Stereo Subwoofer Pairing

Deep bass is not directional, and that has been shown by a lot of research. Yet, as some have argued here, it may sound as if these deep bass notes do indeed emanate from different speakers. The explanation is that the human brain uses the upper harmonics from the main speakers to locate the deep bass tones. You can do two simple tests. The first is to use only one subwoofer. Interestingly, and provided the sub has been integrated properly, even with one mono sub the deep bass will seem to come from the two main speakers. The second test is to use two stereo subs, but turn off the main speakers. You will not hear stereo, because the directional information from the main speakers is lost.

So you can happily connect two subs in mono and not lose anything. However, why not connect them in stereo anywany- is there a disadvantage to connecting two subs in stereo? Indeed there is, because two mono subs will produce the same signal, and will therefore be smoothed more completely by having two subs reproducing the same signal in different locations. This is why most experts and manufacturers suggest to connect two subs in mono.

Two subs are indeed better than one because they reduce room modes. Similarly four are even better, but this is a story of diminishing marginal returns.
In principle you have four ways to deal with room modes in a given room:
1 use dipole subs (Rythmik have a kit for a dipole sub). Dipoles excite far fewer room modes, so you address the problem at source. The disadvantages are that dipoles tend to be larger and require more power.
2 install bass traps. The down side is that they are big and ugly.
3 use multiple subs. Two are a big improvement, and four brings additional improvement.
4 use dsp room equalization like the Antimode 8033. Use this with one sub and the (big) improvement is quite localized. Use it with two and the improvement applies to a much larger area.

Ideally, you may want to use all four methods. In real life, a combination of two or three will already bring vast improvements. If you have a bit of space I would opt for two dipole subs combined with room eq. If the room is small and domestic tolerance limited, I would opt for two really small subs like the SVS SB1000 plus an Antimode. If you have space and the budget, by all means go for four.

Ideally subs should be the same model, but as long as they are not too different there should not be a problem. Two identical ones but with slightly different ages are not an issue at all. Speakers do indeed need to be broken in, but this does not normally take more than a few hours. With subs, you are unlikely to even notice (but wait for a few hours with the Antimode equalization).

willemj,

I'm new to subs and just about to integrate a couple in my system.

A commonly recommended crossover point for subs is 80Hz, which means the sub will also be still putting out sound above 80Hz.

That seems to be getting into the directional territory.  I've read numerous people say they can localize their subs when crossing over at that frequency.

80 Hz is common for HT. For music I prefer to use them below the main speaker's roll off frequency, i.e. rather lower, such as 35-50 Hz. That is also the advice from REL: set the low pass filter quite low, and the volume relatively high.

willemj

Deep bass is not directional, and that has been shown by a lot of research. Yet, as some have argued here, it may sound as if these deep bass notes do indeed emanate from different speakers. The explanation is that the human brain uses the upper harmonics from the main speakers to locate the deep bass tones.

It is absolutely true that harmonics can suggest that the bass you’re hearing is directional and often, when bass sounds directional, that’s the explanation. But it is not absolutely true that all bass is non-directional; differing phase between bass signals can create the impression of direction. That might be obvious if you really think about it, but it’s also been proven through research, including in work done on multichannel audio by Microsoft.

" Deep bass is not directional, and that has been shown by a lot of research. Yet, as some have argued here, it may sound as if these deep bass notes do indeed emanate from different speakers. The explanation is that the human brain uses the upper harmonics from the main speakers to locate the deep bass tones."

Hi willemj,

     Excellent post that really clarifies the forces at work.  I was starting to think that deep bass may be directional because I'm able to perceive the locations of deep bass instruments within my sound stage illusion in a very consistent and convincing manner. 
     Your explanation, that the human brain uses the upper harmonics of the deep bass emanating from my main speakers to locate the deep bass tones within the sound stage illusion, really helped me understand the forces at work and the reasons for my confusion. All 4 of my subs are run as mono signals and mentally making sense of the deep bass localization that I am perceiving was getting difficult.  Our brains may be the most valuable component in all our systems.   It's all clear to me now,  Thank you.
      I'm not aware of the OP's, rjb1101's, room size and budget so I'm not sure if he even has the space or funds for the ideal solution of a 4 sub distributed bass array system but agree that 2 subs are better than 1 as many have already stated.  
     Assuming he wants to use just 2 subs , I think it's more important for good bass response that there are 2 subs instead of 1 than it is that they match. 
     If  he's mainly concerned with good bass response at his listening position,  I'd suggest he run them as mono and position them in the following manner:

1. Position sub#1 at his listening position and play music with good and repetitive bass.
2. Slowly walk across your front wall from right to left and find the exact location that the bass on the music sounds best to you and place sub#1 at this spot. If you prefer accurate and natural bass, avoid placing sub#1 in a corner.
3. Place sub#2 at your listening position and play the music with the good and repetitive bass again
4. Slowly walk along your left wall, from front to back, and find the exact location that the bass on the music sounds best to you.  Place sub#2 at this spot again avoiding corner placement if possible.
5.  Sit at your listening position and play the same music again.  If the bass response is not very good, start again from the beginning.  Remember, even small adjustments in positioning can affect bass performance at your listening position.

    In most rooms, 2 subs perform best with an asymmetrical positioning scheme; that is, with one along the front wall and one along a side wall rather than symmetrically with both placed along the front wall with one placed behind each speaker. 
    If you really prefer both subs along your front wall behind your speakers, however, you could try this by substituting the following for step #4 above: 
   Slowly walk along your front wall, from sub#1 to the left, and find the exact location that the bass on the music sounds best to you. Place sub#2 at this spot again avoiding corner placement if possible.
     The only way you'll discover which positioning scheme sounds best to you in your room is to try them both. 
     I'm almost certain that you'd get better bass response in your room, and certainly better bass response throughout your entire room and not just at your listening position,if you utilized 4 subs in your room rather than 2,  but you may have constraints I'm unaware of.  I hope this post was helpful to you.
  
Best wishes,
   Tim