Sound pressure level and Room size for ESLs

I am trying to calculate how the sound pressure level changes in respond to room size increase or decrease for electrostatic speakers (dipole). For example, if I am moving from a 1000 cubic ft room to 3000 cubic ft room, what increase in speaker efficiency (db) would be needed to generate the same sound level (assuming same distance from the speakers)? I suspect that this may depend on the frequency as bass frequency is omni directional, but I just wanted to get a rough idea. If some sound engineers can chime in, it would be very helpful.

thanks, Jay