Dialnorm is a metadata field in AC-3, E-AC-3, and AC-4 that tells the decoder how loud the dialogue in the stream is, on a scale from -1 to -31 dB. The decoder then attenuates playback to bring everything to the listener's reference level, so channels and programs with different mix loudness all arrive consistent. It is the broadcast world's original answer to the loudness problem, predating LUFS normalization by years and solving the same complaint: shows and ads that jump in volume. Get dialnorm wrong — a mismatched value — and audio is audibly too loud or too quiet, which is why it is a classic source of broadcast loudness faults.
