1db is about the threshold of human perception on a solid test tone. With things like music or more complex sounds it is harder to hear small differences.
One click on a volume knob (if yours clicks) is about 1 to 2 db. 3db is twice the energy, but definitely not heard by a human as "twice as loud". 3db is what you get when you swap out a 300 watt amp for a 600 watt amp. This is why you can point and laugh at people who say their stereo is twice as loud afte getting an amp that is twice as big. It's a log scale.
As for what is heard as twice as loud, you could argue that for decades. It is subjective. 6db? 10db? I dunno. Personally I subscribe to the "about 10db" theory, but even that isn't perfect. 100db might be "twice as loud" as 90db to a person, but 110db may not be "twice as loud" as 100db to the same person. It's a crapshoot. And that is comparing the exact same tone. Two exhausts with simlar objective or measured loudness (db) may seem to a person to have a more drastic difference in subjective loudness .