Critical subwoofer tip

The waves would not reach your ears at the time even if they started at the same point as they are different lengths.

When I was in the Navy, this was known as a gross conceptual error (GCE). You get zero points for the answer. Better luck next time.

@ieales 

The ≈50Hz fundamental starts propagating from the woofer at the same time as the [say] ≈2250Hz beater whack does from the mid. If the voice coil of the woofer is 3 inches back from the mid, the whack is ≈180° out of phase by the time the fundamental starts to reach your ear.

The waves would not reach your ears at the time even if they started at the same point as they are different lengths. And bass takes a few cycles to be heard anyway.

Consider a kick drum on an open field where there are no reflections. The higher frequencies of the beater hitting the head arrive before the full amplitude of the head deflection, yet you still identify it as a kick.
 You can see a kick waveform here
http://192.168.1.160/assets/CblSnkOil/Signal_vs_ACLine.png

Putting the same kick drum in a room does not change your identification as a kick, but you are fully aware it is now a kick in a room

In a multi driver speaker the frequencies which make up a kick are reproduced by different drivers. The sound begins when the electrical signal starts and in the voice coil plane. The ≈50Hz fundamental starts propagating from the woofer at the same time as the [say] ≈2250Hz beater whack does from the mid. If the voice coil of the woofer is 3 inches back from the mid, the whack is ≈180° out of phase by the time the fundamental starts to reach your ear.

Time aligning attempts to minimize these displacement artifacts. Nothing is ever perfect and there are crossover affects to consider as well, but properly aligned systems have a coherence missing in others. Our brain can correct for frequency variation but cannot shift time. 

Compared to live acoustic sound, all playback sucks. But as Nietzsche opined "Without music, life would be a mistake." So we put up with it.

ieales, Trikustik-this is a trip! thanks 

That is achieved by time and phase alignment of the sub, LF, mid and HF drivers. Full Stop.

@ieales Yours is not the first mention in the discussion referring to the importance of time alignment. I don’t think I understand this in the context of low frequency behavior in a closed room.

For example, if my listening position is 10ft away from my main speakers, all frequencies above about 113Hz are arriving at the listening position from the speakers directly because all the frequencies at this level and higher have wavelengths of less than 10 feet. In contrast, frequencies below this threshold will require more than 10 ft to reach full amplitude because their wavelengths are longer than this. This means, to hear these lower frequencies, they have to bounce off at least one or more room boundaries. This would seem to indicate that all these lower frequencies arrive at the listening position later than the higher ones. In other words the higher frequencies would arrive at my listening position after traveling 10 ft, the 30 Hz frequency had to travel at least 37.5 ft, the 40 Hz traveled over 28 ft., etc. Since all the frequencies travel at about 1,125 ft/sec, the lower, reflected frequencies are all going to lag behind direct ones from the mains.

There was some suggestion that a delay could be applied to the subwoofer but it would seem for exact time alignment, you would have to apply a variable delay to all frequencies except for the lowest the system can produce to have them all arrive at the listening position at the same time. This seems pretty involved to me. Or have I missed the whole point again?

Enter MC's room  Raummoden Rechner - Trikustik to view nodes.

View MC's system Millercarbon's System - Virtual Systems (audiogon.com)

A combination of ported and unported subs with 20Hz capable mains are going to produce impressive, but never accurate low end.

Tekton MOAB have 17 distinct arrival time per channel. With 8 widely spaced LF [main woofers to 300Hz 1st order, down only 6db @ 600Hz] drivers [4 main and 4 sub] there will be lots of indistinct bottom.

MC once advised: " Here in Washington we have some sweet green bud that will get you there in one step: inhale."  in The 5 stages of making a bad audio purchase

I like my music as I heard it in the studio and the control room: Neat.

That is achieved by time and phase alignment of the sub, LF, mid and HF drivers. Full Stop.

ieales, I agree.

While its not absolute this generally works for me.

mtmug-

I think it is safe to say that I am one of the confused ones. If we ignore changes in amplitude over time (impulses) and just stick with a sustained tone at a given frequency, how are two wave fronts 180 degrees out of phase not the same as two with the polarities reversed?

They aren't. The are the same. Your confusion stems from reading guys who sort of understand some of it, just enough to keep everyone else thoroughly confused.

If I had a sub with speaker level inputs and no phase control, I would expect swapping the speaker leads to produce the same effect as flipping the phase 180 degrees.

Correct. Same thing.  

Think of a 40Hz wave propagating out of a sub. The wavelength at 40Hz is about 28 feet. https://www.translatorscafe.com/unit-converter/en-US/calculator/sound-frequency-wavelength/  This means your room has to be at least 28 feet in every dimension, or else the wave will encounter a wall and reflect back and cancel itself. Before even one full wave.  

Of course no one's room is this big, and 40Hz isn't even the bottom, we want 20Hz, which use the web page, is 56 feet! Nobody but nobody has a room 56 feet on a side with a ceiling 56 feet high! Nobody! So ALL rooms are small, from the perspective of a sub.  

Take a room most would consider large, 17x30x9. Pretty good size room, right? Those dimensions in terms of wavelength work out to 66, 37 & 125 Hz, respectively.  

You see what happens? No matter where you put a sub the waves are all much larger than the room, and so they wind up reflecting and canceling or reinforcing themselves. You can physically move the sub around. All this will do is move the locations of the modes. You can reverse polarity. All this will do is move the locations of the modes. You can adjust phase. All this will do is move the locations of the modes. Changing location, phase, and polarity are all different sides of the same coin. All any of them are doing is moving around the modes. Same exact problem. Never goes away.

Notice there is zero chance any of this has anything to do with the main speakers. The mains are either too high in frequency to matter, or if they are at the same bass frequency then phase still doesn't matter, because they are just two more sources of the same problem. They still cancel and reinforce because the same wavelength physics applies to them as well.

When people notice profound effects and improved bass by changing phase, it is not because they have matched anything. It is because they have found a setting that is less lumpy at their listening location - for certain frequencies! Same goes for EQ. You can get it pretty good at one spot, but only by making it worse everywhere else.

This is why multiple subs works so well. More subs in more locations results in more modes, more areas of cancellation or reinforcement. Same problem. No getting around the physics. With more subs however then the output of each one can be less. Because each one is less then each mode is less of a lump. Yet add them all together, the result is the volume you want without the lumpiness you do not want.  

This distributed bass array concept by the way is one of the great audio developments of the last 50 years. This one simple move - four subs instead of one - enables anyone to have truly awesome bass in any room and for as little as $3k. Amazing. Yet hardly anyone is doing it, mostly as far as I can tell because the concept is hard to understand.

Well, do a search. Look around. The physics is rock solid. The results uniform and unimpeachable. Everyone who does it is blown away how good it works.

Any recording with a prominent Kick drum will reveal proper polarity.

The kick is a poor arbiter. One might have a preference, but unless one knows the material first hand, it's just a preference.

Since polarity can change from track to track on some more highly produced recordings made at different locations it would be most useful if every modern preamp was equipped with a remote control polarity switch since its more discernible at the listening position

Polarity can vary from instrument to instrument in one track!

Back in the day, the east coast & Europe was mostly pin 2 +ve, while the left coast was pin 3. Some studios varied in both locations. Microphones could be either depending on their history.

Pop overdubs were often done all over town. Sometimes on both coasts or overseas. Not all engineers checked polarity. Hence the drums could be +phase, lead vocals -, percussion +, backgrounds ?, etc.

Some more twisted engineer types carried their own mics, pre's, EQ, amps, speakers & cables. 0:) That way you could get the room without worrying about polarity.

One of my subs remote equalization presets is set for reverse polarity.
Any recording with a prominent Kick drum will reveal proper polarity.
By simply switching the assigned preset button the Kick will sound noticeably more or less forward.
This only changes the polarity within the subwoofer system.

Making the manual switch at my preamplifier does a complete switch.

Since polarity can change from track to track on some more highly produced recordings made at different locations it would be most useful if every modern preamp was equipped with a remote control polarity switch since its more discernible at the listening position.  

I think it is safe to say that I am one of the confused ones. If we ignore changes in amplitude over time (impulses) and just stick with a sustained tone at a given frequency, how are two wave fronts 180 degrees out of phase not the same as two with the polarities reversed?
If I had a sub with speaker level inputs and no phase control, I would expect swapping the speaker leads to produce the same effect as flipping the phase 180 degrees.
BTW, I am enjoying these posts and appreciate the OP for starting it and everyone who has contributed. It has caused me to think more about things I "knew", or should have known, and how poorly I have considered some of those things in practice.

More than a few post confuse polarity with phase. If nothing else, maybe this discussion will point this out.

180 degree phase shift LOOKS like reverses polarity, when graphed. However, 180 degrees phase shifted is actually 1 cycle delayed.

Inverted polarity will substantially reduce LF. (Subs out of polarity with the main speakers) 

Now, after the polarity is correct, we can optimize the phase. 

So, we must optimize sub: level, crossover, polarity and phase. 

I recently purchased a sub to augment the bottom end of my Spatial M3 open baffle speakers. Due to domestic constraints, one sub is all I will have in the living room. 
The sub is front firing, and placed about 18” from the rear of the Spatial. If you follow me here, I was concerned about the back wave (open baffle) cancelling out the sub’s forward wave. I made all my adjustments- xover freq, volume, Q, listening mode (Eq).
Things sounded very good- then I flipped the phase to 180 degrees.
This caused things to snap further into focus- I imagine the back wave of the OB speaker was interacting with the sub at some frequency, causing cancellation perhaps?
I mention this because it may very well be that some sub/ speaker interactions will not be affected by the switching of phase- sealed box speakers maybe, front ported, etc., on and on. Maybe rear ports with subs behind the speaker are affected differently.
This could explain why some people hear no difference when flipping phase- I have a separate home theater system, and for the life of me I never heard a difference when doing so. In this 2.1 channel system, it is readily apparent to me.
As I write, I also realize that the 2.1 system uses a sealed sub- the HT subs were ported. Maybe porting changes things too. Just more to consider...

...and then there's the simple solution of delay for time resolution of waveforms.  Phase matching is easy....

Delay is to achieve the same impulse arrival time at the listening position. 10ms is about 11.25 feet. Delays around 1ms are more common.

An All-Pass Phase control does not affect all frequencies equally. It is used to get the best integration between the non-linear sub and main phase responses. see  PhaseControl.png (868×506) (ielogical.com) for an example

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).   

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'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.

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

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

Everyone rightfully cites REL subs as having high-level inputs---the sub taking it’s input from the output of a power amp. Often not mentioned is that the Rythmik subs having one of the company’s PEQ plate amps provide both low (line) -level and high-level inputs---low-level on RCA jacks, high-level on speaker cable binding posts. Rythmik’s XLR model plate amps forfeit the high-level inputs.

Rhthmik plate amps also provide a continuously-variable phase control, which is labeled "Delay". The knob can be placed anywhere between 0 (no delay) to 16ms (roughly 180 degrees of phase rotation). Delay can also be achieved by physically moving the sub, but the delay control allows one to find the optimal placement in terms of room modes, the control then aligning the loudspeaker/sub electronically.

If the woofer of a loudspeaker and the driver of a sub are both reproducing, say, 80HZ, and the 80Hz wave from the two are 180 degrees apart in phase (opposite polarity), the result will be cancellation centered at 80Hz, resulting in a deep hole in frequency response. It is no different than the tweeter and midrange driver of the loudspeaker being out of phase with each other, except that with tweeters and midrange drivers in a loudspeaker, the designer is free to use x/o parts to boost the output of the drivers to achieve a flat response.

I found a better result in my system by playing a repetitive kick drum and trying to match the beat sharper than softer.

It depends on how the kick drum was eq'd. For some music, there is no low end at all and other's next to no point. Additionally, we often flipped the foot phase on pop recordings.

My favorite piece by ear is a pipe band recorded out of doors. When the phase is correct, focus dials in just like a camera.

I won't criticize anyone who sets theirs subs up by ear. Everyone has their own way to do things. That's all part of the fun.
However, I think it's kind of like watching a car drive past and guessing how fast it's going w/out using a radar gun.
I spent a couple more hours today tweaking my subwoofer(s) setting to try and improve the frequency response. There's no way I could have gotten to where I am without the aid of REW.
I'm pretty happy with the first image, as it's only out by a couple of dB (in one place) from what's acceptable using EQ/DSP (5dB).
Second image shows my previous setup compared to today's changes. It also includes how the 'Mains Only - without Subs' measure.
https://imgur.com/a/aXlCoXh

Worth noting that when choosing source material to tune phase with that is where a sound meter is crucial.  You have to be sure there is bass down in the frequency range the sub is covering.  Many recordings with good bass will not do much below 40 hz or so. So Adjusting phase on a sub with music containing little or nothing in the range the sub is covering will do little or nothing.  Details matter!  🏆

Anyone familiar with phase cancellation technology used in headphones should appreciate the importance of optimizing phase relationships. The problem is that if you have a two position phase switch on your sub, you can only reverse the phase 180 degrees, and if your phase is only off by 90 degrees, you won’t hear a difference either way. I have two subs, and I accidentally had the phase switches in opposite positions from each other because like a lot of you, I didn’t think it made much difference. When I put both subs in phase with each other, the difference was obvious—I got louder, punchier bass. Golfnutz got it right—if your subs are out of phase with the mains, they’re literally working against the mains bass output, and the overall bass will be reduced. So setting the phase is simple—just use the position where the bass sounds the loudest. Just keep in mind that with a 2 position phase switch, you may not hear a difference.

kick drum or many modern pop recordings with synthesized percussion will do nicely. You want to maximize the “kick”.

there I said something positive about modern pop recordings. 👏👏👏

In my experience adjusting subwoofer phase matching to the mains using a fixed sine wave/null points was vexing. 
It was much too room and location dependent. 
I found a better result in my system by playing a repetitive kick drum and trying to match the beat sharper than softer. It seems more likely to result in a phase structure that is consistent from ‘the other room’. If you have inaccurate phasing, when you go elsewhere in the house it seems much more apparent. 
If you get this kick drum right, it seems more correct. Not just from the listening position that has room issues effecting it, but from a broader area of the structure. 

Yes doing anything perfectly is always very hard but with the right sub to fit your system, a sound meter app on a smartphone and streamed white noise, doing it very well is not hard at all.

If you do that versus merely doing it by ear you might be surprised about the difference. 

also the more subs to integrate the harder it will be to get it perfect. That’s basic math. Well enough is always much less difficult.

The YouTube test was really useful. Flushed out a level issue but phase was in sync.  The localization of low frequencies as suggested by MC was quite profound in its absence IMO.

Adding a sub perfectly to an existing system involves all of the challenges that crossover design has, with perhaps the only exception that you are involved in line level vs. speaker level settings. This is why I say it is far too much for the average audiophile, and best left to ARC systems like built into JL Audio subs or top notch digital preamps or integrateds like from Anthem.

Even if you do all the filtering right, the matter of the room remains.  To paraphrase the Bard:

Aye, and there is the rub, for in that room what sounds may come must give us pause and make cowards of us all.

Best,

Erik

Not unless the slope of the cutoffs for both are the same.

Not true. One can achieve near perfect integration with a main 1st and 3rd order sub with polarity inverted.

Slopes must be adjustable. Ideally on both the main and the sub. They do not have to match.

Subwoofer phase adjustments are simply (very small) timing delays applied to ONE of two speakers attempting to producing the SAME frequencies.

Not unless phase adjustment is done with a delay or by physically moving the speaker.

Lastly, one cannot "invert the phase." This makes no sense. Adjusting the phase 90 degrees, 180 or 270 of a subwoofer is adding a small delay in the timing of the sound wave. There is no way to do the opposite and speed the sound wave up, hence no such thing as, "inverted phase."

A typical phase control does not add delay. It shifts the phase relationship in the speaker range. If one inverts polarity, the phase shifts. The timing remains the same.

How to set phase correctly:
Since it's easier to hear a null than a peak do this. Reverse the polarity of one speaker (swap + and -) and play a test tone from 20hz to 200hz. Next, from the listening position, ask someone to adjust the phase so you hear a DIP in sound level at some point during the test tone. This creates a null point. Swap your speaker cable back and you're done!

What does a phase issue sound like?
Simply put, there will be an audible dip in the crossover range. Again, only IF there is a phasing issue, will this be noticeable. Importantly, a phase induced dip may lead you to set the sub volume level too high causing, "bloated bass."

Back to the OP's point: If one believes inverse speaker polarity causes a drop in low frequencies, then one must also believe phase of a subwoofer vs. main speakers may cause a similar drop. No, polarity and phase are not the same at all, however, for demonstration purposes, the drop of sound level IS similar.  

Subwoofer phase adjustments are simply (very small) timing delays applied to ONE of two speakers attempting to producing the SAME frequencies. A subwoofer phasing problem is a timing issue.  A few posts stated the problem well: AT THE CROSSOVER POINT of the main speakers to the subwoofer, phase matters. The goal is to slightly delay the subwoofer's sound wave to match the main speakers' phase and have these competing signals reach the listener simultaneously. How much it matters depends entirely on your specific set up. (Eg. room dimensions, LF capability of the mains, location of the sub to listener) WHERE the sub is located in relation to the main speakers affects phase. Specifically, subs located close to mains = little phasing problems, whereas, subs positioned CLOSER to the listening position and FURTHER from the  main speakers = larger phasing problems.

As was also pointed out, subs often reach up to 100 or 200hz and phasing issues become more noticeable, complicated and real. I agree with MC that under a certain frequency it cannot matter.  

Lastly, one cannot "invert the phase." This makes no sense. Adjusting the phase 90 degrees, 180 or 270 of a subwoofer is adding a small delay in the timing of the sound wave. There is no way to do the opposite and speed the sound wave up, hence no such thing as, "inverted phase."
Smooth LF augments all music performances and movies, at loud volumes or quiet volumes. LF done well is glorious.

Fun little story...  I was helping my brother set up and dial in his 2 Sumiko S9 subwoofers. I played around with them for a few hours and just couldn’t get them to sound right. Turned out that I had one of the subs 180 degrees out of phase and it was pretty much cancelling out all of the bass from the second sub.

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.  

Miller Carbon and all of you that are "amazed by his knowledge" need to come to my house. My Velodyne subs, like the OP’s, also have four selectable amounts of phase delay. If you can’t hear the differences in 0, 90, 180 and 270 degrees with your ears and see them on an spl meter, then you are deaf and blind.  This is a system with two subs rolling off above 40Hz and the loudspeaker pair running full-range.

Thanks @golfnutz I get it and their reason to listen for minimum bass makes sense.  I suppose with two subs, the method would be to measure and adjust each sub separately, one at a time.

I have a question for people who have phase controls for their subs. How do these work?

see ieLogical SubterraneanHomesickBlues

Most Phase controls are All Pass filters. They don't affect all frequencies equally. see http://www.ielogical.com/assets/SubTerrBlues/PhaseControl.png

To properly integrate a 'generic' sub controls for Level, Phase, Polarity, Crossover Hz & Order are required. Delay is icing on the cake. Bespoke subs mating to a specific set of mains may get by with fewer controls.

MC's response is nonsense. But I repeat myself.

New to this forum. Hello All. I would add this:  ybe happy if you can hear the difference cause tweaking the placement or the phase makes audible differences. No argument against science except this is empirical. There are those that can’t hear the difference and those that can. End of story. 

Your suggestion is how lots of people try to figure out where to set the phase.

TAS suggests doing what you’re saying, but reverse your speaker cables so your mains are out of phase first. Using an SPL meter takes any guessing out of the equation, it's that obvious.

Here’s the reasoning sent by Rythmik in their setup sheet:

"Here’s what’s happening when you follow this procedure. By reversing the polarity of the main speakers, you’re putting them out of phase with the subwoofer. When you play a test signal whose frequency is the same as the subwoofer’s crossover point, both the sub and the main speakers will be reproducing that frequency. You’ll hear minimum bass when the waves from the main speakers and subwoofers are maximally out of phase. That is, when the main speaker’s cone is moving in, the subwoofer’s cone is moving out. The two out-of-phase waves cancel each other, producing very little bass. Now, when you return your loudspeakers to their proper connection (putting them back in-phase with the subwoofer), they will be maximally in-phase with the subwoofer. (It’s much easier to hear the point of maximum cancellation.) This is the most accurate method of setting a subwoofer’s phase control. Unless you move the subwoofer or main speakers, you need to perform this exercise only once"

Is there another way to figure out the optimum phase except to listen?  Maybe use an SPL meter at the overlap frequency, play the same program material at the same volume setting, and then set the phase at whichever setting achieves the highest SPLs?

@mitch2 

If you're subs are hitting any frequency same as the mains, then it matters. It's either in-phase or out-of-phase, that is, the sub drivers are moving in the same direction as your mains. When they are out of phase, you run the risk of having frequencies cancel because the drivers are moving in opposite directions. There's lots of illustrations on the internet showing this.

Setting the correct phase is much more important with a single sub, and less important to not important at all as you add additional subs.

Does it also become less important the lower you set your sub's roll-off point?  For example, I have two subs set to roll off at a pretty low 40Hz because of the low frequency extension of my main speakers.

Here’s some REW measurements I did today showing how changing the phase on the subwoofers effects frequency response.
This isn’t about how good/bad the subs/mains/room are.
No changes for volume or crossovers on the subs, phase only.
Descriptions in the images should be self explanatory.
The first image are the speakers, and basically my best attempt to get the frequency as flat as possible without using and EQ or DSP. The results are a combination of crossover/loudness settings and room position.
I tried adding a third sub yesterday, and couldn’t improve on what I have with two subs. Mind you, it’s only an 8"/150 watt sub, compared to 15" and 900 watts.
https://imgur.com/a/YJeNzUh

The effect of phase adjustment (or delay if you will) is clearly measurable if you do frequency sweeps around the crossover point. Setting the correct phase is much more important with a single sub, and less important to not important at all as you add additional subs.

An appropriate topic for me!

I just added the Martin Logan 1600X to my Classic 9 eStats. This subwoofer has all the adjustments you can think of via phone and tablets remotely.

I read the manual front to back. ML suggests (when pairing with their electrostats) to (through remote app.) INVERT the signal, and then dial the phase control to 90 degrees. I set all parameters to ’0’ otherwise and crossed it over with the 3rd order filter at 35Hz. I am really impressed so far.

Today, I await for the ’Bass Kit’ software to arrive. This sub connects to the laptop via USB and runs Anthem Room Control software. I will be eager to see what it comes up with. From the features, it can sample up to TEN positions at your LISTENING AREA. ...and then, once that is set, you can adjust the settings to your liking after the fact. (to taste, lol)

Can’t wait. Oh, and for the nay sayers on subs in 2 channel systems, I like the lowest octave of the symphonic bass drum when tuned low in orchestral music, and being a pipeorgan tech for the same company for 32 years, l sure want to experience those frequencies below 25 Hz. Now, how many PASSIVE stand alone speakers can really reach 16 Hz?

Right, therefore, a subwoofer is a great way to ’augment’ your 2 channel system when done properly.

Why else would Parasound put a point one on their P6 2.1 preamp with a built in LPF via balanced line?

Have a productive one my friends, and enjoy the music (O:

That response from MC was absolute nonsense. For someone who perpetually lectures on the need to simply listen and let ones ears be the judge on whether a difference in sound can be heard from making changes, I find it incredibly ironic that he would claim phase adjustment does nothing in any setup. Sorry MC but you’re wrong. Perhaps in a DBA setup like his the phase adjustment provides a negligible benefit. That point I won’t dispute because I’ve never personally utilized a DBA in my own room. However, when using a single sub the phase adjustment absolutely does make an audible difference at the listening position. I have a JL E110 with continuously variable phase control and it’s a helpful tool for getting the sub to perform at its best. One just needs to shut their mouth and listen to hear the truth. 

I have a question for people who have phase controls for their subs. How do these work? 0 and 180 seems simple enough. Same thing as swapping the speaker leads. How is continuously variable phase control being accomplished though? Is this just a delay circuit? Is it an adjustable all-pass circuit? This is something I have struggled to understand and implement with my DSP module. 

The effects of adjusting phase can be easily measured from the listening position.

The effects of adjusting phase can be easily measured from the listening position.

Phase match is critically important for me when it comes to my Dual SVS SB16 ultra is like day and night