Back when the majority of music was recorded on analog tape, little to no information was actually recorded below 30Hz or so. Fast forward to the digital age and now every bit (no pun intended) of every frequency is captured all the way down to 0Hz. Where this sometimes becomes a problem is with studio users who are monitoring on small speakers, in environments that aren’t acoustically isolated (the case in many home studios).
In this situation, there can be a build up of low-end noise that’s too low in frequency to be heard on the small monitors, yet it is eating up headroom and affecting audio that can be heard. The easy way to get rid of this low-end noise is using a high-pass filter. When you start a mix, as a first step try putting a high-pass filter/EQ across every track, set to 30Hz. Problem solved!
To take this one step further, raise the frequency of the high-pass filter on tracks that have a lot of muddy or unwanted low end. Big electric guitars, guitar solos, keyboard pads — anything that’s potentially interfering with the bass and the drums below 100-200Hz. The only catch is to not raise the filter cut-off frequency too high. The goal is not make everything sound thin, but instead to shave off that unwanted mud. A good rule of thumb is to go track by track, turning up the high-pass filter cut-off frequency on each track to where it sounds like it’s starting to clean things up, then back it off about 20%. Happy mixing!