Around a year ago I came across a very enlightening post by Bob Macc on the sonic consequences of treating your tracks with aggressive filters. You should go check it out yourself, but the big idea is that using steep filters causes significant phase distortion that reaches all the way up and down the frequency spectrum. Phase distortion removes clarity and impact from your mix, so in short, it’s not good. With that in mind, I’ve come up with some good practice points for filtering your audio.
First, use gentle filter slopes. 6db/octave is my go-to for LPFs, and 12db/octave is my go-to for HPFs. This lessens the impact of phase distortion, but most importantly, it sounds more natural and there’s less risk of accidentally cutting out some good stuff.
Second, use a Bessel curve. Bessel filters are linear phase, which means there’s near-zero phase distortion when they’re implemented correctly. While some EQs have linear phase DSP options, remember that if you’re using steep filter slopes, linear phase will cause distortion too; it’s the combination of gentle slopes and Bessel curves that’ll preserve your audio.
This advice is in line with the Hippocratic philosophy of mixing, e.g. “do no harm.” I’m mixing mostly acoustic music, hut it doesn’t matter if you’re mixing orchestral, folk, punk, or EDM; phase distortion from filters is rarely something you want. Following the two points above will save you a lot of mix clarity and let you put the distortion where you want it.