A sone is a recognized unit of loudness and describes the sound output of your hood. The lower the sone value, the quieter the hood.
One sone is approximately the equivalent of a quiet refrigerator. Sone levels are the lowest when your hood is set to the lowest speed, and increase incrementally with each speed. Typically, the sone level is measured at maximum hood speed or CFM. Sones are a linear measure like inches. Doubling the sone value is equivalent to doubling the loudness.