Loudness: What is Gating?
What is Gating?
Some audio clips may have a prolonged period of silence or low volume sound, which can skew the loudness calculation, resulting in any sound in the clip being altered to be too loud. For example, a nature documentary may have long periods of silence where animals are shown. These extended quieter periods would lead to the loudness being calculated as quite low, so that gain would be applied to make the audio compliant. This risks making noisier sections of the soundtrack have an excessively high volume. In order to combat this, a gating concept to filter out low volume sounds or silence was adopted within ITU BS.1770-2. This concept works in two stages: firstly using an absolute threshold of -70LKFS and secondly using a relative threshold that is 10LU below the absolute loudness calculation.
The file is divided into 400ms sampling blocks each with 75% overlap to the previous and following section in order to obtain the best sample of the audio signal. If the loudness of the sample block is below the absolute threshold of -70LKFS then it is discarded. The loudness of the remaining blocks is calculated and a relative threshold defined -10LU below this. Any blocks below this relative threshold are discarded and the loudness is calculated over the entire file.
Loudness is a newer measurement, and attempts to put a figure on ‘how loud’ the material appears to be to a listener. How a person ‘hears’ the loudness is based on many factors, such as type of programme material, the listeners mood and health, and whether they like the programme material, so it is quite subjective. The Loudness measurement attempts to put a number on how an average listener might respond to the programme material.
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