Dipole sub hole size

I am wondering what the minimum size for the two vents of a dipole sub should be, considering the phenomenon of the acoustic impedance.

Rod Elliot (sound.westhost.com) describes the implications for closed box speakers, giving their minimum cone sizes:

...small cones (in small boxes) will be incapable of reproducing spectacular bass, regardless of the claimed Xmax of the driver. As an example (and please note that this is a very rough estimate only) the following table shows the expected minimum frequency of various sized drivers in a sealed enclosure having a small baffle, and radiating into ½ space. Vented boxes work differently, but the vent area still limits the low frequency performance. A very large vent (needed for good performance and low noise at extra low frequencies) in an enclosure cannot be effectively driven with a small loudspeaker cone area.

Driver Diameter Minimum Frequency (-3dB)
200mm (8") 40 Hz
250mm (10") 32 Hz
300mm (12") 27 Hz
380mm (15") 21 Hz
450mm (18") 18 Hz


Consider a system with a 15" sub in an H-frame where the two ends are of course open, but by holes (vents..) of, say, 8". My question is, does the size of the holes now determine the lower frequency in the way described above, or is it only determined by the size of the sub driver itself. I understand these holes need to be of reasonable size in any case to avoid wind noise.
I'm unsure because the 8" holes would pretty much behave like 8" drivers in this configuration, but data from Axel Ridtahler's RiPole Subwoofer seems to suggest otherwise.
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Ripole analogy

Consider a system, ....but data from Axel Ridtahler's RiPole Subwoofer seems to suggest otherwise.

I'm well satisified with the sound of diy RIPOLE playing classical mucic and small group 50/60's cool jazz. The RIPOLE was constructed for a 12-inch diamater narrow driver and used biamped. However, the noise from the bass driver when really low notes are played (George Wright pipe organ, etc.), indicates a lack of driver Xmax. I believe Axel Ridthahler stated that the chambers and hence the openings were to be minimzed. I used 1&3/8-inch chambers ( by 13-inches )and the same size openings. Since the RIPOLE is a highly specialized type of enclosure, I doubt that RIPOLE info can be directly translated to other types of enclosures. best wishes for the frames you are working on, and hope you will post the resuts, and listening exteriences, ....regards, Michael