A closer look
The ruined underwater city of Rapture isn't just visually decaying; its soundscape is mathematically calculated to sound miserable. Audio designers implemented a technique called physical occlusion tracing for the game's diegetic music (music that characters in the game can actually hear). When a 1940s jazz track plays from an in-game gramophone, the game engine constantly calculates the density, thickness, and material of the virtual walls between the player and the audio source. It then runs the music through real-time low-pass filters to authentically muffle and distort the brass instruments as you walk through the flooded corridors.
