed the screw holes with water putty. I have had success with drywall compound in the past, and I imagine auto body filler would work well too. Then I sanded, painted, stuffed the boxes with fiberglass insulation, and mounted the speaker and terminal cup. I used T-nuts for the speaker, I highly recommend them.

Installed and connected, tested to make sure at least sound is coming out :)

As you can see they are pretty stealthy. Just imagine the other one on the other side of the couch :)

So here comes the interesting part. Those 12" speakers have a Vas of 4.25 cubic feet. So how are they not going to sound terrible in a 1.7 cubic foot enclosure?

Well, the magic of linkwitz transform.

This chart explains it. If I stopped here and simply hooked my subwoofer amplifier up to "subwoofer out" on my home theater receiver, I would get the blue line. That's an F3 of 42 Hz. I am missing an entire octave, and my front channel speakers can play that low, which would make the subs basically worthless.

I would rather get the green line, with an F3 of 20 Hz.  But how? Adding the frequency dependent gain of the red line, I can reassign the system parameters to make the green line a reality. What is this witchcraft?

Signal processing. I need to create a precise equalization curve that counteracts the terrible response of the system as is. The gain, as mentioned above, is frequency dependent, and goes up as the frequency decreases, but in a very accurate manner. With a "brute force" approach of just increasing the gain at some cutoff, there would likely be a lump in the response, which shows up as "boom".

Here is the magic box I just finished. It makes my subs respond to 20 Hz. In that steel and wood enclosure is a power supply, the linkwitz transform, and  +12 dB of gain buffer. I designed and built the transform and gain buffer on PCBs, populated the boards, and installed them. It feeds into the stereo amplifier below it which runs both subs.

I have to be careful as I can probably damage the framing in my house. These subs absolutely thunder when called upon.

1 comment:

Dave said...

Very cool build - I have been kicking around building some subs for my own home, might have to "borrow" some ideas from you.