Port velocity

Based on the studies mentioned in this papers overview, ports with a large radius end termination (NFR = 0.5 as an optimum) are superior to non flared ports both in gain and distortion. However there are some culprits, as most of the studies discussed in this paper focus on the difference between equal diameter ports and not on the difference between flared versus large diameter ports. They also seem to focus on listening levels 10 - 30 dB lower then PA output levels.

The studies note that the effect becomes smaller with increased tuning and increased port output. The effect is mainly large on small input signals below ~10 V. As port velocity increases to near turbulence, the effect diminishes greatly. The only way to keep the results of these studies valid, is to increase the port diameter or surface area, in order to keep the port velocity low

The largest port velocity reduction I saw in these studies was near 3.5 m/sec. (in the middle of the port area) which is obviously easily over matched by an increase in port area.

So the actual conclusion is that you shouldn't use flared ports to overcome the short comings of ports with insufficient port area. You should first get the port velocity away from any turbulent level by choosing a sufficient port area and then preferably choose a sufficiently flared port (small flares can actually be worse in effect).