Bathroom fan noise is usually measured in sones. A lower sone rating means the fan sounds quieter to the human ear. This is important because people are more likely to use a quiet fan long enough.
A very quiet bathroom fan may be rated around 0.3 to 1.0 sones. A moderate fan may be around 1.5 to 2.0 sones. Older or budget fans can be much louder.
Do not choose the lowest-sone fan without checking airflow. The fan still needs enough CFM for the room size and moisture load. The best choice is usually the right CFM range with a low sone rating.
Installation can also affect noise. Loose housing, restrictive ducting, tight bends, or an undersized duct can make even a quiet-rated fan sound louder.
Bottom line: for most comfort-focused bathrooms, look for the right CFM and a sone rating around 1.5 or lower. For bathrooms near bedrooms, consider even quieter models.
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