Dialnorm
A number in the Dolby bitstream's metadata stating how loud the dialogue sits, so the decoder can turn the programme down to a common level.
The field is present in every Dolby Digital stream and takes values from −1 to −31 dB. The decoder attenuates the audio so that dialogue always lands at −31: a programme tagged −24 is pulled down 7 dB, one tagged −31 is left alone. The idea was that viewers could change channels without lunging for the remote, and that films could keep their dynamics instead of being squashed flat.
The scheme assumes someone measures the programme and fills in the right number, and that's where it fell apart. With the wrong value, adverts and trailers came out deafening. The US eventually legislated with the CALM Act in 2010, using ATSC's A/85 recommended practice for measurement. Europe solved the same problem by measuring the audio itself in LUFS under EBU R 128.