Is sound "additive"?

Doug- I am pretty sure that you are correct. It is additive, and not arithmetically. You would need an additional channel of amplification of course to power the second (third, etc)speaker. There are actually noise models that allow you to "add" noises of different dB level to give a total noise level. Same would be true for music.

Thanks guys. That helps.

Felthove, I am curious as to where you are going with this? Just a question, or is there some experiment in the works?

This is not an additive hobby. I can keep it to under 16 hours a day. No problem. Well, Ok, Fridays I listen 17-18 hours, and Wed about 20, but I'm good on Thurs and hold it to 16. But that doesn't make it additive. What do *ewe* tink, dude?

Actually this question is virtually impossible to answer with the information at hand. The sound coming out of your speakers can not be represented by a simple scalar value. It is not as simple as saying the output is 89db. The sound is however, a complex quantity where the sound possesses magnitude and phase. If the speakers, cables, electronics etc. were all exactly the same, it is theoretically possible they would all be in phase with each other and add perfectly. But because we live in an imperfect world where the signal paths would not be the same, some of the sound pressures could theoretically cancel each other out if they were 180 degrees out of phase which is exactly what noise cancellation scientists are working on right now.