Saturday, March 23, 2013

Tact good for tweeter alignment

While I have concluded that the low frequency resolution of the Tact 2.0 measurement program (or more precisely, the display of the measurement program, but it might be the program itself, because of the type of pulse it uses and the number of bins) doesn't work well for subwoofer time alignment.

But it works very well for supertweeter time alignment.  A tone burst (which may be partly digital artifacts) appears in the supertweeter channel.  And using two channels is fine, I was wrong about the delay being auto-adjusted to make the leading edge of the tweeter signal line up with the leading edge of the Acoustat signal.

After the first measurement, it appeared that the supertweeter was lagging by about 0.16 ms.  So I adjusted that exactly (for some reason, I used the "short time delay" menu in the Behringer) in the right channel and got this picture, where white is the tweeter and yellow-orange is the Acoustats:

Yes, for some reason the leading edge of the acoustat signal appears to go down, out-of-polarity.  But I believe the main part of the pulse is what follows, and it goes up.  I still don't understand this.  But looking at the above picture you see that I have lined up the leading downward pulse from the panels with the leading edge of the squiggly burst from the super tweeter, which might go up (depending on which pixel you look at, some of the leading edge of the tweeter signal looks like digital artifact pre-ringing which can be ignored.  So I ignored tiny pixels, but chose the first decent looking line as the leading edge, which I admit is a judgement call.)  There is a bit of ambiguity here as to where the tweeter signal really starts, but we are within a few 0.01 ms here.  When I started, the tweeter burst was half way further down the screen, and that was a mere 0.16 ms difference.  For absolute perfection here, a better measuring device and/or listening may be required.

After doing the above measurement, I realized that the two supertweeters were not correspondingly positioned for the two panels.  I would have to adjust the other one to match this one.  But then it also occurred to me it might be better to push out both supertweeters all the way, so that the front edge of the stands for supertweeters line up with the stands of the Acoustats they are next to.  There is nothing magic about getting the two stands to line up, but it is a more reproducible positioning than most others (except having them line up on the back side) which is helpful for practical reasons.  I often move one or both supertweeters out of the living room for  parties.  If I have them calibrated for any particular position, it helps to make that position an easily reproducible one.

And since I am adjusting the delay anyway, I don't have to time align the positions of the two speakers for the reason people not having digital systems must.  I can choose to optimize the relative position of the panels and supertweeters for other reasons than actual time alignment.  In addition to the reproduciblity issue described in previous paragraph, there are also issues related to dispersion and diffraction.

Basically you don't want the supertweeter firing from behind other speakers, inside a hole as it were, because it's like talking through cupped hands.  If the tweeters are actually slightly forward of the Acoustats, that is helpful in reducing edge diffraction related to the sound projected by the supertweeters.  On the other hand, it could increase edge diffraction related to sound eminated from the Acoustats.  But that doesn't matter as much here for several reasons, the most important being that as a figure 8 speaker the acoustats don't signficantly project sound to the hard right or left, that's a null.  For another, the supertweeters are omnidirectional, which is exactly the opposite, they will product loads of diffraction and other undesirable addition effects when there are nearby boundaries.  Plus, one takes advantage of their omnidirectionality if they are slightly forward of the other speakers, getting more that 180 degrees of free radiating angle directed toward the user rather than in the other direction.

So I moved both the supertweeters out like that, and calibrated both channels like the above for correct time alignment achieved by digital delay.

I listened a bit to radio, KRTU because KPAC was playing opera, and the new setup is wonderful.  Somehow it is both more transparent, more spacious, and more relaxed.  I can also move my had a bit either way without the image seriously distorting, instead the image shifts gradually as I move my head.  This is all to the good, and I think having the supertweeters moved out and time aligned digitally is a big improvement, and a successful day's work.

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