I'm not talking about crossover frequency. A crossover is merely a filter that cuts off certain frequencies before it even gets to the speaker. When you're dealing with box specs of a ported box, the port itself has a tuning. For a daily ported box, most are tuned around the 30's range. That means you'll have a nice bump around the tuning frequency. When you calculate airspace, you have to subtract out the displacement of sub, thickness of wood, port, bracing, and anything else that's inside the box. The fact that your box is big, it'll make your sub very efficient(loud) with very little power.