Critical subwoofer tip

In the bass region, the in-room frequency response is by far what matters the most to the ears.

You see, the ear’s time-domain resolution is very poor at long wavelengths: We cannot even detect the PRESENCE of bass energy from less than one wavelength, must hear multiple cycles to begin to detect pitch. So by the time we BEGIN to hear the bass, the room’s effects are all over it (think of how long the bass wavelengths are in relation to the in-room reflection path lengths), thereby swamping any initial phase coherence or lack thereof. The transients and overtones which give us the timing information of bass instruments are much further up the spectrum.

On the other hand, the ear is very good at hearing changes in sound pressure level at low frequencies. This is predicted by the way equal-loudness curves bunch up south of 100 Hz: A 3 dB change at 40 Hz can be perceptually comparable to a 6 dB change at 1 kHz.

To the extent that the phase of the subwoofer(s) affects the in-room frequency response, it matters. For those who have a phase control on their subwoofer’s amplifier, it is a useful tool for getting a good blend between mains and sub(s). It is not as powerful as the some of the other controls (such as gain and crossover frequency), but it is still useful. Adjust the phase control for smoothest frequency response (after adjusting the gain and frequency controls) and you will have made the biggest improvement it has to offer in the area that matters the most.

Duke
subwoofer manufacturer

I wouldn’t advocate against the use of REW or other methods of DSP available if you have them. 
If you are attempting to dial in your sub to a system, phasing does have some importance, maybe especially for odd room acoustics or multi sub environments. 

I found the utilization of a sub/main sound wave null measurement at the listening position for a particular set of frequencies near crossover point,  then reversing polarity of the main speakers didn’t achieve the anticipated result. I used two identical dB meters, and varied the locations taking notes. I don’t have a perfect listening room, that was quite obvious. 
I found at the lowest apparent dB a general agreement, then main speaker phase reversal  would indeed show an approximate ‘highest output’  but the ‘walk around’ test still felt somewhat disjointed. 
After retrying this with an ear also to the timing of impact, I found an interesting thing that slightly shorter sub delay (I’d have to guess of several msec) built a more cohesive setup- both at the listening position and from various parts of the room and in the adjacent areas. 
This was using several types of recordings with dynamic sharp impact at an appropriate low Hz level output as called for in the situation. 
One would think it not to be so critical maybe especially using several subs. But this was the way I approached setup after finding the other methods to not quite nail it. 
It’s obviously only a suggestion to those trying to dial in using the sub/speaker bass null method that they may also find a slightly shorter (or maybe longer, depending) delay to attain the desired result. Cheers,
RW

I've got 2 DIY 10" subs.  They are made out of nice birch plywood.  The cabinets do not vibrate. They are ported.  They are, heaven forbid, wired in stereo.  They are crossed over at 100Hz currently.  They will play 105dB @ 30Hz. All my crossover has is level for highs and lows to compensate for gain from the amps.  They have sat in the same place for 20 years.  I always thought I had good bass.  Then, when I retired I started taking this seriously and put bass traps and other sound treatments in my room.   Now my bass is really nice.  It was actually boomy before when I thought that is what it should sound like.  I can walk around the room and the sound is the same.  I think room treatment is much more important than these expensive subs with all these controls.  The only reason a sub should cost more is better cabinet construction and nice drivers.  All this other stuff leads to chasing your tail.  What is much more frustrating to me is how recordings are made and bass levels vary.  My true test is to find a recording where they hit a timpani as hard as they can and it still sounds right.  I've got that.

It is not just Phase but Time also. The sound of the woofer has to get to your ears at the same time as the sound from your main speakers. The only easy way to match both is digital subwoofer management. You will never get it dead on without it. It takes the sting out of subwoofer matching.

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).