The human ear is not a linear instrument. We are naturally more sensitive to mid-range frequencies (where human speech lives) and less sensitive to very low or very high frequencies.
$$ N = 2^\fracL_N - 4010 $$
The dynamic range of the human ear spans approximately $10^12$ units of intensity (from the threshold of hearing to the threshold of pain). To manage this vast range, the decibel scale is employed, defined as: phons and sones
From the formula, we can derive the critical rule of psychoacoustics: An increase of 10 phons corresponds to a doubling of loudness (in sones). The human ear is not a linear instrument
The is a unit of loudness level. It is designed to compensate for the ear's frequency dependency. How it works: the decibel scale is employed