Now it's looking like I can get much nicer system response in both channels...with the polarity of either subs or panels (but not both) inverted from the present settings. When either was unintentionally inverted with the chairside DEQ, I got nicer looking curves, and notably with NO dip at or around the crossover region. With the front panel DEQ, which was not inverting, I struggled to get such nice curves and came closest when I adjusted the delay to be around 4 ms more than previously determined settings (setting which make sense and they also approximate the sub backward distance, as I couldn't see it any better than that anyway). That 4ms is just enough to cause a polarity inversion at the 125 Hz crossover frequency.
Both were checked before in a variety of ways, including a phone app combined with its downloaded test file. (I never trust such things very much.)
But to really know what's going on, it would be best to measure polarity myself. And now I have an oscilloscope in the living room just ready for such a task. I just need to hook up a microphone and microphone preamp.
I'm sure the sub output, even without equalization, will look quite different from the original polarity test signal I have been using in the past few days (either of my self produced files: SoftPolarityTest or LoudPolarityTest2, which are a series of pulses spaced by about 5ms). It will be long and flat and even cyclic because of the rolloff and crossover of the sub. Turning off the EQ (but not the crossover, which is not a good idea on my system) may be helpful in making the correct polarity more visible. I might also need to record a test with longer pulses, the current ones probably don't generate much low frequencies. My original tests were made by analog recording one of my oscillators (not the present one, which might make it easier) because I don't know of any software that does this sort of thing, not Audacity for sure, and I couldn't find any such plug-ins either.
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| SoftPolarityTest output from panel DEQ (including crossover) |
The 4x orders of the Linkwitz-Riley filters permit both drivers to be in phase with one another. Thus it is true for the 4th order (24dB/octave) however for that crossover, less group delay is achieved with the out-of-phase connections (I never used them anyway with 4th order, preferring to have everything in-phase just in principle). Now that I have 8th order Linkwitz Riley, I believe not only can I use the in-phase connection, but it achieves less group delay that way as well, because that characteristic alternates with higher orders. (IIRC.) However, I am not sure if this is affected by the phase corrected (FIR) filters I currently use. It was my understanding that they would simply output everything in phase all the time. So therefore phase would not be optional--in phase would be required.
If there is some flaw in this which means I must use out-of-phase connections, I want to know that too. Perhaps I can change the crossover to fix that. Because the current pair of speakers lacks problems in the 80-100 Hz region, I could use a 4th order crossover again if needed (though...I think it also causes complications with wall bounce). I could also sacrifice the phase correction thing too (if I went to 4th order, for which phase effects are still inaudible).
So the best case would be if the current sub polarity is incorrect. It was actually somewhat ambiguous with the phone app. Then I could just fix that. But if it turns out that the ways work better being out-of-phase with one another, well I will do that for now but think about changing the crossover as well.
And if I do make them out-of-phase for now, which should be the in-phase part? Opinions vary. I don't think I could hear a difference but I suppose I should try, if that becomes necessary. My preconceived idea is that the sub should be out-of-phase and the panel in-phase, which is the opposite of opinions I have heard from the Polarity Pundit, my friend George Louis.
The kind of solo trumbone and other asymmetrical sounds where absolute polarity has proven in some cases to be detectable mostly contain a lot of high frequencies. In the bass, it's all a jumble of whooshing reflected sound. The leading edge of anything, even a low frequency sine, but with acoustic harmonic sounds even more so, represents a quick blast of high frequencies because the starting and stopping of anything, it's abruptness, represents high frequencies, whereas the extension, duration, of everything represents its lowest frequencies. The only such thing as a "pure sine" would be one that started at the beginning of time and never ends. We normally don't think in such terms, we truncate into bits, and say "see sine wave here" and imagine that duration in isolation from the world--a world unto itself--but in the real world a sound must start and stop, and those are not actually the same as the rest.
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I doubt absolute polarity is reliably identified by ear. High quality experiments published in JAES indicated otherwise with some exceptions, such as solo trumbone on headphones. Mixed music, or virtually anything played in a room, has too much complexity for polarity effects to be audible.
But nevertheless, I do take some effort to preserve it. It was a shock therefore to discover this week that my long trusted (since 2006 !) digital processor was out of polarity, at least in the earliest version (which I may well have had several of at the time).
I had never tested it before, I just couldn't believe it would be wrong, especially through the digital connections.
Even this time, it took a long exploration of other ideas before I finally saw with my own eyes that it was a simple polarity difference, not so much a difference in latency.
So while I named 'Eisenson's Revenge' the kind of issue where you are at some moment having made some major mistake, but thinking about so many other things you haven't caught it yet, not so much that you are deaf or whatever, this isn't really that. In this case I presumed my device to be correct for 18 years. I even used it to do polarity experiments! (However I failed to even hear a difference, so it wouldn't matter if I had the conditions reversed.)
George was constantly telling me I needed to test these things, everything. Of course, test it his way, by listening, which is highly unreliable at best, if not completely random. But he was right I should have tested this before now. So this error needs to be named after him, when you just assume the polarity is correct and it isn't.
And further, by mixing DEQ's of different vintages over time (as units burned out and were replaced) further compounds the problem because some units would be out of relative phase with the other units, causing issues in systems with multiple ways.
Leading to the situation in the past two months, where I was adjusting with one way out of phase, so compensating for errors caused by the phase inversion, a complete waste of time. Though it's only the points closest to the crossover frequency which seem most affected.
However, then the same is true about the sub inversion. Which I'd have never discovered if the chairside DEQ weren't out of polarity. In addition to the faulty DEQ, somehow the subs seem to be inverted, and have been for some time.
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Getting the microphone hooked up to scope hasn't worked yet. I can't find the 9V AC (!) adapter for my M-Audio microphone preamp. It's not clear my Focusrite is working right without a computer connected but just a USB power supply. I've seen no microphone output on my scope yet.
Meanwhile, I have a few Oscilloscope apps on my phone, so I tried one, SignalScope X.
When I ran my "SoftPolarityTestRight" through the right channel (I don't run the LoudPolarityTest through speakers because it's dangerously loud but works ok for electronics) I was able to get an image which shows a very nice 1ms impulse looking almost square and perfect, trailed by bass and reflections
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| Right Channel playing Soft Polarity Test |
But when I ran the same test signal through the right sub by itself...it's simply inaudible. The test signal simply doesn't have any bass at all, it's all 1kHz and above. The test signal does have DC since it only goes in one direction, but that DC gets eliminated in many ways, including the servo loop on my Aragon 8008 BB amplifier, and the bass transformer in the Acoustat. For that matter, I don't think the Emotiva Stealth DAC passes DC either, and it's certainly filtered out by the crossover for the panels and the DEQ.
So I ran a different test signal, the one that goes with the SpeakerPop app. It's not a series of spaced pulses, it's a series of very long duration (OTOH one second) sawtooth waves. Running that, the subs produce a very weak sounding "buzz" sound. Once again, there is very little bass at normal frequencies 20-200 Hz in such a signal. And again, the near DC part is filtered out in many ways. (In fact, to avoid stressing my amps and speakers which I did a few years ago with this signal, I created a "normalized" version with no DC offset and reduced amplitude, and that is what I actually played.)
Still, there was sufficient output to capture it unambiguously in SignalScope X. But the result is not easy to understand.
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| Right Sub playing normalized Speaker Pop triangle wave |
Is that in normal polarity or inverted polarity? There's no easy answer.
At first I thought this showed the phase shift above and beyond the crossover frequency all the way up to 20khz, some of which might be leaking through the crossover and sub speakers. But that doesn't make sense because such output should be too weak to measure, I'm barely able to measure anything at all.
But then later I realized this is showing the effect of room reflections on the bass. Bass tones are so long that you can't even play one full cycle without the room reflections butting in.
I verified that the source of the wrapped sub tones is NOT the crossover or DEQ. I viewed the analog output of the front sub DEQ (which follows the crossover so that's in there too) on the scope, it it was a low frequency sawtooth, just one positive impulse fading into the background on each cycle.
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| Speaker Pop electronic output from Sub DEQ |
Studying the sub output again, I pondered whether I should consider the very first stroke as the polarity of the output, the first large stroke, the last large stroke, and so on. I couldn't make up my mind about this. But then I also noticed that the relative widths and sizes of the up and down strokes varied a lot with position in the room. If you were taking the first large stroke, or something like that, the polarity would invert going from very close to the subwoofer, to the listening position, and then once again to the kitchen doorway in the back of the room.
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| Right sub Speaker Pop at different location |
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| Right sub Speaker Pop at different location |
Which is making all these stroke things seem like reflections.
Well, how can you tell what the polarity is with a jumble like that? I should note that I got similar results with the Speaker Pop app itself (which uses this same test signal). I got one polarity near the speaker, a different polarity at the listening position, etc. So maybe the app is having the same trouble here as I am in interpreting the waveform because of all the included reflections.
Way back before subwoofers, I first learned to test the polarity of speakers by connecting a battery to the speaker terminals. I think my friend Bro showed me that. I was leery of the idea, thinking the battery might harm the speaker since the speaker had very low impedance at DC. Bro assured me it was not a problem, the battery itself would be limited by its own internal resistance to not put out enough current to damage a speaker driver.
But I can't do that with subs now, it seems. I can't just connect to "the speaker terminals." I must go through the very fancy electronics that forms the "plate amplifier" of the subwoofer. The latest generation on my right sub has only line inputs, balanced and unbalanced. If there were speaker terminals, they would just feed in through the amplifier in the same way, but with more attenuation. And most certainly the amplifier does not pass DC, etc.
Now the other way of figuring out the correct polarity was to see, experimentally, which gave you the loudest output at the crossover frequency. I worry that might not be correct in some cases, but generally speaking it should be correct.
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I tried using the Speaker Pop app which was easy because I was already playing the (normalized) Speaker Pop tone
The Right channel was very inconsistent, though it mostly seemed like negative polarity close to the subwoofer, positive polarity at the listening position, and negative polarity in the kitchen doorway.
The Left channel was a bit more consistent, almost always positive polarity near the subwoofer.
I tried playing both sub channels together and inverting the polarity of the right sub. At first, inverting the polarity of the right sub made it sound "weird" and I thought that couldn't be right. But when I measured the "weird" sound, it was about 2dB louder. It was more like a thrack than a tap, and I was beginning to think this sawtooth like impulse *should* sound like a thrack.
I also noticed that indeed it seemed more centered with the Right channel set to negative polarity in the Tact.
I went to the SVS app for the right sub to change the polarity there...and as I was changing it, I could have been wrong, but just before I "changed" it I noticed that it seemed like it was already in the "inverted" polarity so nothing changed when I "changed" it. I repeated the previous tests in both the Normal and Inverted to be sure, and confirmed that the previous measurements corresponded to the "inverted" mode in the SVS sub app. So it appears that until now I had been running the Right sub in the "Inverted" mode of the sub electronics without knowing it.
This might have happened when I replaced the plate amplifier a couple years back. There was a wire harness and the best thing was to test it both ways. Back then I used the Speaker Pop app and getting the usual ambiguous result, I decided that "Inverted" was correct because that was how it measured at the listening position. Well, now I see that having the right sub set to Inverted in the SVS app was exactly the wrong thing to do, and was at the bottom of the DEQ inconsistencies and the fact that the inverted DEQ actually worked better on the right channel than the correct polarity DEQ I normally use. But not so much on the left. (So now I believe that the Right sub had been out of polarity, but not the Left. The Left channel was different but not worse when I used the chairside DEQ. When I started this post and investigation I had believed that both subs might be out of polarity.)
All these years though, I must point out, that it actually sounded fine, even with the imperfect EQ and out of polarity sub. Making one of these mistakes isn't like leaving the water running. It's more like it's hardly noticeable in the overall scheme of things. And this is more true for me than usual because I'm using such a steep crossover, the effect of panel and sub being in the incorrect polarity has relatively small effect, localized around the crossover frequency, where I could easily equalize it out.
But now, with both SVS subs in their "Normal" polarity as they should have been, and using a non-inverting DEQ, I believe the polarity issues have been corrected. (Even though I still don't have a good test signal that makes this easy to see on an oscilloscope. There's just a preponderance of evidence that points this way.)
Speaker Pop has been a very useful tool in these regards despite it's ambiguity in some cases.
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It occurs to me now that I *can* use the polarity inverting DEQ as a chairside DEQ so long as I invert the phase of the particular way it is operating. So if I use the chairside DEQ on the panels, I simply invert the panels, or I could invert the subs and the Tact to achieve the same effect (ignoring absolute polarity, which I think I can do mostly). It's an inconvenience, and it also means I can't do fast A-B testing where I had one adjustment in the chairside and a different adjustment in the front. But it's still possible for month long adjustment sessions, as I am just now (hopefully and finally) completing soon.
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Oops! I thought it was all done, I was now going to "finalize" the PEQ's for both channels not that I figured out the polarity issue which was dooming the right channel.
So for the "End of Eq'ing" post I was going to lead with the measured spectra with the front panel DEQ's for both subs and panels. But something was clearly wrong with the right channel again!
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| Right channel, no panel EQ |
I hadn't re-loaded the '2024' preset into the panel DEQ. For doing some test, I had loaded the 'Inital Data' preset, and just left it that way.
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| Right Channel, latest 2024 presets |
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| Left Channel, latest presets |
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| Left Channel, Design 10ms Delay |
And that gaping depression could be solved with either 14ms delay (which I had just removed) or polarity inversion, just as the right channel.
But what about my that/thrack test which made me believe only the right sub must be inverted and the other one not?
Well, instead of repeating that possibly erroneous test, I played the Phasing Test from Stereophile CD2 with the panels disconnected.
That also was very ambiguous. It did possibly seem that with both subs inverted, the out-of-phase was more out-of-phase, and the in-phase more nicely centered. But when I measured loudness, when I played the out-of-phase with only one sub inverted (which now seems wrong) it measured about 1-2dB softer on peak, and the in-phase about 1-2dB softer on peak, exactly as it should have, compared to having both subs inverted.
I am thinking we can't go by the phasing test here because Atkinson's bass has lots of weird harmonics and phase shifting going on. If I switch the phase during one bass tone, the results are nearly unpredictable. I also think some out-of-phase cases are louder than they should be because they excite certain twisted tangential and oblique room modes more. I should mention in all these phase loudness tests, there was a significant amount of room rattling going on.
I'm thinking the numbers here which are unequivocal are that the right sub must also be inverted from it's former condition to avoid a depression in the crossover region which happens otherwise. And the effect on what little imaging I can hear from the stereo subs is (because it's highly ambiguous) is either acceptable or slightly superior. I suspect on low continuous bass tones around the crossover frequency the improvement would also be as noticeable as with the pink noise.
When I re-measured the Right Channel with the spurious extra delay removed (so back to the 10ms design delay which every other speaker way uses as a baseline) it looked even better, what I thought I had achieved with the chairside DEQ before discovering all the polarity issues:
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| Right Channel, 2024 preset, 10ms design delay |
That passes with flying colors!
In the older generation SVS left sub plate amplifier (prior to bluetooth app) there is no "polarity" control, only a "phase" control. I'd rather not use such things especially not knowing how they work. Is 180 degrees the same as inverting polarity? It might be adding some internal delay instead, or performing some other kind of complex operation. Anyway, I set it to 180 degrees, with the delay back to 10ms, and it wasn't looking as nice as with the 14ms delay. In the right channel, no delay was as good as the real polarity change.
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| Left Channel, Design Delay, SVS Phase 180 |
I recall using the SVS "Phase" control before and determining that it was useless. This is only barely as good as the 14ms delay version shown above and only slightly better than the 10ms design delay with no inversion. Similar results could have been achieved by adding about 2ms more delay, which is perhaps what it actually does.
What needs to be done is to invert the polarity through the balanced cabling. I think I had and WAS using (and perhaps still?) a polarity inverter in one of the channels. It was mounted midway between two balanced cables so not as to hang on any xlr connection. It will take some work to see if there is such a beast already connected now (and remove it) or needs to be added (in which case, I will have to find the old one or buy/make a new one).
Oh boy.
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| Left channel, XLR polarity inverting adapter |
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