There are things called tactile transducers that vibrate based on the frequencies fed to them via the LFE/sub channel. Look for Aura Bass Shakers as an example. They can be found in the $25-30 range, though you do need an amp to power them.
Jonathan: Unless you have an unusually tall sofa with a lot of room underneath, how is it even possible to do what you suggest? At any rate, no: The subwoofer must be able to radiate unrestricted into your listening environment. If you want a gut-thumping "shaking" situation, do as John suggests and get the Bass Shakers. JB
And since this thread is going to lead to specific recommendations, I'm moving it to the Speakers section (read the disclaimer at the top of the Basics-area thread-listing page).
If you want to improve the bass response in your room, look into bass traps. Depending on your room dimensions certain freqencies of bass are enhanced in one area of the room and cancelled in other areas. Since sound is really just a pressure variation in a room, this impedes the effectiveness of your entire system.(room included.) Since the room itself is part of your "system" wouldn't it make sense to address the problem there? Bass traps will convert the sound energy into heat. By so doing you reduce the rooms ability to interfere with the bass. When you turn on your system it will sound cleaner and punchier, and be even throughout the room. It has worked wonders for me.
I did Dual Adire Shivas in the bottom of my Hide-a-bed couch. The bed portion had seen better days and this allowed me to build a 10 cubic foot ported box that holds the 2-12 inch subs. Yes, you can feel the bass.
I drive it with a 320 watt plate amp and have it tuned to 17 hz for great home theater bass.
Thnksguys. I really appreciate the information that you all have given me. If anything, I woould have put 2 10" or 1 12' subwoofer in a very slim box placed as low to the floor as posssible underneath the couch. There's a wide piece of wood between the cushions and floor so it doesn't sink.
I bet the question that I asked, you guys can tell I'm an amteur at ht.
I have been running my HSU 1225 behind my couch (between it and the back wall), and I can tell you that it really does shake the couch when there is low, loud bass.
I have cement floors, and when the sub was in the front corner, I never got any sort of tactile sensation from the sub. The bass was there, and it measured pretty flat (after EQ), but I never felt that the room was shaking, as so many tend to describe things.
But with the sub behind the couch, I am actually getting better integration with the fronts, and get a really good shake in the seat of the pants.
I don't know how that would compare to real tactile transducers and bass shakers, but for movies where lots of stuff goes BOOM, its great fun.
FWIW, in the paper work I got with the 1225, Dr. Hsu suggests behind the couch placement as as alternative, and I have to say, its working out quite well.