How loud are portable ACs really, and which are quietest?

Most portable ACs run 50–58 dB on low. Inverter-compressor models (like the Midea Duo and Whynter NEX inverter line) cycle more gradually and are commonly rated near 42 dB on their lowest setting, which reviewers describe as library-quiet. Older single-speed compressors are noticeably louder, especially on the compressor cycle.