Is sound "additive"?

If I kept adding speakers playing at say, 70 db to a particular room, would the sound keep increasing? Wierd question, but I was thinking about how it sounds when a room fills up and people are talking -- it just gets louder and louder (though some of this is can be attributed to people speaking louder to compensate for others' talking). I was going to try and relate this to the effect (in SPL) of adding multiple subs to a room. Would it just keep getting louder and louder?