Kerf bending has taken my interest. I could cup the OD of the woofer frame from behind, at least a half moon, and then flare it out to the height of the slot.
With bendy plywood, no kerfs would be required to bend to the radius of an 18" driver.
OK there's some information I must disclose, sorta embarrassing, but whatever
.... I knew about this, just haven't dealt with it. Thats the threaded rod singing ~172hz.... I am working on it! I've got plans for all the things!
After mentioning that ringing is one (of the many..) problems with threaded rod between drivers years ago, glad you that wasn't just BS
There are lots of recent posts regarding the use of the Axi2050 down to Fc, and using the Klipsch Jubilee K-402 conical/tractrix expansion horn crossover of ~340Hz as an example of why you shouldn't.
Comparing the K-402 to your (mostly) undersized mouth exponential horn is like comparing a pancake to a pineapple, but lets do it:
As can be seen in the green trace, your horn has a sharp drop of >24dB/octave below it's Fc (cutoff frequency) of ~160Hz, while the K-402/Axi2050 (red trace) drops off at about -12dB per octave below 500Hz, similar rate to a cone driver in a sealed box.
The TAD TD 4002 (blue trace) lighter diaphragm has a higher Fs than the Axi2050, so drops off at a steeper rate below 500Hz.
The -8dB dip ~200Hz in the green trace is evidence of its undersized horn mouth relative to the low Fc.
These "features" would be enough for most to heed the usual advice for exponential horns to "cross over ~2x Fc", ~320 Hz.
That said, you have proven your huge, beamy, oval exponential(ish) horn has plenty of output for you at your one meter (!?!) listening distance at Fc.
With "brick wall" filters and enough EQ, it can be crossed to your slot loaded bandpass 18s" that low and still exhibit flat on axis response.
But as Airvoid says:
"looking at just one axis is like looking at the situation through a straw"
At any rate, no amount of filtering or EQ will make a HF compression driver sound "clean" (compared to a cone driver capable of smooth response) when it's forced to cover such a wide range.
I wish I could back up that subjective statement (shared by most listeners and designers) with some extensive measured evidence.
The best I have is this simple two-tone test using 523 +932Hz), using the lowest distortion HF compression driver I'd tested, the EV DH1A (yellow trace) compared to a 3.5" "full range" cone driver (TC9FD, blue trace) on the same horn (250Hz Fc):
In retrospect, the drivers should have been equalized to the same response prior to the tests.
The 532Hz level is nearly identical for both drivers, the DH1A is louder at 932Hz.
If the drivers were perfect, those two frequencies would be the only ones seen.
The test shows both IMD (intermodulation sum and difference distortion) and HD (harmonic distortion) products.
Easy to see how much more IM the compression driver has generated.
It was also very easy to hear a large difference when playing music when equalized to similar response.
Subjectively, even order HD is fairly benign, odd order less so, but I think IMD may be what is behind the negative sound quality difference we have always been able to easily hear when comparing cone drivers to compression drivers going "too low".
Or it's something else we haven't yet been able to measure
Art