What is prefer position of port (front, back, up firing, down firing or side firing)
Here is example up firing
https://www.lautsprechershop.de/hifi/mark2.htm
Here is example up firing
https://www.lautsprechershop.de/hifi/mark2.htm
Assuming the internal location of the duct (if any) is at the desired offset within the line, 'any' is the short answer, with a longer being 'it depends on exactly what you're doing and where your priorities lie'.
For e.g., with smaller, relatively highly-forced MLTLs, I often prefer a rear duct location because there is limited capacity from the eigenmodes generated in the box to damp the vent tuning & thereby keep its length down for a given pipe CSA (Av). So in those cases, you usually end up trading off Av against duct length for your intended Fb, just as you do in a regular vented box. Or to put it another way, you're trading / balancing air velocity in the duct with its self-harmonic modes. It's a matter of opinion, but I tend to find that in those conditions, a slightly higher vent-velocity is easier to deal with than duct harmonic resonances. Within reason, a rear vent exit location usually keeps all but the most obnoxious flow-noise inaudible. I can't always say the same about a strong harmonic mode. YMMV as always, but that's my preferred compromise in those particular circumstances.
For less highly forced enclosures, and again assuming the duct internal location is at the desired point in the line, then you don't have this consideration & it's a question of the alignment & its interaction with the intended positioning, and what your aesthetic preferences are.
For e.g., with smaller, relatively highly-forced MLTLs, I often prefer a rear duct location because there is limited capacity from the eigenmodes generated in the box to damp the vent tuning & thereby keep its length down for a given pipe CSA (Av). So in those cases, you usually end up trading off Av against duct length for your intended Fb, just as you do in a regular vented box. Or to put it another way, you're trading / balancing air velocity in the duct with its self-harmonic modes. It's a matter of opinion, but I tend to find that in those conditions, a slightly higher vent-velocity is easier to deal with than duct harmonic resonances. Within reason, a rear vent exit location usually keeps all but the most obnoxious flow-noise inaudible. I can't always say the same about a strong harmonic mode. YMMV as always, but that's my preferred compromise in those particular circumstances.
For less highly forced enclosures, and again assuming the duct internal location is at the desired point in the line, then you don't have this consideration & it's a question of the alignment & its interaction with the intended positioning, and what your aesthetic preferences are.
Preferred down firing since with the drivers of the day I could keep the vent a simple baffle thickness reflex and tune it with spacers and since most were in carpeted rooms or at least had accent/area rugs the vent automagically
got 'critically' damped good enough for even the most 'picky'/'finicky' females that otherwise didn't much care for vented speakers.
Extreme ends.
Higher, air has weight too and why bass horns have to be so big for good efficiency.
Yes.
As I've pointed out periodically, driver upper mass corner (Fhm) dictates box acoustic loading BW, so for a given midrange/whatever, the lower the Fs, the lower its Qts = 2x Fs/Fhm must be.
Higher, air has weight too and why bass horns have to be so big for good efficiency.
Yes.
As I've pointed out periodically, driver upper mass corner (Fhm) dictates box acoustic loading BW, so for a given midrange/whatever, the lower the Fs, the lower its Qts = 2x Fs/Fhm must be.