What is a High Pass Filter?
A high pass filter (sometimes called a low cut filter) allows high-frequency signals to pass unchanged, but reduces the amplitude of signals lower than the cutoff frequency. The amount of attenuation is defined by the slope. Using a high pass filter on parts can help focus a song’s low end by only allowing the most important low end sounds to exist in the mix.
To start, decide what part or parts of the song should provide the sub bass frequencies. This would usually be the kick drum and bass synth or bass guitar. Use a high pass filter on all the parts of your mix except those. Start at approximately 80Hz, then sweep up slowly until you start to hear the sound change and then turn in back down slightly. Experiment with different slopes; with 6db/Oct sounding more gentle and “natural” and on the other extreme; 48db/Oct sounding modern and “digital”.
Advanced mixers should experiment with a resonant high pass filter on parts to make it poke out. Somewhere between a Q of 1 and 1.4 should be all that is needed, but higher Qs could yield interesting results, too. A resonant high pass filter can even be used on sub bass parts like bass or kick drum to make them really stick out. This is how the famous Little Labs Voice of God works.
My all-time favourite high pass filter is FabFilter Pro Q. The second generation of this plug-in was recently released and it sounds amazing. For producers just starting out, I suggest checking out iZotope Alloy. It packs a great EQ, Compressor and more in a single plug-in so you only have to learn one GUI. Alloy is both quite musical and a good value.