What is the quietest air conditioner for a bedroom?

Noise is measured in dB(A). A quiet split indoor unit on a low setting can be around 19 to 25 dB(A), which is barely audible and fine for a bedroom. Around 30 to 35 dB(A) is a soft hum that most people accept in a living room. Portable monobloc units are usually 52 to 58 dB(A) because the compressor sits in the room with you, which is closer to a dishwasher and can disturb sleep. If quiet matters, a split almost always wins; if you only cool occasionally, a portable may be acceptable.

A portable monobloc keeps the whole machine in one box in the room and vents hot air through a hose to a window. It needs no installation but is louder (typically 52 to 58 dB(A)) and less efficient. A single split has a quiet indoor unit and a separate outdoor unit connected by pipes, so it is much quieter (indoor unit often around 19 to 35 dB(A)) and far more efficient, but it must be professionally installed. A multi-split is one outdoor unit feeding several indoor units, which is the right choice when you want to cool two or more rooms without a separate outdoor box for each.

A real air conditioner has a refrigerant compressor and can lower a closed room below the outdoor temperature, and it is rated in kW or BTU of cooling power. A fan only moves air and removes no heat. An evaporative 'air cooler' blows air over water, adds humidity, needs a window open, and cannot reach a set temperature. compare.ac lists only units with a genuine refrigeration cycle, so everything you see here is a real AC.