[JLM]

The concept is that bass travels in waves and a room is like a big bath tub with a few inches of water in it.  If you move your hand along the length of the tub you'll form waves that move toward one end, crash into it, then return.  The next wave hits the return of the first, possibly doubling it or canceling it.  With bass the effect is +/- 20 dB peaks and valleys in frequency response.  This effect is frequency and location dependent.  No amount of room treatments (short of 100% absorption and that sounds really weird) or EQ can resolve it.  The best solution is multiple subs (search "swam" here at AC or read 'Sound Reproduction' by Floyd Toole) or this: 

http://www.spatialcomputer.com/page9/page10/page10.html

The 2nd best solution (only partial) is to have the biggest possible room with the most advantageous length/width/height ratios.

[beowulf]

The concept is that bass travels in waves and a room is like a big bath tub with a few inches of water in it.  If you move your hand along the length of the tub you'll form waves that move toward one end, crash into it, then return.  The next wave hits the return of the first, possibly doubling it or canceling it.  With bass the effect is +/- 20 dB peaks and valleys in frequency response.  This effect is frequency and location dependent.  No amount of room treatments (short of 100% absorption and that sounds really weird) or EQ can resolve it.  The best solution is multiple subs (search "swam" here at AC or read 'Sound Reproduction' by Floyd Toole) or this: 

http://www.spatialcomputer.com/page9/page10/page10.html

The 2nd best solution (only partial) is to have the biggest possible room with the most advantageous length/width/height ratios.

Hi JLM, thanks for that link!  I will read up on it.

[JeffB]

I know about the swarm concept, but one thing I don't understand about it is how time and phase come into play. 
I have also read something that seems the opposite advice which is that it is best to have the bass built into the speakers to help preserve the timing/phase or if using separate subs to place them next to the main speakers, or if a single sub then between the speakers.
Perhaps the swarm concept works best below a certain frequency?  If so, is there an ideal frequency where the transition from mains to subs should occur.  Would this frequency be the 80Hz that THX recommends.

[DaveC113]

/\ read poisoidonsvoice post in the 1st page of the thread I linked in my post above.

[beowulf]

/\ read poisoidonsvoice post in the 1st page of the thread I linked in my post above.

There's a heap of good info in that thread for sure. 

[JLM]

I know about the swarm concept, but one thing I don't understand about it is how time and phase come into play. 
I have also read something that seems the opposite advice which is that it is best to have the bass built into the speakers to help preserve the timing/phase or if using separate subs to place them next to the main speakers, or if a single sub then between the speakers.
Perhaps the swarm concept works best below a certain frequency?  If so, is there an ideal frequency where the transition from mains to subs should occur.  Would this frequency be the 80Hz that THX recommends.

I also have a hard time getting my head around the 'time and phase' sort of issues, but much research has gone into the phenomena, such as you mention THX and references included the Floyd E. Toole's 'Sound Reproduction'.  It has been approached from psychoacoustics, speaker design, and room acoustics.  Faster subs allow for higher crossover (up to maybe 200 Hz for smaller main speakers, but most audiophiles want to stay lower).  The human ear is most sensitive to phasing errors between 400 - 4,000 Hz (roughly) and below (again roughly) 120 Hz the ear cannot locate sound sources.  The room cutoff between orderly low-frequency wave behavior and disorderly higher-frequency (created by room irregularities) is called the transition zone or Schroeder frequency (and are room dependent but are in the 120 - 160 Hz ballpark). 

So overall the concept of separate sub(s) is valid.  As mentioned above a single sub will provide deep bass, but with peaks and troughs (a particular place in the room can only give flat response at a given frequency).  With more subs (sources, in the right places) the peaks and troughs even out for all room locations/frequencies.  Built-in subs only make sense for set-up along the 'wide wall' of the room (where center sound staging can easily collapse).  Bottom line, the best speaker number/location to generate mid/treble frequencies unfortunately does not coincide with that for bass frequencies.