ITU-R BS.1770 is the foundational recommendation that defines how loudness is measured, and almost every loudness rule in the world is built on it. Its algorithm has two key parts: a K-weighting filter that shapes the signal to approximate the ear's frequency response, and a gating stage that ignores silences and very quiet passages so they don't drag the average down. The output is a single loudness figure in LUFS (called LKFS in the ATSC world — the same thing). EBU R128, ATSC A/85, and every streaming target are essentially BS.1770 with a chosen number attached. Later revisions refined the gating and added true-peak measurement.
