Picturing the New Alignment

 As previously described, I've settled on something like the simplest alignment method for now.  Align for identical listening position wavefront alignment.  And for now, alining so that the positive wavefronts match, I'm ignoring some negative polarity phase lead which seems to come from every driver.  I am currently investigating what causes that.  All my drivers are nominally in polarity, and so where they begin positive polarity movement represents the true beginning (or so I believe now).

[Update: Days later I determined that it appears the phase lead seems to be an artifact of the Focusrite or the the ARTA program.  I see about 4ms delay in a pure loopback signal from the inputs to the outputs of the focusrite.  But I remember seeing a very similar phenomenon years ago using an EMU 0202 interface with REW program.  And the magnitude of the phase lead seems to be far lower in magnitude in the loopback measurement than it appears in these acoustic measurements.  I will show this in a subsequent post.  I'm still considering it a puzzle.  Anyway, for practical reasons, my current method of aligning to the first positive motion still works best because the beginning of the tiny phase lead is much harder to determine unambiguously due to its fairly low frequency and magnitude similar to low frequency room noise.  The beginning of the upwards motion is a signal change which is relatively well determined for all drivers and should measure identically from all drivers, though it emphasizes the higher frequencies passed by each driver rather than the lower frequencies, a topic that may be worth additional investigation sometime, though I use linear phase crossovers anyway, AND the higher frequencies are what's most important to align for the subwoofer anyway.]

Near the beginning of this series of measurements (for which the microphone might have been slightly nudged from previous measurements) I found a total latency of 98.396 ms for the panels (including the standard 10ms delay I have dialed in to the midrange DEQ).  There also seemed to be about 4ms of phase lead, with an extra little drop before the rise which may be about 0.2ms which I had previously believed to be all of the phase lead.  But I have decided mostly to ignore the phase lead for now.  For most of the screenshots I set the yellow cursor to the exact point of the beginning of positive movement, so you can read the number at the bottom of the graph.

For the supertweeter, I measured a delay of 98.458ms, ignoring a tiny bit of pre-ringing, and also the chair reflction about 2ms later.

For the subwoofer, which had been previously aligned using the now discredited "maximum summed amplitude at crossover frequency" method by setting DEQ delay to 12.32ms (vs 10ms for panel), I measured delay of 102.271ms

Calculation would have then indicated I should have set the bass DEQ delay to 8.44.  However, I included another bass measurement which was 0.1ms less, and tried to average in my head, but ended up miscalculating and dialing in 8.72ms.  However, as it turned out, determining the bass adjustment is hard within +/-0.4ms or so because of low frequency room noise, and 8.72 bass vs 10.00 ms panel seemed to work out just right in subsequent measurements.  Here are the 3 bass measurements I did after the adjustment, and you can see they bracket the panel delay of 98.396, with the last measurement falling on that number exactly.  I decided that was as good as I was likely to get it.

BTW, you might notice the phase lead for the bass is also around 4ms, so if I had consistently aligned tweeter and bass to the beginning of the phase lead, at least as shown in these photos, the results would have been the same.  It is not always this clear.  However that method doesn't work with the tweeters which show no appreciable phase lead only a few 1/100ths of a ms of pre-ringing.  Also it's hard enough to see clearly the beginning of the positive rise, the beginning of the phase lead is in many cases hard to see clearly as it is obscured by low frequency room noise.

I dialed in the 0.06ms correction for the tweeters, and they now also lined up with the panel at 98.396:

Now adjusted to 98.396ms delay, all three drivers together (left channel) look like this:

Left Channel

Ignore how the leading transient looks like an M.  The second hump of the M is really the chair reflection.  Then what you see is that the positive transient of the bass blends into the decay of the positive transient of the panels, a little late to keep the top of the summed response level...but the fact that it doesn't even go negative first is an accomplishment.  The bulge of the bass transient peak is about 4ms later than the leading peak of the tweeter.  I believe this is the "most correct" alignment.  (I experimented with many other alignments in at attempt to keep the top of the "M" level but could only make it look worse.  But I haven't done those measurements yet as they should be done...with the chair out of the way.)

The fact that this 20ms pulse doesn't look square all the way across the top is probably mostly due to room reflections.  But if you let your imagination wander, you can still imagine the 20ms pulse, with the leading and trailing edges marred mostly by the chair reflection which makes them look like an M instead of an I, but otherwise looks remarkably good for an acoustical measurement in a small room.

Measuring the impulse response using sweeps, then calculating the frequency response and distortion, I got this:

Now the crossover frequency at 125 Hz may be close to the bottom of a tiny dip.  But the amazing thing is how tiny that dip is.  From 50-150Hz we see nothing but tiny wiggles around a gradual decline.  This is far better than previous measurements, which typically showed a 10dB or larger peak centered at 150 Hz, which happened to elevate the magnitude at 125 Hz, and possibly a 10dB notch between 90 and 110 Hz.  The tiny wiggles represent the tolerances of my fine tuning the bass response with digital EQ, without which there would be huge peaks and suckouts.

In fact, this is the best measurement I've ever seen in ARTA, with a smooth decline across the spectrum until the tweeter rise prevents total drop.  Distortion looks a bit higher than I'd like below crossover frequency, but this may be largely a result of room noise from the air conditioner running near maximum with outdoors at 105F.

The two previous alignments showed that if the microphone is centered, everything in the right channel measures the same, except the right sub is about 0.32ms delayed because it's farther back.  So I dialed in the same values determined for the left channel with that 0.32ms less for the sub, or 8.40 (vs 8.72).  Then did a measurement of all three drivers in the right channel:

Right Channel

The droop between the panel spike and the peak from the bass is just a bit deeper.  Then I turned off the panels just to look at the sub and tweeter.

It's a bit hard to see where the bass actually starts rising...but actually it seems to start rising at the exact moment after the pre-ringing of the tweeter, though the bass only rises slowly at first so that part might be the tail end of the phase lead.  It's possible the bass would best be moved forwards about 1ms because my previous alignments were not correct.  I was tired of doing these measurements in the presence of obscuring room noise and decided that, for now, this was good enough.

Then, I finally tried moving the chair, and so the M artifact went away, giving a somewhat cleaner looking impulse response where you can see leading and trailing edges pretty well, just some reflection stuff in between.  I showed this picture in a previous post without explaining all the work that went into making it:

You might think that moving the bass fowards in time (by setting less delay in the DEQ) could make the top of the transient just past the initial rise say risen longer.   Earlier measurements of this were inconclusive, it appearing difficult to do much better than this, either in removing the gap between first and second peaks, and in the pulse staying positive as long as possible.  A systematic exploration of this might be interesting.

But meanwhile I was seriously bothered by the phase lead issue, and in my head (not having seen all these measurements together) I was thinking the panel phase lead was far shorter, and so if I aligned all drivers to the beginning of their phase lead, if any, I might get an actual "better" alignment, more like I had gotten using the maximum output at crossover frequency method.

So instead of trying to optimize the right channel any better, as I had done for the left, or exploring the transient shape with different delays systematically, I then started investigating the phase lead phenomenon electrically, to be described in the next post.