Bottom line, 10 db SPL doubles the output. 70db is twice as loud as 60 db. 80 db is twice as loud as 70 db. 90 db is twice as loud as 80 db, etc, etc, etc.No, no Jmc.
As Peak & others note, it is generally accepted that 10dB SPL "SOUNDS" twice as loud (or half, depending how you look at it).
In sound pressure terms (SPL), +6dB is double the sound pressure.
In order to achieve +6dB SPL you need 4x the power.
Also, looking at the formulae Peak's posted, you'll see that 20dB (SPL) is 10x louder (3,16^2).
A difference of 20dB SPL= 10 times "louder" (i.e. sound pressure level). A difference of 40dB SPL= 100 times louder... etc.
Ultimately the confusion stems from the fact that there is a difference between intensity and sound pressure level. You need 4x the intensity (energy/watts) to achieve 2x in spl terms. A difference of 10dB (SPL) is 3,16 times louder... a difference of 20dB (SPL) is roughly 3,16 x 3,16 louder...
You need x10 watts to achieve +10dB of sound pressure... so, 10watts => +10dB spl, 100watts => +20dB spl, 1000w=> +30dB spl (and the speakers have kicked the bucket long ago).
Cheers