Ok been digging a bit, it was a long time ago I was messing with Dbs and power as it was with car hifi and I have been outta that game a while. I managed to trawl up a decent fact that its what your measuring thats the key. It was definately never going to be 10 though.
An old electronics engineer always used to say it was 6, yet a lot of others said 3, now I understand why.
Shamelessly ripped from interwebz
"I was recently giving a class on using digital audio workstations (DAWs). As I was describing how to make smooth crossfades and ramp outs, one of the students asked a question that led me into a lengthy dissertation on the nature of sound. Since this information is not widely covered, I think it is particularly useful for most picture and sound editors.
First, we need to understand our friend, the decibel. The decibel, or dB, is one tenth of a Bel. A Bel is a unit of sound named after Alexander Graham Bell. In addition to inventing the telephone, he did a great deal of research into the nature of how our ears perceive sound. As it happens, a difference of 1 dB of sound power is the smallest change our ears can perceive under ideal conditions.
Now, our ears are very sensitive. In fact, the total range of our hearing is 130 dBs, which can be expressed as a ratio of about 10 trillion to one. Because that number is so large, the dB is set to what's called a logarithmic, or log, scale to make sounds more convenient to measure. On this scale, if you double the power behind a sound you get a change of 3 dB in the power level, which is a change of 6 Db in the sound pressure level (our ears sense changes in pressure, and a change in sound pressure level, or SPL, of 6Db is a doubling).. Double the power again and it increases another 3 dB (6DB SPL), for a total increase of 6 dB (12Db SPL). As an example, let's say you have a 100-watt amp and you replace it with a 200-watt amp. Your total power increase is just 3 dB (6Db SPL). Replace that 200-watt amp with a 400-watt amp and your increase again is just 3 dB(6Db SPL). Thus, a 400-watt amp has just 6 dB more power (or 12 Db SPL) than a 100-watt amp.
The meter on a console or DAW is measuring voltage, and it takes a 6 dB increase of voltage to make the power increase by 3 dB, but that makes a 6 Db increase in sound pressure level, and a correcponding 6 Db increase on a consoleÕs VU meter. In other words, if you raise the level of a sound on a console by 6 dB, you are doubling the volume. A change of 12 dB higher would make a sound four times louder. I realize that it's a bit confusing to have these two different scales, but since we are normally working only with VU and peak meters we can just remember that a change of 6 dB is either twice or half the level of a sound."
Which makes sense to me as we were messing with car hifi amps we did need to double the power to make 3DB more power, but that was in fact 6Db SPL (what we hear).
But its certainly not 10. From what I can pick up somewhere between 6 and 10 is the perceived doubling, where as 3 and 6 are very exact measurements depending on the situation you are measuring.
so 6Db (SPL) is double
12Db (SPL) is 4x
Still the fact remains the vast majority (in fact all) sound reviews of graphics cards I have read are not conducted with any science behind them and have to be taken with a pinch of salt. The actual frequency will make it sound a lot higher or lower in volume that the true Db SPL.
By their very design PC cases are going to be poor for absorbing sound and as such a lot will pass through the case, but some also will rebound internally. But I bet one case to another could deviate quite significantly depending on how the case was filled with cables, devices etc.
Its quite amazing how some materials will absorb some frequencies, this is basically what a home cinema system is overcoming when you run a test mode with a mic. Its going to pass various frequencies via the loud speakers and measure how much can actully be "heard", and hence boost or decrease that frequency to achieve a flat tone sound.
Flat materials reflect a lot, eg a wall, a pc case side, a laminate floor, "softer" materials reflect less externally and more internally, eg a comfy sofa, a carpet. Thats why a empty uncarpetted room sounds "loud" and also why a sound studio has "egg box" type walls to reflect and absorb the sound so low reflection = more pure sound.
So take the Db readings on reviews with a pinch of salt, your individual PC and pc placement may make a significant impact on those numbers, eg if you have similar frequencies it may combine in effect to your hearing, or alternatively a card that is fairly high in real measurement may not seem so bad for you due to your PC placement. This is why people don't say the same about how noisy a card is.