I believe the difference between 2v and 6v output is 12db. Thats a quasi educated guess. I do 'know' that the difference between 2v and 4v is 6db so I assumed an equal increase from 4v to 6v. :-)
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This is getting all too confusing.
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.
Amplifier power, twice the amplifier power will give you 3 db more SPL. So a 200 wpc amp will give you 3 db more output than a 100 wpc amp. So you would approximately need a 1000 wpc amp to create sound twice as loud as a 100 wpc amp.
As for voltages, I'm not sure of the formula, but if indeed twice the voltage gives you 6 db more SPL, then Peak's formula sounds accurate. 6V would sound roughly twice as loud as the standard 2V output. The reason for this is normally so that a preamplifier is not necessary. Since the BAT preamps provide 17 db of gain, your Cary already provides 10 db of gain more than a 2V output, you could easily drive your Pass amps to their full output w/o a preamp.
All this really means is that you have much more gain than you need, and will probably be listening with the volume control turned down pretty low. Careful you don't crank it by accident, you could damage your speakers.
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).
This link to Stereophile may help clarify things. On reading through the various posts, I can say that jcmcgrogan2 has got it right. There is something in Gregm's post that might confuse things more...but I'm not sure. I find myself turnig to Wikipedia more and more these days as it is peer review and continuously edited to fairly accurate on most issues...truly an amazing resource enabled by the internet.