Tweeter alignment for linear phase crossovers

While the linear phase crossover didn't require much or any change to the subwoofer time alignment, there was a huge change required for the tweeters, as shown by these transient delay measurements:

Midrange transient delay time measurement

Tweeter transient delay time measurement

The tweeter response is delayed 55.3 ms less than the panel response, relative to the delays which had been dialed in during the first time alignment with ARTA in June.

Somehow, I had been thinking  the tweeter would be delayed more, not less.  But now it's obvious it should be delayed less, for one or both of these reasons: (1) the 96kHz plug in runs at a faster sampling rate, and (2) it has way fewer taps, less than 2000 available as it turned out.

I dialed in the 55.3 ms change suggested by these results, and then tried to maximize the cancellation at the crossover frequency.  

This was very difficult, possibly at least in part because I was still thinking about what I had always believed to be the acoustic crossover frequency, 17220 Hz (which is what I have dialed in for the tweeters).  True, I dial 20kHz into the panels, but they roll off before that, and the crossover has little effect on the panel output, they must effectively roll off at 17220 Hz I presumed, because that is thwat I dial in to the supertweeter response.

Little did I figure, or have I ever figured, that the tweeter response might affect the the 17220 Hz dialed in crossover.  I figured the tweeters didn't have much highpass or lowpass near that frequency.  But I should have thought more carefully.  For one thing, I'm using a cap having a 40kHz crossover on the actual tweeter inputs.  (I do this to provide "boost" all the way up to 40kHz, and also protection and hum elimination.)  That is little more than an octave away, so probably has some effect on the acoustic crossover of the tweeters.  As does the response of the tweeters themselves, which I had always assumed to be about 40kHz as in the advertising literature from 1980, but as we shall see...I should have thought of that also, it may be more like 22kHz because of deterioration of the damping oil in the tweeters.

Meanwhile, you will notice in all the attempted time alignment graphs I'm doing, there IS a big notch, but it's way below the 17kHz I was struggling to look at.  It's more like either 12kHz or 14kHz, depending on the graph.  Perhaps there is a separate phenomenon at 12kHz or something, and the real joint acoustic crossover is 14kHz.

Anyway, I was trying not to notice the 14kHz or 12kHz notches as I was making these measurements, with it only slowly sinking in that I should have been paying more attention to them.  Instead, I was trying to minimize the response at 17kHz, which is far into the area where the tweeter output is far away dominating the panel output, and therefore the time delay adjustment has little effect!!!

One thing I decided to do right off was figure out how to make screenshots on the PC itself rather than using my camera.  There are at least two very different ways to do this: using the ARTA "copy" button at the bottom of the page, or what I've found far more convenient, the Windows+PC Screen buttons.  What I didn't think about doing was keeping a separate logbook log of each photo I was taking.  This was a big improvement over the camera approach I used in earlier posts.

First I took a look at the response with the supertweeter turned off, so I could get see the panel acoustic crossover.  The Acoustats are out of phase so the subwoofer crossover cancellation is still shown.

But deciding where the acoustic crossover starts, the 6dB down point, depends a lot on what you define as the baseline.  If I take 1kHz as the basline, the 6dB down point might be only 8kHz.  But that clearly isn't the crossover, it's simply a slight general downward slope, the kind shown in most measurements of good sounding speakers.

Still, ignoring such absurdities, the question is really unanswerable just from this graph.  It could 12kHz, 14kHz, or the 17.22 kHz I had previously assumed.  Restoring normal polarity, the response from the subs down to the supertweeter crossover is remarkably smooth and uniform, then the supertweeter seems to take over around 12kHz.  Somehow that didn't yet sink in to my thinking.

Then, taking a look at the cancellation, with panels polarity reversed, it did not look good at all to my way of thinking.  There was no detectable cancellation at 17kHz if at all, suggesting to me things were way off.

This seemed to be a little better at 17kHz, and I noticed the notch below 14kHz was getting wider--which also looked like maybe a good thing, or maybe not.

But I was worried about going in the right direction based on this kind of scanty evidence.  I hunted around back and forth around the measured transient time with no avail at creating a notch at 17kHz. Finally I decided to look at the time record (note that Step Response was very confusing, time record was much clearer):

The thicker portion is where the tweeter output begins, and it is clearly about 2ms delayed.  Dialing that change in made it clearly better:

But then it occurred to me, the reason which this looks like a dual impulse is because I had the subwoofer turned off.  Turning the subwoofer on made it start looking like a real 2ms transient!

After a few more attempts to improve the tweeter overlap, I got this:

This was looking so much like a real impulse (as I'll show in a later post) that I was shocked.  In fact this would not at all been possible without the linear phase crossovers.  You would not see an outline of the 2ms transient as clearly as this.

So now I went back to the cancellation display (note that all the impulses above are with Acoustats reversed, so there would be some difference if that were corrected) and it looked like very little progress had been made at getting a 17kHz cancellation.  But there was a noticeable dip or something at 14kHz.  It might not be better than a previously measured response, but it was late and I quit here.  Perhaps some different engineering might be useful on this crossover, also considering the rise above 14kHz, which is not really correct, and some more of the fall above 8kHz might be corrected.  From the transiet picture above, it appears that this is already about as good as it's going to get without other changes.