The sonotubes are approximately 4' in length, and 20" in diameter. Each endcap is three layers of 3/4" MDF, with two layers inside and one layer outside. Having three layers for the endcaps posted a problem because you cannot purchase binding posts long enough to go through that far. So what I did is run brass bolts in from the inside to meet with short binding posts run in from the outside. The end caps, dowels, and protective grille over the top driver brings the total height of each subwoofer to approximately 4.5'.

Most people cover their sonotubes with a black fabric "sock". I wanted something that would look a little nicer, so I covered mine with cherry veneer. Unfortunately, due to bumps in the outside of the tubes, there are some ridges in the veneer. And Alla and I had some issues with the glue, so there are some glue marks on the outside near one of the endcaps. We also had glue on our hands for a few days afterwards. So overall the finished look is not commercial quality when seen up close, but from a distance they look very nice.

The project as a whole took a few weekends to complete. A lot of time was spent waiting on the endcap paint to dry, and sand, and recoat. I purchased a high quality mask to protect myself while cutting the MDF, as MDF creates a very fine sawdust when it is cut, and contains carcinogens. It's really best to cut MDF outside in the open air, rather than in the garage. I wish I'd found and purchased a larger circle jig though, because my makeshift one was not perfectly accurate.

I am driving the finished subwoofers with a Behringer EP2500 subwoofer and giving it low-pass boost and equalization to mimic a Linkwitz-Transform using a Behringer DCX2496. You can accomplish the same thing with a cheaper unit, but the DCX2496 provides some additional flexibility. The cables were cut from a spool of Impact Acoustics 14/4 (four runs of 14awg in the sleeve) I won in a contest a while back. I'm doubling up the runs so the signal path is 14/2 each. Each channel of the EP2500 is driving two drivers in parallel, for a ~2Ω load.

Using a Behringer ECM8000 microphone with an M-Audio MobilePre, I calibrated and equalized the subwoofer response flat down to somewhere below 5Hz, possibly even 2Hz, because of room gain benefits. The subwoofer can be driven to reference levels without clipping.

With the flat response, you have to bump up Black Hawk Down pretty high to feel the 5-7Hz 0dB signal from the Irene scene. I've included a copy of the waterfall chart posted by MingL here:

At reference levels, it's a tiny bit noticeable. It's possible to increase the low-pass filter to boost those frequencies several more dB, in which case it really feels like there is a rotor passing over you. I'm not really sure what the dB level is supposed to be; 0dB implies 115dB to me, which I don't necessarily think I can reach at 5Hz, but at ~105dB it's already vibrating the entire house. The whole movie plays way loud at reference levels though, since it's all gunshots and explosions.

Regardless, at this point I feel like I've accomplished building the ultimate traditional subwoofer, and can reproduce all the infrasonics I might need. Now I just need to get much better speakers that have flatter overall response and can reproduce high frequencies well. :)

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