The sone was proposed by Stanley Smith Stevens as a unit of perceived loudness in 1936. Loudness is the subjective perception of sound pressure. According to Stevens' definition, the sone is equivalent to 40 phons, which is defined as the loudness level of a 1 kHz tone at 40 dBSPL. The number of sones to a phon was chosen so that a doubling of the number of sones sounds to the human ear like a doubling of the loudness.