With subwoofers, it is important to set them up correctly. A rudimentary understanding of wave physics can help tremendously in realizing a good setup. Keeping the listening position away from extreme energy zones of a room is paramount to excellent sound quality. Such energy zones exist at room walls, corners, and even fractions of the dimensions of the room (1/2, 1/4).
Setting up a subwoofer in the corners of the room offers, generally, the most boost to a subwoofer's response because the tri-corner of a room is the highest energy zone for bass. This placement causes all 3 room modes associated with the room's height, width, and length to be fully energized causing a significant boost in output for these 3 frequencies and some of their harmonics. The flip side to this arrangement is that modal ringing at these frequencies is worsened and this results in one-note bass (or 3-note bass) where many of the bass tones are hard to hear, and other singular tones stick out and hang around. Of course, setting up subwoofers in each of the 4 corners will ameliorate this problem as discussed below...
By placing a subwoofer mid-wall, you are minimizing the room gain to the subwoofer since these are lower energy zones. This placement, while decreasing the apparent output of the subwoofer, actually helps to stabilize the energy within the room so that there is less one-note bass since less room modes are energized.
Setting up multiple subwoofers within the room to optimize based on the energy within the room has been done both theoretically and experimentally. One such method is known in some circles as the Harmon method and the following link does a great job at describing the phenomena better.
http://www.harman.com/EN-US/OurComp...hip/Documents/White%20Papers/multsubs.pdf
Every room is different and there are some proponents of room setups where the subwoofers are placed to maximize the output while also achieving linear frequency response. This is known in some circles as the Geddes' method, but requires far more guess and check.
http://mehlau.net/audio/multisub_geddes/
Personally, I tend to prefer the Harmon method because it requires far less guess and check.
If you cannot optimize the placement of subwoofers, then one subwoofer is damaging enough. If you can optimize the placement of subwoofers acoustically, then 2 subwoofers presents a good possibility as shown in the Harmon presentation linked above (setup #6). For 3 subwoofers, you can try the Geddes method, although it requires guess and check (measurements and data analysis). For 4 subwoofers, you can go back and try the Harmon Method etc.
Some notes:
1. Multiple lesser subs setup correctly may get you closer to the best response that the lesser subwoofer can offer in the ideal setup.
2. Single better subs setup correctly will get you deeper frequency response not possible with lesser subwoofers, more bass power that is not possible with lesser subwoofers, but at the cost of less frequency linearity due to the room modes not being canceled out by mulitple subwoofers.
3. Mix and match subwoofers: Do the Geddes Method. The best subwoofer gets the corner spot because its strengths are amplified. The rest of the subwoofers basically act as carefully placed phase canceling "instruments" that help to fill in nulls caused by the corner subwoofer due to its room placement. This requires guess and check (and many hours of experimentation).
Personally, I've tried all of the above. I've settled on a compromise between the Harman and Geddes method while using very high quality subwoofers. This compromise was taken because of the physical limitations of the room (not being a closed shoe-box dimensioned rectangular prism).
. I like his suggestion as well if you need something right away.
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