sorry for late reply, hardly have time for the hobby...
do I understand correctly, recommended/needed crossover point depends on rather sidewall depth, not on the U-shape or H-shape ?
Yes, the cabinet depth is critical for U-, V-, and H-shaped open baffles (OB). Approximately, the cabinet can be considered a half-open tube (a lambda/4 resonator).
The resonance frequency can be estimated with the formula f = c / 4*Depth. For a cabinet with a depth of 0.4 m, this results in a resonance frequency of about 214 Hz, and for a depth of 0.2 m, about 428 Hz in a rough approximation (in reality, the resonance frequency is slightly lower than theoretically calculated).
For U- and V-shaped OBs, the crossover frequency should be well below the resonance frequency (as a guideline, I’d suggest 0.5 to one octave lower, depending on the slope of the crossover filter). So for U-shape cabinet with 0.4m depth, resonance frequency about 214Hz, one octave below would be 100Hz as XO frequency - as you suggested.
In H-shaped OBs, the resonance is symmetrically radiated, which doesn’t impair the dipole character as much, but for the same resonance frequency, it requires double the cabinet depth.
Reason being - depth makes kind of "tube" which acts like horn or TL and shorter "tube" lessens the effect ?
Yes, as mentioned above, for U-, V-, and H-shaped open baffles, the resonance frequency can be roughly estimated by considering them as a half-open tube.
An U-, V- or H-shaped OB design resembles a transmission line (TL) more than a horn when considering the effect of the air's radiation resistance.
In my next post I will provide a comparison of a 1m wide classical/flat OB and a 0.3m wide but 0.35m depth U-shape OB, the "folded version" of the flat OB, to show the differences in more detail.
