augerpro said:

Could you elaborate on end correction please?

Sure. The air inside the tube oscillates, but also the air outside both ends of the tube contribute to the effective mass of air. There is a simple formula for calculating the effective mass of the air outside a baffled tube, and it can be expressed as if the tube was a bit longer than the actual length.

For each side of the tube, one should add 0.85*r to the length (r is the tube radius).

Obviously, having two (well separated) vents, and equal amount is added to both vents, and the amount is the same as for the single vent in the small box. However, if a larger single vent is used in the large box, the end correction becomes larger (due to a larger r), and so the tube length has to be shortened.

marchel said:

**correct me if I'm wrong, **

I think , simply doubling the volume and doubling the number of the same port is wrong, I think you'll gonna have to recalculate the port dimension for the new box volume and a new port size altogether.

One can also think like this: Take two subwoofers and stack them on top of each other. Then drill a hole through their walls, so that the air can flow freely between the two box cavities. Due to symmetry, no air will flow between the boxes. This means that the wall between the two boxes has no effect. This in turn means that a box with double box size and two drivers should have two vents, identical to the single vent in the single box.