Regarding the inverse square law: Be careful! This law applies to idealized situations such as point sources in isotropic, homogeneous media. For a speaker in a tunnel, for example, it does not hold true. Speakers have radiation patterns as do electromagnetic devices such as antennae. Boundaries are also important. You will also have interference effects, etc. I would anticipate that the inverse square law to be quite inaccurate, but being outside will help. I love theory myself, but this situation calls for the voice of experience. (Now if that voice says, "Always use the inverse square law," please let me know.) The best thing I can think of from a theoretical perspective (sort of) is to obtain the speaker patterns if you can.