Past the critical distance, gain cannot help.

Direct sound falls 6 dB per doubling of distance. The reverberant field does not — it is, near enough, the same level everywhere in the room. So the ratio between them falls with distance and then flattens, and the distance where they are equal is the critical distance: Dc = DF × 0.057 × √(Q V / RT60).

The engine is free and needs no account. An ordinary painted meeting room, 6 × 4.5 × 2.7 m: 110.7 m² of surface, 5.5 m² sabins of absorption, RT60 2.12 s and a critical distance of 33 cm. A table microphone at 1.8 m records −14.6 dB of room over talker — and turning the gain up changes that figure by nothing at all, because it raises both sides of it equally. Replacing the whole ceiling with 50 mm absorber takes the room to 0.41 s and the critical distance to only 76 cm, because Dc goes as one over the square root of RT60: double the absorption, gain 41% more distance. A shotgun instead of an omni moves it to 84 cm for nothing, and a lavalier at 20 cm in the untreated room is +4.5 dB — better than the treated room at 1.8 m. Every lane ships a worked example that costs nothing. Writing the report needs credits.