a unit of loudness, subjectively determined, equal to the loudness of a sound of one kilohertz at 40 decibels above the threshold of hearing of a given listener
See sone in American Heritage Dictionary 4
(sōn)
noun
A subjective unit of loudness, as perceived by a person with normal hearing, equal to the loudness of a pure tone having a frequency of 1,000 hertz at 40 decibels.