Help me decide what to do

Hello all,

First off, I'll have to confess to zero experience with subwoofers, but I want to build one. I'll describe my needs, and hope to get some suggestions from you.

I recently moved into a house with 10' ceilings and hardwood floors. The living room (where my av system is), dining room and kitchen are all connected. It makes a room roughly 40 X 15'. Needless to say, it's hard to get my music to sound as good as I would like. Since I have a Jones to build something, I'm thinking of a 12" down firing sub in a sealed, cabinet. I have been reading about Rythmik servo amp/speaker combinations. They also have a nice cabinet design that I might use. I'm hoping this will be a good first step toward a good system. Presently I have a Yamaha RX 550 AV amp, Definitive Technology 8040 towers and 8040 center. Surrounds are 2 way bookshelfs I built 10 years ago.

Let me have it folks. Where am I going wrong?
Room acoustics may be a major driving force to your development (as it should be for everybody). Hard to offer ideas in the abstract as your space may prove to be favourable or not. Good chance you have to think of ways to contain echo. For sure, you'll want to read Floyd Toole's great book.

As home theater, good chance you'd like earthquake rumbling bass and good sound at several seats. As far as subs, you'll certainly need two or more subs that are heterogeneous in design and location. Upper freq might be best served by dipoles which float the music out the back and all around and create great ambience.

First of all - get ready to spend money - this is not a small room, and if you want to fully fill it to "realistic" levels below 40Hz. I'd imagine at least 12 units would be a starting point.

Secondly, there is no single universally best design - subs probably more than anything are very room and application dependent.

Funny you should mention the Rythmik name, as a buddy & I are presently discussing them - most particularly the performance and versatility of their direct servo drives and custom EQ'd plate amps. Then there's the option of something they call Master/Slave mode - oops, my mind started to wander for a moment there. ;)
What's 12 units?
I see what you mean about 'get ready to spend money' Started out wondering if I needed a sealed 10 or 12 incher. Now I'm thinking about double 12s or single 18. Sheesh! Still undecided about sealed or ported, but space considerations might decide for me. Let me know what you and your friend think about the Rythmik systems. Master/Slave - uhh maybe not..
Funny you should mention the Rythmik name, as a buddy & I are presently discussing them - most particularly the performance and versatility of their direct servo drives and custom EQ'd plate amps.

Way to go! These speakers use motional feedback (the final frontier) with a second custom voice coil. Great design and cogent website, but they seem to use just a small feedback fraction... which is always the case for commercial products where stability even in the hands a nutty owner is important.

How many speakers? If you assume your system should be able to destroy your hearing at all frequencies, a person can compute the number of speakers to produce the number of megBells needed to accomplish that. Don't take this kind of arithmetic seriously.

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Larry that should have been 12" - you know, the old John Holmes joke - "I have 12 inches, but can't use it as a rule" - or was it Harry Reems?

Based on a few past builds, I'd generally opt for sealed at these frequencies - the boxes still need to be well constructed and braced, but with the right drivers sealed can provide a lot of advantages.
I decided to pull the trigger

I'm ordering a Rythmik DS 1200CI servo subwoofer and amp. Picked up a sheet of 3/4 MDF today to get started on the box. Two questions: What is a good sound deadening material to line the box? Stuff the box? Is it really important to coat the mdf for moisture protection? Since it's a down firing cabinet, I wonder if I will need to add a net to prevent stuffing from falling into the speaker.

Thanks for the advice so far; everything helps.
From Rythmik's site:
Our plate amps when installed correctly are suitable for installation in a sealed box without a separate chamber. There are no air leaks. If you do choose to install it in its own sealed chamber, ensure that there is adequate air space to avoid heat build up. You may choose to ventilate the chamber.
The subwoofer does not come with damping material. We recommend 2" polyfill to cover all walls.
Their published plans are pretty decently drawn - and while I'd normally use BB plywood on any build, if properly braced - very important for a sub- I'd be be surprised if you didn't get better results than almost any commercial built unit at the same net cost.

Note that on a sealed enclosure, you can feel free to modify the dimensions to your heart's content - provided the net cubic volume is unchanged, and you have room for the amp module. I do find it interesting though that they recommend you can get away without isolating the amp.

To repeat something I've mentioned before, a buddy of mine who's been a car installer / speaker repair tech for over 30yrs notes that the highest failure mode in consumer grade subs units is poor thermal dissipation for amps in either vented or sealed units - and even when still within manufacturers warranty, repairs are often "not timely" - as in "oops, our apologies - can you tell your customer we ran out of replacement amps ? "