If you were to add more speakers, yes, the sound will get louder. Since dB are meassured on a logarhitmic scale, you can't just add the dB numbers up. The sound will keep on increasing and increasing. Now, some speakers will probably end up canceling out some of the others, but if you have very large numbers of speakers that are placed well, you get a truly awesome effect. You know how a bunch of people wispering in a room generates a presence, well, a bunch of speakers playing at very low volume create a similar presence. It is very stimulating to the brain, and can also become very tiring after a while.
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?