Noise levels explained
Air conditioning noise is measured in A-weighted decibels — dB(A) — and both the indoor and outdoor units carry a rating. Indoor units are generally very quiet, especially on low fan speeds; outdoor units are more noticeable. Because decibels are a logarithmic scale, small differences in the number matter more than they first appear.
What the numbers mean (rough guide)
- ~20 dB(A) — a quiet whisper / rustling leaves
- ~30 dB(A) — a quiet bedroom at night
- ~40 dB(A) — a library
- ~60 dB(A) — normal conversation
How decibels work (and why small numbers matter)
Decibels don’t add up the way ordinary numbers do — the scale is logarithmic. As a rule of thumb, every increase of about 10 dB(A) sounds roughly twice as loud to your ears. So a unit at 40 dB(A) isn’t “a bit louder” than one at 30 dB(A) — it sounds about twice as loud. When you’re comparing units, a few decibels is a meaningful difference, not a rounding error.
Typical noise levels
Indoor units are designed to be unobtrusive: on their lowest or “sleep” setting, many sit somewhere around the high-teens to low-20s dB(A) — quieter than a whisper — rising into the 30s or low-40s on full fan speed. Outdoor units, which house the compressor, are louder, commonly in the 40s to low-60s dB(A) depending on their size and how hard they’re working. Running a unit harder — higher fan speed, or extreme outdoor temperatures — makes it louder.
What to look for: for a bedroom, check the lowest fan-speed or sleep-mode figure rather than the headline maximum — that’s what you’ll hear overnight. For the outdoor unit, where it’s positioned matters as much as the rating: think about windows, quiet rooms and, importantly, your neighbours, since outdoor-unit noise is a common cause of complaints. Siting also feeds into whether you need consent — see planning permission.