The sone is a unit of how loud a sound is perceived. The sone scale is linear. Doubling the perceived loudness doubles the sone value. In acoustics, loudness is the subjective perception of sound pressure. The study of apparent loudness is included in the topic of psychoacoustics and employs methods of psychophysics  <wikipedia>.

GOOGLE DEFINITION: What is a sone?
A sone is a measure of loudness. The higher the sone rating the louder the sound. Understanding how loud an exhaust fan will be can be difficult, largely because bathroom exhaust fans are measured in sones, while most common noise levels we are familiar with are measured in decibels.

Their are other factors that can effect the perceived sound level: wind direction, humidity, moisture, air movement including volumes of air moving through in this case the active ventilation.