Window troubles [blocking sound - solved]

So I'm making a map in ZDoom (Doom in UDMF format) and I have a window between two areas. There are monsters in both areas and while I set the "window" linedef to "block everything", that doesn't stop them from seeing through it and killing each other trying to shoot the player. I know there's some sector trickery that would let me make a window that only the player sees through, but I can't remember how it works and short of manually building a REJECT lump, I don't know any other way to make the monsters on one side of the window not notice anything on the other. Could someone help me, please?

Never mind, I was dumb and forgot you needed TWO "block sound" lines to actually block sound, and "block everything" still only counted as one.