Nice DIY LT Home Theatre SubWoofer
I have a Home Theatre installation which has about 20 cu. ft. space (flush with the TV and electronics) for a good subwoofer. Originally I was using a double-tuned bandpass with cutoff around 12 Hz (the lower-tuned chamber, about 16 cu. ft.). This never sounded good, although the frequency response measured correctly - it was very muddy. So I decided to go for a Linkwitz Transform sealed box. I decided to stick to the guidelines re satisfactory subwoofer output laid down in the old AES paper, i.e. 103 dB at 15 Hz. with output within Xmax ( the room is about 40 ft. x 15 ft.). This doesn't turn out to be easy to do, eh. If you use the usual 15 inch long-throw subwoofers, you need at least three plus a lot of power because of the LT circuit boost requirements. In the end I settled on 2 Peavey Low Rider 18 inch drivers. These are Kevlar-coned PA units, with an Xmax of 9.6 mm and efficiency of 98dB. The enclosure is built with sand-filled panels (15mm MDF with 2 inches of sand between), with the back wall (concrete) providing the rear of the enclosure. The drive units are fixed to the back wall and resiliently interfaced to the cabinets. I chose the Peaveys partly via recommendations from local musicians, who rate them highly here (Spain), partly via good reports on the internet. For the sub amplifier, I bought a Rhythmik Audio A370 with Linkwitz Transform board. I used transformed TS specs of Fp at 12 Hz and Qp at 0.6. The simulation (LspCAD) showed that the required level was met with Xmax for the drivers at 7.5 mm - comfortably within my target. The Home Theatre system is equalised via Audyssey.
The sound is pretty much as I expected. At the listening position 17Hz can be heard quite clearly. The main improvement is in LF transient response - the bass impact can be startling, even at mid listening levels. A surprising discovery was the extensive amount of really low frequency sound (ie around 20 Hz) on most of our satellite TV channels. It's worth the effort, I think, to get good infrabass - it really does transform the listening experience.
I'd like to thank the people who helped me along the way - in particular Lynn Olson and Stanley Linkwitz, who graciously set me straight on some critical aspects. Also Brian Ding of Rhythmik Audio - as far as I can see, the A370 amp and board is the standout choice for this type of application. Also Alexander Arnold at Parts Express, when I had problems with one of the drivers. We Brits tend to really like Americans - it's not hard to see why.