What do “sones” refer to in exhaust fan sound ratings? If nearly silent is 0.9, how loud is a 4?
That is a great question. I had to do a little research on this. A sone is a measurement of sound in terms of the comfortable hearing level for the average listener. The lower the sone value, the more comfortable the listening environment will be. For your question, a 4-sone rating is about four times a loud as a 0.9 (~1) sone rating.
Sones are not decibels or volume, but rather how sound is sensed. Sones are a linear measurement, like inches. Doubling the sone value is equivalent to doubling the loudness (i.e. one to two, two to four is doubling the sound level twice). Your 4-sone fan will be more than four times a noisy as a 0.9 sone fan.
One sone is equivalent to the sound of a quiet refrigerator in a quiet kitchen. Typically, the sone level is measured at maximum cubic feet per minute (speed), however some newer products are also being tested at normal CFM settings to provide consumers with typical sound level information.
Always look for a quiet fan, or it will not be used. Broan makes fans below 0.3 (really quiet). If you want more information on fans for the bathroom or kitchen, go to broan.com. They have a great site with lots of information and the sone ratings.