tl;dr: Level match before and after version of whatever you do!!

This statement is crucial and fundamental to everything you will do in your musical journey. Among all the tips and tricks available over the internet this is the only one which you should always do. Let this be your natural habit and instant reflex!

Why is it so important to any comparison you’re gonna make during music production, mixing and mastering?

Our brains are funny, and they have a long evolution history which influence us even this day and age. There are many different phenomena, one of them – which is crucial – is the difference of frequency balance perception depending on the sound pressure level we are exposed to. The changes are shown by Fletcher-Munson Curves which are established by series of human hearing measurements. The lines show what is the sound pressure level needed for our brain to interpret given frequencies as equal in loudness. And as you can see the shown curves change significantly depending on the sound pressure level.

But what that means in practice? I’ll leave the theoretical stuff from wikipedia article and graph interpretation as a homework for you. Meanwhile make yourself an experiment. Put any music into your DAW, duplicate the track, change the volume of duplicate by -0.5 dB. Compare the differences you hear when switching between the two. Spoiler alert, the louder version will sound fuller, deeper, wider, punchier, with more highs and lows, but the only thing that changed is just the 0.5 dB of volume. This is always mind blowing for people who never experienced such experiment.

And that’s what can be the source of bad decisions while working with sound. Every process we apply somehow changes the perceived loudness. Doesn’t matter if the result is sounding louder or quieter, the louder one will always „win” in such comparison by the sole issue of just being louder as described above. Imagine applying the limiter to your master bus, you gain +6 sB of loudness, now it’s sounds much better you think, but wait, level match the before and after just for sake of comparison. And now magic happens again, the limited +6dB version doesn’t sound impressive any more, you hear pumping, you hear no transients and other issues that were “masked” by sole fun of just having shit louder.

So what are the conclusions here, well there’s few.

• If you won’t level match before and after processing, comparison does not make any sense, because the difference of the sole processing will be mixed with the psychoacoustic effect of the change of level. No matter if you’re comparing your eq, compression settings or comparing your mix to some reference music. 
• Use plugins and tools which have the output gain control, so you can easily level match by hand.
• Use plugins which have auto gain feature, but be aware that there are some plugins in which this feature works really bad and overshoots the settings by a lot!
• Mind that plugin developers know the psychoacoustics and all what is described above. There are some plugins notorious for having their default setting just louder and not even doing anything at this stage.

Tools to help you with the issue:

• Your ears, in fact this is the most important tool, you have to learn how to spot level differences, it’s not easy at the beginning, sometimes it’s not that easy when you’re more experienced. Practice as much as you can.
• Make the loudness matching your automatic habit and the thirst thing you do before making decisions
• Plain old LUFS meter, any will do, just measure before, measure after and the difference in value will be your output correction of the processing. But be aware that even using specialised metering might fail in some very corner case situations. More on metering in future posts …
• Ian Shepherd’s Perception AB Plugin.