I thought it all depended on what you were measuring. I think it depends if you're interested in magnitude or power. Power is related to magnitude squared, so that's where the 6 db comes from. 3db is equal to doubling normally, I think. db = 10log10( mag / refMag ). The only problem is, I can never remember when you ought to use 20log10(...).

Anyway, the point is that if we perceive a, b, and c as being "equally spaced" volume levels, then when you measure them, you find that a/b == b/c. This could mean a couple of things, and I don't know which. It could mean that our ear responds logarithmically to volume, or, it could mean that our ear responds roughly linearly, and our brain decides that it would be best to compare ratios.

Sorry this is so vague.

Richard.