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?
Convert?fit=crop&h=128&rotate=exif&w=128felthove
Felthove-- you're right-- weird question, and thinking about it makes my head hurt, but presumably much of it would depend on room size and acoustic properties, ie the way sound "bounces" off the walls and around the room. IMO the bigger the room and the "deader" the acoustics, the less the effect. But in a smaller more "live room", you may have chaos?? Of course theaters use multiple speakers in very large rooms, but they must be strategically placed?? :>). Cheers. Craig
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.
i'm not sure if my math is correct but i *do* know that, basically, ewe have to double your power to get an additional 3db of gain. so, perhaps, if one subwoofer is playing at 90 db, adding a second at 90db (which is another way of doubling the power), would give a total of 93db?

even if my math is off, two subs at the same level will definitely be louder than one - one of the reasons (besides imaging & soundstaging) that i recommend using a stereo pair of subs instead of yust one: at any given volume level, the distortion will be measurably - and audibly - lower.

doug s.

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.
As much as I enjoyed your posts, I am glad that I asked something that did not have a simple answer. I was not asking for a particular reason -- just because the quandry arose in my mind. I have no experiment planned.
liguy, there is no theoretical reason why a properly set-up 2-channel subwoofer system would not be in phase - unless one subs' speaker leads were hooked up backwards from the other - just like w/a pair of regular full-range speakers, being driven by a stereo amp. of course, you could also have your subs out-of-phase from your monitors, if the subs' leads were reversed relative to the monitors', but this mix-up is also possible w/a single sub. in fact, assuming proper connections, there's actually a chance of signal-cancellation when using only *one* sub, w/the two-channel signal summed to mono. in this set-up, there's the possibility of left/right-channel signal cancellation...

regards, doug s.

btw, the proper reason to use two subs in a system instead of one, is *not* to increase the spl of the bass - this wood only make the sound too bass-heavy in relation to your monitors. as said before, a second sub allows the *same* spl's w/reduced distortion, as well as improving imaging/soundstaging.

doug s.

Sedond, agreed! I was only pointing out that the answer to the question is not simple. In order for sound to be perfectly additive they must be exactlt in phase. If they are a degree or 2 different the sum will be different. I made no attempt to relate this to a subwoofer.
liguy, i have no disagreements w/yer last comment... ;~)

regards, doug s.

Thank you Sedond. You are a gentleman and a scholar.