'Loudness' is the perceived change in amplitude. 'Loudness' is a psychoacoustic metric so its not an absolute and so is subject to individual differences. There is some other oddities such that while many people perceive a 10 dB increase as a doubling of loudness, they perceive a 6 dB reduction as a halving of loudness. And this is all frequency dependent too. Increases in loudness changes its scale depending on frequency, so it depends on whether we are talking about bass, midranges, or upper treble. The don't react the same to human perception, even for the same corresponding changes in SPL. For my own perception, I would say that a 10 dB increase is quite a bit more than a doubling.