Room gain is the slight rise in upper-bass and mid-bass frequencies that naturally occurs when sound is reinforced by the boundaries of a room: the floor, ceiling, and walls. Research has shown that the human brain is aware of this gain, but we filter it out of our conscious experience. Scientists theorize that we evolved to push it into the subconscious. Otherwise, we'd be constantly distracted by it.
When we walk into a room, we are aware of room gain, but our brains are processing its effect in the background. When we listen to a music system or home theater system in a room, research has shown that our subconscious brain expects to hear the effect. When the sound has a flat frequency response (i.e., with room gain removed), our minds are aware that something is wrong. Because of this, understanding the natural bass reinforcement of a room and preserving it is crucial when correcting the room’s sonic problems.