When talking about power, 3dB represents a ratio of two to one or a doubling of power.
When talking about voltage, 6dB represents a ratio of two to one or a doubling of voltage.
When talking about Sound Pressure Level, 6dB represents a ratio of two to one or a doubling of sound pressure.
Interestingly, our perception of loudness is not the same as sound pressure level. Although the actual formulae is somewhat complex, as a rough rule of thumb, an increase of 10db SPL is perceived to be approximately twice as loud.
link