Tom,
You are correct about the matching of dispersion at xover. Major point!
This was a feature of the very first two ways from Western Electric around 1932-33. They put the 13.5" woofer in a short horn mounted in a flat baffle to induce controlled narrowing dispersion with frequency. More of a waveguide than an impedance transformer concept.
This design was the grandmother of the Voice of the Theater cabs, which were originally meant to be mounted in large flat baffles, but that notion got lost. The woofer horn in an 825/828 cabinet is not really there for impedance transformation...in fact the 120hz bump this small horn creates is a problem! but it does narrow the midrange dispersion of the woof to match that of a 500hz horn, maybe 60 deg horizontal.
Note that early RCA theater horns were stenciled "Directional Baffle"
As for the 1200hz round horn. Hard to say without knowing more. Perhaps a low slope xover was used and they needed loading down to a lower than expected frequency.
Also, the impedance of a horn usually flattens out and is optimal somewhat above its cutoff region, so this could be searching for the sweet spot.
Finally, given that it is a round horn, it may be a L'Cleach horn, quite popular in hifi circles. Jean Michel worked out the math for optimized terminations of the horn mouth to the free air. Generally, this is a critical point for horns. Poor termination leads to a plethora of mid-to-high order modes caused by reflections back from the mouth to the walls of the horn and the driver, visible in the horrible HF waterfall performance of the Altec 511 I posted earlier, for example. This is how the real (acoustic) ringing of horns is brought on. Major time smear and dispersion anomalies ensure.
The L'Cleach termination adds a lot of material to the edges of the mouth. Here's a pic of an Iwata profile horn with JMLC termination that we set up at ETF in Denmark last week. That I believe is a 500hz horn, but could be 300.
Here's one JMLC lecture: https://pearl-hifi.com/06_Lit_Archive/1 ... TF2010.pdf
I'll try to find a link for another good one. I have it on my computer as a pdf somewhere...
In any case, there may be very good reasons to use a 3' horn at 1200hz crossover but I think it could be done with less drama. Many pro 1200hz horns are like 8" wide!
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