EQ & Compression: Chicken & the Egg??
If you use a chain of multiple processes on an instrument, you might wonder what the order should be regarding EQ & Compression. Equalization is primarily about changing signal levels, albeit in carefully specified frequency regions, so pre‑compression EQ can alter the gain‑reduction action of the compressor, but post‑compression EQ won't.
If you're happy with the way your compressor is working, just put any equalization after it in the processing chain, but if you find that frequency‑based problems make it difficult to achieve the compression you want, dealing with this problem via pre‑compression EQ makes sense. For example, extreme low‑frequency thumps from a vocalist tapping their foot on the mic stand can play havoc with attempts to compress the vocal itself. Filtering out these low‑frequency level peaks with EQ, pre‑compression, can immediately make the compression sound much more predictable. Sporadic low‑frequency resonances from acoustic guitars or guitar/bass cabs can also be tackled in this way.
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When combining an EQ and a compressor the order in which they are placed in the signal chain makes a big difference in the overall effect. These differences are due to both technical and psychoacoustic factors. Placing an EQ before a compressor can have the effect of exaggerating the applied EQ, due to a phenomenon similar to the psychoacoustic effect known as "frequency masking".
Frequency masking is a phenomenon by which louder sounds (or louder frequency ranges within a sound) tend to draw our attention away from the less audible sounds (or frequencies) nearby. Any frequency boosted by the EQ will cause the compressor to lower the overall level whenever the signal source contains frequencies in the boosted range, and this sudden level shift can make the track "sound EQ'ed" without actually producing the frequency specific level changes intended by the EQ. This phenomenon explains why many people think that compressors sound "dull".
This phenomenon explains why many people think that compressors sound "dull".
Placing an EQ after a compressor you can often attain more audible results with less EQ, (and therefor fewer EQ artifacts), producing results that often do not "sound EQ'ed". Most mastering engineers EQ post compression in order to enact the most change with the least EQ.
To understand this technically, think of a compressor and EQ as one integrated unit. Placing the EQ before the compression in this view is like a having a compressor with a frequency dependent threshold. An EQ boost (for example) will send more signal at that frequency to the compressor, which in turn will react to this increase in level and try to control the output level by compressing more, often thwarting the intention of the knob-turner.
From the perspective of Psychoacoustics, the human nervous system is designed to detect changes in our surroundings as a survival skill: A loud sound in a quiet environment, a sudden silence in a noisy environment, a breeze on a still day, etc... These changes in our environment command our immediate attention. In the example above, the compressor will react more to the boosted frequency and we perceive the overall change in level as a sort of "focusing" on the boosted frequency.
courtesy of https://www.softube.com/index.php?id=eq_before_compressor