"Correction" - the HPF

The EQ the most abused/neglected section on a mixing console.  I always try to teach people at square one that the EQ should primarily be used to subtract, or "sculpt" the sound.  When have you seen a sculptor pack material on to create a statue?  Yet, I still walk into our student Chapel every couple of weeks and find almost all of the vocal channels with low-mid and high-mid frequencies boosted.  Thats a muddy mix, and its asking for feedback.  I rarely boost EQ on any source except for some playback channels or to accent transients on drum channels.  

On some old Neve audio consoles that were built to “military spec”, the term used for this section on the mixer was “correction”, instead of “equalizer”.  That gives a little bit different picture of what this section of the mixer does, right?

The HPF (High Pass Filter) and the LPF (Low Pass Filter) may cause your brain to hesitate because the HPF is used to subtract “low” frequencies, while the LPF  is used to subtract “high” frequencies.  The HPF is literally my FAVORITE filter.  

I use it on almost everything in my mixes (if the console has them available).These filters are best used when trying create ‘space’ or 'separation' in your mix.  The idea is to cut out the mud on everything except the low frequency instruments that end up in the subs.  This way, those instruments have priority in that low frequency ‘space’.  The HPF cleans up vocals, guitars, piano, most orchestral instruments, etc.

I usually set my HPF to cut starting WAY up high around 150Hz and adjust one way or the other depending on the voice.  Many times I've had someone looking over my shoulder asking, "why are you cutting way up there?  
#1 - there isn't much that is useful/critical for articulating a voice below 150Hz for MOST instruments/voices (check the chart to the right for a reference for "useful" freq ranges). 
#2 - its not literally cutting it OFF there.  That is where the slope BEGINS.

The HPF "correction" filter is not a brick wall.  These are typically 6dB, 12dB, or 18dB slopes.  The image below shows the HPF applied at around 160Hz with a pretty steep slope.  On the right side you see a high frequency "shelf" cutting at roughly 10k.  That is NOT a LPF. Sorry, couldn't find a better pick than this.