This mode corresponds to one half of the wavelength that will fit into the room's length. It can be easily calculated by ½ *speed of sound / Length = Hz. In our example we will select the Length to be 30 feet and thus it will have the 1L mode at 18.8 Hz.
Since this is such a low frequency and few speakers have significant output at this frequency the 1L mode is not likely a concern, but some of it's harmonics, such as the 2L mode could be a concern.
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.