Streaming services do not play a track as loud as it was mastered; they adjust it to a target level. A loud master is turned down, a quiet one is raised only as far as its peak level allows. The meter measures the values this decision rests on and recalculates the normalisation for Spotify and for the AES and EBU recommendations.
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WAV and AIFF are read sample by sample; FLAC, MP3, AAC, Ogg and Opus go through the browser's decoder.
Integrated loudness: the average over the whole file, K-weighted and gated at two thresholds (absolute −70 LUFS, relative 10 LU below the average). Normalisation is based on this value.
True peak: the highest level of the signal between the samples, found by four-times oversampling. It often lies above the sample peak.
Loudness range (LRA): the distance between the 10th and the 95th percentile of the short-term values according to EBU Tech 3342, a measure of dynamics over time.
Momentary and short-term: loudness in sliding windows of 0.4 and 3 seconds, searched here on a 10 ms grid.
PLR: the distance between true peak and integrated loudness. A small value indicates heavily compressed material.
Sources: ITU-R BS.1770-5 · EBU R 128 · EBU Tech 3341 · EBU Tech 3342 · Spotify for Artists · AES TD1008 · Apple Digital Masters
The file is read and measured in the browser and never leaves the device. Measurement follows ITU-R BS.1770-5 (K-weighting, gating, four-times oversampling for true peak) and EBU Tech 3341 and 3342.