How does the 'Block Sound' flag work?

[Amuscaria]

I noticed that many areas in DE that had the 'block-sound' flag on the line segment doesn't seem to work, while other areas they seem to work. Does that flag only block sounds coming from the direction that the line segment is pointing? Also, does it actually stop sounds from playing (meaning a player behind the line wont hear anything)? Or does it only apply to alerting monsters?

Finally, the 'Block everything' flag, does it also block sounds? If so, does it work the same way - have to point in the direction of the sound source.

Thanks.

[Project Shadowcat]

It takes two "layers" of Block Sound lines to stop sound. That way monsters can funnel in from smallish areas instead of in just the small local area.
Facing doesn't matter.

As for Block Everything, I don't think it blocks sound, so if you want to block sound with it, just check that, too.

EDIT: Block Sound lines never affect the player, it's only for the monster.

[Zippy]

It takes two "layers" of Block Sound lines to stop sound. That way monsters can funnel in from smallish areas instead of in just the small local area.
Facing doesn't matter.

To try and give a clarifying visual...

Think about a Doom map which is maybe a collection of rooms, with each hallway connecting rooms being completely sealed with a single Block Sound line. The reason it takes 2 lines to completely block a sound is because now this gives you some flexibility: when you make a monster alertable noise, it will wake all the monsters in your room and the immediate neighbor rooms but no further. In other words, the Block Sound flag on a line is used to delineate the map into sound "zones" and noise in any given zone will wake monsters in that zone and every neighboring zone.

This also avoids funny little issues like being able to walk right up next to a monster facing away from you and no waking him up when you blast away with your firearms into the open air just because he's on the other side of a Block Sound line.

This means if you want a spot that definitively stops sound, always, there's two simple ways to do it: 1) Zero height sector (a sound doesn't pass through a closed door, for instance), 2) just put two Block Sound lines right in a row.

[Xaser]

Well... almost. Monsters will always hear you if there's a direct line of sight (not counting facing direction) between you and them. Even "deaf" monsters follow this behavior -- it's just how the engine does it, though admittedly it'd be silly any other way.