OK this is how it is! Hypothetically say I have a jl 12w6v2 with a 500/1 going to it in a sealed enclosure. Now if I were to go with a spec tuned ported enclosure with the same amount of power I would gain roughly 3db's. OK now if I were to get another 12w6v2 and run it in the same spec ported enclosure with a 1000/1 now I would gain 3db's for doubling the sub, and 3 db's for doubling the power. All together going from my origanal 12w6v2 sealed to the two 12's ported and the 1000/1 over the 500/1 I gained about 9db's total. OK now 3db is deffinetly noticable if you dont think so just add another sub and you will notice a difference unless you are running your amp at 2ohms and adding another one runs it at 4 and you cut the power in half but thats a whole nother topic, so if you have 1 sub running at 4ohms with 300 watts and you add another so you run it at 2ohms with 600 watts you will gain about 6db's and for about every 9db's you gain you roughly double the audible level's. hope this helps.
going from one sub to two subs is +3dB double the power is also +3dB so if you add a second sub and drop the load on the amp from 4 ohms to 2 ohms, thus doubling both driver displacement and power, you're talking +6dB which is approximately twice the perceived volume.
yup but those are round about #'s not exact. if you go sealed to ported,double the sub,and double the power you will roughly gain about 9db's and like I said before roughly 9db;s is supposed to be doubling the audible volume. Glass said 6-10 so it is somewhere around there.
yeah, a lot of figuring out car audio is math when ya get down to it. the devibel isn't a linear scale either, so it helps if you know a little advanced math for doing log scales if you really want to get into figuring out sound pressure and understanding how it works. this is why most kids stop at "ok that's loud. I'm happy" hehe