Loudness Standards

December 30th, 2010 by Ian

It seems like there are a dozen different loudness “standards” floating around.  So which ones are legitimate?  In this post, we will give a brief overview of several documents.

ITU-R BS.1770-1 – “Algorithms to measure audio programme loudness and true-peak audio level”

This document introduces the fundamental algorithms used for measuring loudness and True Peak levels.  The algorithms are based on the results of listening tests, where participants were asked to match the volume of various test signals to a reference signal.  The loudness algorithm works by pre-filtering each channel (up to 5) to attenuate bass frequencies and account for the acoustical effects of the human head.  The loudness of each channel is measured and then averaged to produce a single loudness value.

Link: ITU-R BS.1770-1

ATSC A/85 – “Techniques for Establishing and Maintaining Audio Loudness for Digital Television”

This American document recommends the use of the loudness algorithm found in ITU-R BS.1770-1 and provides further recommendations for controlling audio loudness in different situations: post-production, live events, file based, etc.  For example, when exchanging audio content, they recommend that loudness be -24 LKFS +/- 2 dB with True Peaks not exceeding -2 dB TP.  As per the requirements of the CALM Act, the Federal Communications Commission (FCC) will make loudness recommendations based on this document in the near future.

Link: ATSC A/85

EBU R128 – “Loudness normalisation and permitted maximu level of audio signals”

The European Broadcast Union, and specifically the PLOUD group, also recommend the use of the loudness algorithm from ITU-R BS.1770-1.  EBU R128 takes ITU-R BS.1770-1 a step further, defining an “EBU Mode” for audio meters that includes a loudness range indicator, gating behaviour, as well as various loudness scales and integration times.

Link: EBU R128

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