Noise ratings explained
Air purifiers are fans, and fans make noise. The spec sheet gives a range, like 24 to 52 dB: the lowest speed to the highest.
The decibel scale
Decibels are logarithmic. Every 10 dB sounds roughly twice as loud, and 3 dB is about the smallest change most people notice. Some reference points:
- 20 to 25 dB: a quiet bedroom at night; the unit is inaudible
- 30 dB: a whisper; fine for sleeping
- 40 dB: a quiet library or a fridge hum
- 50 dB: a normal conversation; noticeable in a living room
- 60 dB: a window air conditioner; most people won't live with it
Why the top speed matters
The advertised CADR is measured at top speed, and on most units that is 50 to 60 dB. People turn it down, and at the lower speed the unit delivers far less than its rated CADR, often a third. Our quietness rating uses the top-speed figure for exactly this reason: a unit that delivers its full CADR at 48 dB is more useful than one that delivers the same CADR at 58.
The practical fix is to buy more CADR than the room needs and run it on a middle speed. The quietest list sorts by top-speed noise, and the bedroom list requires a lowest setting of 26 dB or less.
Rated vs measured
Maker figures are measured in a quiet lab at a stated distance, usually one meter. Reviewers who measure at three feet in a real room read a few dB higher. The independent measurements on model pages note the distance and speed so you can compare like with like.