In simpler terms, a sone is a linear measurement. This means that if a sound is rated at 2 sones, it is perceived as as a sound rated at 1 sone. If a sound is rated at 4 sones, it is perceived as four times as loud as 1 sone.
The relationship between sones (S) and phons (P) is governed by Stevens' Power Law: [ S = 2^(P - 40)/10 ] Where: sones meaning