@D.A.R.K.:
A more robust explanation of the above would be helpful to less tech oriented TBrs. Thank you in advance.
This would be a massive thread unto itself, and likely just confuse the less tech oriented folks.
For those who know, not really necessary.
But as you requested, I'll give it a shot, based on my personal experimentation with sub tunings and over 25 years of touring as a FOH mix engineer.
Disclaimer: I'm not a designer, I just use the stuff. Hopefully the more educated weigh in and correct any of my mistakes.
High pass filters reduce the audio frequency spectrum below the chosen frequency point.
The db per octave setting (usually found in increments of 6 db in most audio gear) is the gradient amount of roll off that occurs from the chosen frequency. The higher the db per octave, the more frequency is removed.
Ported cabs: The tuning frequency for the port is for low end extension of frequency response. This is good for getting extended range response. However, this design also has the increased potential of frequencies below the port tuning frequency to cause over-excursion of the drivers.
Utilizing a high pass filter set at or below the port tuning frequency will remove or minimize the frequencies that are likely to cause over-excursion which can damage drivers. The amount needed (db per octave) can depend on variables which include source frequency response, equalization which may enhance frequencies below the port tuning, and the amount of power being used to increase volume of the frequencies below the port tuning.
Ideally, the use of a high pass filter will save amplifier headroom (fundamental frequencies take lots of power to amplify, we generally hear the 2nd harmonic on bass and psychoacoustics fill in the rest in our heads, which a whole other discussion), and allow speaker cabinets to function towards a more maximum efficiency while reducing risk of damage to drivers.
I'm a fan of the more extreme use (48db per octave) to maximize efficiency and minimize damage. As mentioned earlier, a more minimal slope is great for tonal applications, but I personally approach that as a different part of the process, using high pass filters in a more mechanical way.
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