Acoustics is a complex study, with the additional difficulty of trying to quantify that every person perceives sound differently. Any sound source is a mix of multiple frequencies, each with different power levels. In response to this, acoustical engineers have developed single-digit parameters like sones and decibels (dBA). Engineers prefer the dBA, a logarithmic measure, to sones, since they are meant to correspond to the sensitivity of the average human ear and its varied response to different frequencies.
A common measurement for fans is to measure dBA 1 meter away from the inlet grill at 45 degrees. Actual installation conditions can affect the sound level in a room. For example, a “live” room with hardwood floors and wood furniture will be much noisier than a “dead” room with carpeted floors and upholstered furniture.
Fan noise ratings vary from 50 dBA to 70 dBA. Note that 10 dBA is considered a doubling of perceived sound.