What is LUFS and LKFS?

Introduction LUFS and LKFS

LUFS (Loudness Units Full Scale) and LKFS (Loudness K-weighted Full Scale) are units used to measure the loudness of sound. They serve as international standards to quantify the relative loudness of audio, based on how humans perceive it. LUFS/LKFS is utilized to measure the average loudness across a variety of audio content like music, films, and broadcasts, applying a K-weighted filter in the process.

Link to Loudness document

What is Loudness?

Terminology and Standards

LKFS is an abbreviation for Loudness K-weighted Full Scale and is used in the ITU BS.1770 and ATSC A/85 standards. The European Broadcasting Union (EBU) uses the term LUFS. Essentially, LUFS and LKFS refer to the same concept, although the use of LUFS is increasingly recommended over LKFS.

Relative vs Absolute Measurement

LUFS is an absolute measurement. However, for easier understanding, a relative measurement called Loudness Units (LU) has been defined. Broadcasters usually set the target level at 0 LU, making it easier to discern differences compared to a reference. For example, if a broadcaster’s target level is -23 LUFS, this would be set to 0 LU, so a reference of -27 LUFS would have a -4 LU difference.

Loudness Range (LRA)

The Loudness Range (LRA) describes the range of loudness from the quietest to the loudest parts, measured in LU. The top 5% and bottom 10% of the overall loudness range are excluded from the LRA measurement for more accurate assessment.

Program Loudness

Also known as Integrated Loudness, Program Loudness describes the overall loudness and is represented in LUFS or LU. Measuring Program Loudness solely based on the overall average level can lead to inaccuracies. Therefore, a gating method has been developed to temporarily halt measurement when the audio level falls below a -10 LU threshold.

Short-term and Momentary Loudness

Short-term refers to the loudness measured over a relatively brief period, usually within 3 to 5 seconds. It is particularly useful for detecting quick changes in loudness in dynamic music or audio signals. Momentary measures the loudness over an extremely short period, typically ranging from 400ms to 500ms, and is used to capture rapid changes or peaks in music or audio signals.

Advanced Features

Many loudness meters include built-in true peak meters for more refined peak level representation.