If there's sufficient cross-sectional area and geometrical smoothness that turbulence doesn't set in, the only significant result is an extension of the effective port length and therefore a lowering of the tuning frequency. The "slug of air" is fluid enough that it doesn't mind changing direction or shape; any resulting loss of energy is negligible.
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This PDF is Alex Savatti's work, "Maximizing Performance from Loudpspeaker Ports" done while he was at JBL.
It is the 2nd release of his earlier 1998 white paper.
This paper goes into depth on the fluid dynamics of vented boxes, including test setups and extensive test cases.
For those who are math challenged, they will still get get a clear understanding of what is going on.
Adhering to the one diameter spacing guideline keeps the port tuning frequency much more predictable.
Especially so, when not using asymmetric flared vents.
Most DIY'ers use simple ducts, cut to a random length, and "good enough for government work"
Consider a slot port as a squashed round port: If the effective "diameter" is the average of its width and height, there seems to be no ill effects from ignoring the one "diameter" rule of thumb regarding distance to the internal walls of the enclosure.
Area = Height * Width
Dia = ((Area/Pi)^0.5) * 2
Slot ratios exceeding 1:12 become erratic and difficult to predict accurately by calculation.
And the net result of the proximity to these walls is what we might expect - a lowering of the effective tuning frequency (though this isn't the only mechanism at play there).
The slot vent has a length correction factor of 2.227, which is much higher than a simple vent (0.732) due to friction from three of the enclosure walls.
A much shorter slot length is required to provide the same tuning as a simple round vent.
The wild card here, as always, is vent velocity and its associated turbulence.
This reinforces the need to use the largest area (least velocity) possible for the design at hand.
Slot vents are ideal for production environments, where the investment in multiple design/build/test iterations is warranted.
Slots offer the least invasive use of baffle board real estate.
The fEarful is the end result of these iterations.
Passive Radiators can work well, but radically increase the cost and weight of the project.
Due to the amount of tuning mass required, a pair (read: $$) of horizontally-mounted opposing PRs is required to keep the cab from walking about.
Think BMW opposing cylinders for perfect balance.
Ideal performance is realized when each PR is the next size up, i.e. 12" PR for a 10" woofer.
PRs have a signature sound, and a definite notch in bass performance.
Last, one must remember this is an electric bass cab, used in a performance level environment.
Nobody is going to hear the difference between an assymetric flared port, and a hole cut in the box.
