Kim Lajoie's blog

How loudness is measured

by Kim Lajoie on August 11, 2014

The meters on your DAW channels or your mic preamps aren’t telling you the whole story.

When sounds are recorded, the microphone captures the continuous vibrations in the air and creates a continuously varying electrical signal that mimics the vibrations. Louder sounds have wider/stronger vibrations. When you have level meters on your gear, it usually shows you the ‘peak’ level – the strength of the electrical signal created by the microphone. Or the strength of the electrical signal that will be turned into air vibrations by your speakers. (Digital meters work much the same way – they just measure the digital signal, which is just a numerical representation of an electrical signal)

The trouble is, the peak level doesn’t exactly represent how loud we perceive the sound.

One of the (several) ways in which our perception differs from ‘reality’ is that we don’t hear extremely short sounds as loudly as longer steady sounds. Some level meters compensate for that (to show us a more accurate representation of how we hear) by slowing down the meter. By making the meter more sluggish, it doesn’t react as strongly to quick changes (short, sharp sounds) but still reacts strongly to longer steady sounds. This is often referred to as the ‘average’ or ‘RMS’ level (RMS stands for ‘Root Mean Square’ – a mathematical way to measure the signal slowly).

The crest factor of a sound is the difference between the peak level and the RMS level. Sounds with a high crest factor typically have a lot of short sharp peaks (e.g. a drum kit). For these sounds, a peak meter would show a high level but an RMS meter would show a much lower level. Sounds with a low crest factor are the opposite – they have fewer or lower peaks, or no peaks at all (e.g. an organ). For these sounds, a peak meter would show a similar level to an RMS meter.

-Kim.

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