Sones are a measure of sound level similar to decibels. An exhaust fan rated for a high number of sones is louder than an exhaust fan with a lower number.
A typical economy exhaust fan emits 4 sones, but some exhaust fans are rated at as low as 0.3 sone. For reference, rustling leaves measure 0.5 sone, a quiet house is about 2 sones and a typical TV volume is 4 sones. Two sones are twice as loud as 1, and 3 sones are twice as loud as 2. The scale continues to double with each additional sone. An exhaust fan's installation can affect the actual sones produced.