Critical subwoofer tip

Good thread on a tough, often debated topic.  MC correct about ability to sense phase of say 20hz and 40ft wavelength using his numbers but two points.  First, thats the extreme case. For most folks xo is up say at 60hz.  So that means a wavelength of 13 ft  for room length.  At 80hz then 10ft is the length. That's real world and pertinent at the top of the sub's range.  Run in phase then, if overlapping some with mains, will produce a peak or boom (best not to use node here)  that will be corrected if the phase is rotated.  At 180 degrees it's a notch and we aren't bothered as much by missing freq.  So yeah, while MC seems theoretically correct at the lowest frequencies, in a modest room at a typical xo point, it could matter.  The OP didn't mention his room size.  The bigger, perhaps the more phase indeed matters.

Second, and this is a question that pertains to rotating the phase, but if a first harmonic is above the sub xo, say 120hz and is therefore played by the mains' low drivers, does our perception treat it as inferior to an in-phase harmonic?  This has prob been addresses ad nauseum in the speaker builder's threads for mid to tweeter crossover design.

Prob over my head now so I'll just sit back, spin some music and see what the group thinks.

While it would seem that getting the phase to match the mains is ideal, theoretically this isn't necessarily going to result in no peak or notch at the crossover point.  Not unless the slope of the cutoffs for both are the same.  For a mains woofer, in a cabinet, this isn't actively controlled and is prob more gradual than a sub.  Perhaps this is why best sound and SPL may be something other than zero.  It mitigates that complex interaction, peak or notch.  One of the advantages to wiring speakers to a (some) sub's speaker output and using its high-pass.   Also, distance from sub to listener vs mains to listener further makes this issue more complex.   Adjustable phase is an aid for both issues.  Good stuff on this (in discussions of tweeter to woofer xo designing) at Linkwitz Lab site.  

I have to agree with Erik and others.  The room is the elephant... in the room.  At freqs being  discussed the room resonates in many modes at exactly a sub's typical range.  Doing SPL is likely swamped by this.  Some cool links out there that show the locations and freq of a room based on dimensions.  It's humbling. DBA?  Place subs at room nodes.  

I believe timing delays are meant for a sub placed at a distance different than the mains but prob more importantly,  different than another sub.  The one further would need the delay so the behave coherently.  I'm not sure about the EE implementation in all circuits but phase does not ideally equal time delay. Phase change is applied to all freq equally. Timing is a shift that can look like phase change but not equally.  Since freq is 1/time period of a wavelength, a simple delay in timing is therefore freq variant.  A 20hz wave is 50msec, 100hz is 10msec.  Applying say, a 10msec shift is like opposite polarity for 100hz but is less than a quarter shift for 20hz, or equal to less than 90 degrees.   So a sub placed near mains or equally distanced to listening position really shouldn't need timing.  If timing improves things in this freq range, I'm putting money on effectively de-tuning a sub-mains XO bloom or notch, or a room node.   That's not at all implying it didn't work.  But then this is why around 90 deg will usually do this, isn't freq variant, and works for the other bobbles in the xo area equally (but not room nodes).