Dual Subs

Let's look at a the situation with a single sub first. The primary bass issue in most rooms is the inevitable peak-and-dip pattern as the subwoofer interacts with the room boundaries. We can shift this peak-and-dip pattern by moving the sub or the listening position, but we cannot make it go away for any listening position without equalization (which can actually make things worse elsewhere in the room). The smaller the room, the farther apart (and more audible) these bass region peaks and dips; and the larger the room, the denser the peak-and-dip pattern. If the peaks and dips are dense (close together), the ear tends to average them out - giving smoother sound. So in general the larger the room, the smoother the bass.

Another way to get a smoother and denser peak-and-dip pattern is to use a second subwoofer, located asymmetricaly with respect to the first. This way each sub will produce a unqiue, dissimilar peak-and-dip pattern at any given listening position. The sum of these two dissimilar peak-and-dip patterns is considerably smoother than either one alone. This is the main advantage of using two or more subs. There are other possible advantages as well, but I won't go into them now.

Since your room is fairly large you probably start out with a smoother than normal peak-and-dip pattern anyway, but that doesn't mean you wouldn't benefit from a second sub.

I disagree with what your local shop told you. There is no magic location where a single sub will produce smooth in-room bass. There will be a "best" locaton where the peak-and-dip pattern is the least objectionable. One minor advantage of using multiple subs is that their positioning is relatively non-critical compared to positioning a single sub.

Duke
dealer/manufacturer

My experience is that one sub can add bass, but two (in stereo) in "live" recording venues, will open up the soundstage and the "volume" of the room, even before you hear the first note of a performance. Balance can be easily upset and locations of the beasts will be important to achieve these positive effects, though.

Two are better, I have used both. Look at Sumiko web site for good info on subs.

I just added a 2nd sub to my system and I am totally happy.Balanced out the room with a better soundstage,and musicality...Music seemed to be better focused..I too have a large room..21x40 with a 8 to 12ft vault though my listening sweet area is 21x14

It would add that little extra as described above. Do you think its worth the money for that little extra? Some do some don't. That IS a large room. You could always sell it if you feel it wasnt worth it.

Other comments are correct, even bass typically is very localized with one sub. For an indepth review of this 'two sub' issue please check out Toole's 'Sound Reproduction' book. Floyd cover this issue better then anybody else. I have two subs and find it is better than one, but the 'hearing' is always related to the size and shape of the room itself. Setting two up for the best sound is very time consuming, but worth it.

Am I correct in assuming that dual subs in stereo would have to be symmetrical to the sweet spot which may not, according to Duke's comments above produce the best results? Is "stereo" a big issue when it comes to low fequency reproduction or is it more an issue of placement resulting in the best sound possible ?

Byfo, very few recordings are stereo below 100 Hz, but the final answer in my opinion is related to the low-pass filter frequency and slope. (The low-pass filter is what rolls off the top end of the sub's output, and usually the frequency is adjustable but the slope is fixed.)

In general if the subs are audible as distinct sound sources (while the mains are also playing), then they're putting out significant energy high enough into the lower midrange that they should be positioned fairly symmetrically and near the main speakers. If you can't detect the subs as separate sound sources, that means they're crossed over low enough and steep enough that they can be positioned with priority given to in-room bass smoothness - and they're probably operating below the region where stereo is a factor.

As a ballpark rule of thumb, if the subs are rolling off at 24 dB per octave around 80 Hz or lower, I wouldn't worry about positioning them with stereo in mind. But you can't necessarily trust the frequency knob on your plate amp, so if your ears tell you the subs are audible reposition them accordingly.

As a rule, this is what I do to adjust the symbiotic relationship between my sub and speakers...adjust your crossover on the sub approximately to 100hz. Then, while listening to music, adjust volume to JUST where the sub is overpowering the lower frequecies of the speakers. Tweak your crossover position now. The outcome will be the optimal settings for the system. The additional sub should be adjusted the same way, and placed in referrence to the other sub and speakers. IF you want boom boom...turn the sub up and scratch accuracy. Two subs placed accordingly will creat a balanced, lower frequency field distribution, which will not exactly sound "stereo"...but will "balance" the emitted waves traveling to your ears. Subs are not directional (lower frequencies), but when used as described, will provide a tonal balance. I didn't write the book on this stuff btw...I just read it a million times. LOL.

Thanks for the clarification Duke.
Byron