The 2L mode corresponds to one full wavelength that will fit into the length of the room. It can easily be calculated as speed of sound / Length = Hz. In our example using the Length of 30 feet, it will be 37.6 Hz. The modes continue, for example the 3L would be 3/2 * speed of sound / Length = Hz.

This is the same frequency as the 1W mode. The combination of having to axial modes at the same frequency will give a very large boost at this frequency.

If room modes are a concern, you may want to learn more about the PARC. A Parametric Adaptive Room Compensation system developed by Rives Audio to nearly eliminate bass mode problems.