Every now and then, people ask about upwards compression or upwards expansion. Often when this is raised there is some confusion about what “upwards” means, and how it might be different to regular compression or expansion. This confusion is sometimes exacerbated by a misunderstanding about how compressors work (as distinct from what they’re used for).
Essentially, “regular” compressors and expanders both work by reducing gain – that is, lowering the volume. Compressors do it by reducing gain when the volume level of the audio goes above a certain amount, and expanders do it be reducing gain when the volume level goes below a certain amount.
Limiters and gates are extreme versions of the above. A limiter is an extreme version of a compressor – reducing the gain so much that the output level never excedes a certain amount. A gate is an extreme version of an expander – reducing the gain so much that the output level is silent.
Because compressors and expanders work by reducing gain, they can be referred to as operating downwards. You might refer to a regular compressor as a downwards compressor and refer to a regular expander as a downwards expander.
(Sometimes this is a source of confusion because compressors are often used to make sounds louder – because the loudest parts of the sound art turned down, the whole track can be turned up without distorting.)
In contrast, some compressors and expanders can operate upwards. They work by increasing gain. Upwards compressors work by increasing gain when the volume level of the audio goes below a certain amount. Upwards expaders work by increasing gain when the volume level goes above a certain amount.
Upwards compressors and expanders aren’t too common – possibly because they can be unstable and produce really loud levels! Still, sometimes these terms arise and it’s good to understand their meaning. In essence:
Upwards compression – reduces dynamic range by making quiet sounds louder
Downwards compression – reduces dynamic range by making loud sounds quieter
Upwards expansion – increases dynamic range by making loud sounds louder
Downwards expansion – increases dynamic range by making quiet sounds quieter