Originally Posted by V63
If the air in the rear flare "belongs to the enclosure" does this mean that the port is "practically" 3.5 inches from the back of the cabinet? I've read online that for a 4" port the port end should at least be 4" from the back of the cabinet. What happens if its closer than that? I see four inch ports closer than 4 inches to the side walls of cabinets all the time. Is this breaking the rules also?
The "still" air in the rear flare belongs to both the port AND the enclosure. As soon as that ports starts to produce sound the air immediately starts vibrating back and forth from the interior of the port (including the flares obviously) to the "enclosure's" air space.
That aside, on the designs that you've seen with 4" ports closers than 4" to the side walls (assuming they were professionally designed and built cabinets), the shift in port tuning (Fb) is accounted for, measured, and re-adjusted to hit the initial targed Fb (or at least it should
The shift that happens as ports get closer and closer to side walls, the more the tuning shifts downward. This can actually be a good thing because this artificial lengthening will allow you to shorten the port requirements and still hit your particular Fb. Unfortunately, it is you that will have to do the empirical testing to see how much it affects the Fb, and then work with either equations or modeling software to figure out a new (shorter) port length to achieve your desired tuning.
Now when a port is closer than it's diameter to a rear wall is a slightly different thing. At low volumes nothing is happening all that much, because (frequency aside), the velocity in the port is directly dependent on the power fed to the woofer. But at high power levels where the air velocity in the port is great enough, not only may
it change the Fb, but you may suffer 'port compression' if it is severe enough, meaning you lose port output dBs as well as shifting the effective tuning, even a complete loss of proper port operation if it is severe (where the port now only acts as a leak in a sealed box and not a resonator of sound.
It's not as detrimental as it may sound IF you have enough port area to maintain as low of a port velocity as possible (and this depends on getting enough port cross sectional area). The lower the port velocity at the power you are going to throw at the speaker, the higher that this threshold is raised before noticeable (negative) effects start to rear their ugly heads.
Simulation is key, but more importantly, testing/tuning the final product is just if not more important.
Almost forgot to state: what I mean by you doing testing/tuning, I mean verifying the actual Fb of the final product via any of the accepted ways to determine port tuning.