Deus Ex: Invisible War Dev Diary #2
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So now we had our collisions making noise, but how does that noise reach your ears? How does that noise reach guards' ears? The original Thief games made use of Looking Glass' proprietary sound engine, which modeled propagation, the effect of sound moving through the world in an accurate manner. Many games, including the original Deus Ex, allow sound to go straight through walls, and just get quieter over distance. So when you, the player, close a door, it doesn't actually affect your ability to hear that guard, the one on the other side of that now-closed door. And, significantly, it also doesn't affect his ability to hear you. Sound goes both ways, after all.
Well, we wanted to make sound a more important part of the gameplay this time around, so that wasn't going to cut it. So a big part of the sound engine for Invisible War is a propagation system very similar to the one used in Thief 1 & 2. If you're standing directly above a guard on a floor below you, and you say something, your speech has to make it down the hall, down the staircase, and back up the hall on the next floor down if that guard is going to hear it. Sound can pass freely through open doorways, but if you close a door, the sound on the other side suddenly gets much softer, if it's a solid door. Frail or battered doors won't block as much sound, and solid metal doors might block all sound entirely when they're closed. Windows block less sound than doors, generally, but if a window is shot out, well, sound passes through unabated. The point is, it works the way you'd immediately expect it to work.
