Zonal sound?

[chewmyheadoff]

When I add an ambient sound to a zone, the sound escapes outside the zone making it quite pointless. Is there a way to contain the sound inside the zone? I'v tried BSP occlusion, but the sound still escapes. This is important because the sound is from a tunnel underground, and I don't want people to hear it above ground.

[SHeld]

try doing more ambient sounds with less radius