Which type of box would you build based on this frequency graph?

[Cokewithlime]

Looking at these pictures there's a huge volume inside the sofa. Only trouble is some walls are very flimsy, and some walls are just the sofa's foam. No way to calculate anything I guess. How about a big thick sheet that seals off the entire bottom section with the driver in the middle?

It will work well enough (in fact may surprise you)

Here are the downsides, having tried this myself.
- the back and arm outside areas of the sofa are generally only supported by cardboard or very thin plywood - not at all rigid and will definitely resonate at various frequencies - that coupled with the close proximity of the fabric/leather sofa covering causes loud buzzing / slapping at those resonant frequencies.
- the LFE effects will definitely pressuize the inside of the sofa and anyone sitting on it will "feel" the LFE effect as well as hearing it - my wife did definitely did not like this during normal music playback and we both got tired of it after playing the second movie - so it got ripped out and went back to a "normal" sub box design

 

[weltersys]

It will work well enough (in fact may surprise you)

The OP already finished his sofa sub.
http://www.diyaudio.com/forums/subwoofers/202787-lfe-sofa.html

 

[bjorno]

How can the distance between S3 & S4 be less than 1/2 the diameter of the speaker?

Hint(s):

b