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Old 16th December 2008
Lives for gear

I believe masking is a function of our brain that filters out softer sounds at certain frequencies in favour of the louder sounds at those frequencies.

The sounds are never "not there" - summing is accurate, whether analog or digital, but we just don't hear them, or don't focus on them.

E.g. - a background AC hum that is very audible when a guitar is not playing, but seems to disappear while the part is playing. The hum is always there, and is always summing with the wanted signal, adding to the sound and creating sum & difference beat notes, etc. We say the guitar playing 'masks' the hum, but it's really still there. We don't really know how much damage it's doing until we remove the hum, and it just sounds much better. Or not. Perhaps it's adding something desirable, in some situations.

I feel it's an important distinction to make about 'masking' - never forget that the lower level signal is still there. And maybe our subconscious can decode it, even if our conscious hearing is filtering it out ...

Our brain filters a lot of audio information out all the time - think about how we can follow the conversation of somebody running around a big empty hall ... in reality, the voice is swamped in a constantly changing wash of reverb - but our brain can focus on the voice, and filter out this confusing information. But record it with a microphone, and the filtering is gone ... our brain is then focusing on the reproduction of the recording, and we hear all the changes in reverb, phasing, etc.

So I consider masking to be a psychoacoustic effect, not an electronic effect. And some people think it can be exploited for nefarious purposes. Subliminals.

As far as mixing goes - the low level stuff is energy. You have the choice of removing it, to clear a space for the louder/wanted energy, or leaving it there, because it adds to the total sound we want.