[collo]

Make sure you're using WinISD Pro as it will give you a graph of port airspeed rather than just a Mach no.

If you're not going for a slot port, you'll be using multiple 4 inch ports or a 6 incher. The maximum velocity you should design for depends on the port size and whether you are using port flares. With the amount of air those drivers will move, I would recommend the PSP flares.

Four inch flared ports are good for somewhere between 14 m/sec and 20 m/sec.

Six inch flared ports are good for somewhere between 21 m/sec and 26 m/sec.

Choose a port diameter and then increase the number of ports until the airspeed is within this range.

The problem you'll come up against is fitting the ports in. As you increase the number of ports, their lengths increase. This also eats into the effective volume of the box.

For a given tuning frequency, increasing the box size actually shortens the port length, so you may have to go to a larger box or accept a higher tune.

If you have a look at the "LLT Explained" sticky thread at the top of this forum there is an extensive article by Steve Callas on large subs. One of Steve's pet designs is a 320 litre sonotube with a 6 or 8 inch port tuned below 20hz using one of those drivers. For two drivers, you would build a pair of sono's.

regards
Collo