Stereo Subs - Do they need to be identical?

As you go towards small ported subs you tend to get more harmonic distortion thermal compression, lower overall SPL output and poor transient response. It is just physics and heavy helpings of compromise. Larger is better when it comes to subs. Sealed box usually has the best transient response even if output SPL is correspondingly lower.

Two subs or one - I don't think it matters that much - for best sound I would not recommend to place a stereo sub next to each main speaker. Definitely an asymmetrical placement will be better.

Again, if using monitors, stereo subs next to them is definitely the general recommendation. And it most assuredly does matter that much

The reason I suggest to avoid stereo sub placement like you describe is to
avoid quarter wave rear wall cancellation in the bass that you always get from symmetrical placement of full range speakers. The great advantage of a sub is that you can reduce this problem with an asymmetrical placement.

I agree that stereo subs next to speakers is definitely the "general recommendation" though. With so many free standing full range speakers out there, I suspect most people don't worry about these details or are blissfully unaware. No wonder there are so many room acoustic bass problems and speaker demos never sound the same twice and some companies have special "setup" technicians trying to figure the best sounding compromise in placement of large full range speakers.

Funnily enough the link I gave above came from a manufacturer...

The link also shows up to a 20 DB SPL hole in the bass in some cases due to quarter wave cancellation and a succession of harmonically related cancellations further up in frequency (a comb filter)...

Furthermore one might wonder why studios often soffit mount large full range speakers ?

These same studios often leave small (less than full range) near fields out far into the room on the meter bridge.

Perhaps there is some logic ....perhaps is is all just aesthetics...

Here is another speaker designer's comments on the reasons for soffit mounting speakers.

Read and draw your own conclusions. Dismiss the science if you like...

Here is the link to calculate the wall bounce from reflections. If you use stereo subs then you can assume the same thing will happen up to a couple of octaves above the crossover (when the effect starts to become negligible compared to the many other room effects/anomalies)

Duke,

Thanks - Earl's paper proves what I said - symmetry in the bass is likely to
give you stronger (more coherent) peaks and suckouts than an asymmetrical
placement.

I would add that Aldavis point about phase is a real concern. I'd prefer to have
the timing of the primary sub signals to arrive at the listener all at the correct
time. So in practice this limits the amount of asymmetry that can be achieved
(to the left or right side wall at a similar distance to the listener as the mains).