Limit chat range

How do you limit the range people can see what a person say?
both text and voice?
like people thats more than 1000 units away can see/hear what i type/say.

Take a look at how DarkRP does it.

**[Gamemode.PlayerCanHearPlayersVoice

http://wiki.garrysmod.com/favicon.ico](wiki.garrysmod.com/?title=Gamemode.PlayerCanHearPlayersVoice)** for voice.

do you know witch file?
thanks raBBish