The sound level is measured from 1 meter away from the Air Purifier with 33dBA background noise.
The decibel level of the fan depends on its speed.
While in Auto Mode, the fan speed is adjusted according to the PM2.5 reading.
So it can vary from 39 dBA to 54 dBA depending on the PM2.5 reading.
The same decibel level range applies when the fan speed is set manually, in accordance with the level set.