Romy the Cat wrote:

Make an experiment, and I will simplify the case quite assertively. Take a typical compression driver, cross it at 800Hz, second order and load it into a proper contemporary horn (Tratrix or JMLC) of 300Hz. Listen that horn, you will get some sort of sound that let accept as OK Sound. Now begin to very slow lower the crossover point and keep listening the channel. While you lowering the crossover point, somewhere around 550Hz (I took this number purely arbitrarily as it would depends from VERY MANY circumstances) the horn will begin to demonstrate what I call “choked sound”. The “choked sound” is HOW HORNS SOUND IN 99% OF ALL HORN INSTALLATIONS OUT THERE – people just too damn to deal with it. The “choked sound” is the satiation when Sound produced by a driver can’t be “processed” by horn. In this “choked mode” a horn produces the “sonic boom” that was made by the horn’s bell and that “sonic boom” screw up the enter band-pass of the channel - the game is over.  Increasing of the crossover point for ¼ octave (for instance) will fix the situation - so we have approximately one octave between horn’s rate and mix crossover point…

The proximity of crossover point to the horn rate is related (besides being related to many other things) to the ability of the given channel to mask out the amplitude imperfections.

A brief explanation: pretend we have a 300Hz horn and 600Hz 6dB crossover or one octave gap; also we have 300Hz horn and 450Hz 6dB crossover or 1/2 octave gap. The channel with one octave gap has +/- 12bB in its band-pass. The channel with 1/2 octave gap has +/- 6bB in its band-pass. Here is where the Rule is applied: both channels will have an identical impact from the perspective of response flatness. The larger horn gap masks out subjective perception of amplitude’s peak and dips. The Rule is NOT depending from the bandwidth of the channel and works all the way down of the auditable region. (Though I presume that at the region of frequencies, wavelength of which is co-measurable with throat diameter) other things will override the Rule)

I spoke about the minimum proximity of a crossover point to the horn rate but there was one very fine point that I did not address and that I never have seen voiced. We have a horn and the driver of the horn is properly crossed, far enough to escape the horn choking and the “sonic boom”. There are many factors that would describe the reason of the minimum proximity (drivers type, horn curve type, crossover type and any other) but among all factors one is always overlooked – the reference volume.

It is paradoxical but with increase volume of signal the minimum proximity between crossover point and horn rate increases. I would hardly be able to put it in any kind of formulation but subjectively I feel that we lose 1/3 octave of proximity with each 6dB. I do not know what would be an explanation of the phenomena but the phenomenon is there.