And please do, take good measurements at ~5deg intervals so we can all laugh at how wretched even a well-designed toppled MTM is.
No need. The measurements of the 1266 show that +/- 15 degrees shows no problems.
That said, I will take measurements just the same. You should keep in mind that mine is not
a well-designed toppled MTM - it isn't designed for horizontal use. Thus, they may not perform quite as well as a center designed for it. However, the measurements I will
take will illustrate that the reflections from further off-axis responses are not frequency response problematic if it performs well within the listening window. In short, what should be examined is the delta frequency response of the measurements before and after reflection-point absorption. This will illuminate how much of a problem these severely off-axis reflections are, in the frequencies in which the midwoofers will interfere with one another. I've done these measurements in the past, and have found that treating the reflection points does just about nothing to the frequency response in these frequency ranges. The only significant effect is in the impulse response and decay times, and the majority at lower frequencies (as I've mentioned earlier). The best way I can explain the results, is that the reflection point SPL is much lower than the direct SPL, due to distance travelled (and possibly minor absorbtion), that the additive effect is barely measurable, and thus, inaudible. Remember, adding decibels is not linear, in any shape or form. The majority of problems of the first reflection points are in the 100hz-500hz range, where midwoofer interference shouldn't exist. The longer decay times due to the reflections at these frequencies muddy up the frequencies associated with speech intelligibility (located in the low khz). This is why first-order reflection treatment is important - not because of frequency response (in a typical room).
On a side note: an expertly designed horizontal MTM will generally have a crossover of ~2khz (preferably lower), and the tweeter will be off centered to minimize the center-to-center distance of the two midwoofers. However, the latter is rarely done, mostly to save money so the same crossover and cabinet can be used (a mistake at some price points).
It's not a strawman. As others appear capable of realizing, an MT will have more constrained vertical directivity than a toppled MTM will, regardless of whether or not there are woofers on either side of said MT playing below ~300Hz or so.
Not at all. This is just plain false. There's nothing magical about an MT that constrains vertical directivity more than an MTM. As you get off axis from a TM, all you see is the typical HF roll-off and a narrow dip at the crossover point, where the center-to-center distance of the tweeter and midwoofer become large enough such that they create significant interference (wider dip = shallower crossover). This is hardly constrained vertical directivity. Note that this is the same dip mentioned in the quote that Curtis made in his last post.
little change in the HF over that 30deg window
This has nothing to do with the directivity of an MTM. This has to do with the dispersion characteristics of the tweeter. The same tweeter in any alignment (save for a horn/waveguide) will show the same HF dispersion characteristics.
For the record, here is the response of that center in the horizontal plane, from the above-linked article. What unmitigated f'n garbage. Even by the nearly nonexistent standards of toppled-MTMs, that's ridiculously poor performance.
Which should have nothing to do with this discussion. The off-axis measurements are at 45 degrees and 60 degrees, not +/- 15 degrees. Of course you'll have problems with a horizontal MTM when your listening window is 90-120 degrees wide! You need to do a better job analyzing the graphs (you continuously and conveniently leave out the important details). I'd prefer Ultimate AV to select off-axis measurements that mirror the typical listening window, rather than the extreme 45 and 60 degrees (at least for centers). This would illuminate which MTM centers were poorly designed, and which were well-designed. Until then, one has to rely on the text (that which Curtis quoted).