Bandpass Effect of ports?

Is there any information of the band pass of effect of speaker port. The one band pass sub I've owned claimed to drop off @6dB octave above 200hz. I'd assume the sound coming from the port of a normal vented sub would offer similar low-pass characteristics.
Tuning frequencies being equal is this filter slope affected by other port characteristics?

e.g. if an 80 x 125 and a 50 x 280 port offered a tuning frequency of 50hz would the high-frequency filter effect be the same? Is it always 6db per octave ?