On Using Filters

By: Tony Williamette

Date: 5/17/22

High-pass and low-pass filters, specifically. Real filters, as I think of them- if only because I don’t want to get into the muck and mire of other EQ curves.

An Introduction

High-pass and low-pass filters are easy to understand. Then why were they so difficult to understand when I was 18 and studying audio in college? Professors and text would throw around the terms “high-cut filter” and “low-cut filter” which further confused me.

To simplify things:

High-pass filter = low-cut filter

Low-pass filter = high-pass filter

Additionally, most filters have some sort of slope settings wherein you can decide how steep you want the roll off to be.

Overall Cleanup

With the growing amount of great recording being tracked at home, I’m hearing ceiling fans, dogs barking, and a higher noise floor overall. Often times a high or low pass filter can clean this up without having to use sometimes-destructive noise reduction plugins.

Helping a Compressor

Sometimes we’ll find compressors like the Empirical Labs Distressor or buss compressors that feature a high-pass filter. This can help keep the compressor a bit more honest and not react to the high energy of the low frequencies, which can often trigger the compressor in undesirable or extreme ways.

Frequency Slotting

I’ve long believed that you can get about 26% of the way through mixing a record by just setting rough levels and using some smart filtering.

For example, I routinely find myself high-pass filtering all instruments below 60-100hz, save for the kick drum and bass/808s. This may seem very simple, but it can clean up a lot of that low end and create nice space for the bass and kick. This isn’t any sort of hard and fast rule and depends on the genre, song, and sound of the recorded instrument.

The same can be said for using a low-pass filter on kick, bass, guitar, vocals, and a wide range of other elements. I find this to be particularly helpful when working with sample or MIDI-based productions as these tracks seem to feature an increasing amount of material in the 15K-18K range.

Simple filtering like this helps me quickly start “slotting,” allowing each element/instrument to have its own natural place in the song. Separating elements of a mix like this can often be done with levels or time-based effects, but filtering is another great method and fairly easy to wrap your head around.

I was first formally introduced to frequency slotting by Bob Katz’s fantastic book Mastering Audio. Bob inserted a print of the Carnegie Hall Musical Pitch Relation Chart. We’ve got a copy of this thing framed and hanging in our studio. There are a few other versions floating around the internet, but this one just looks so damn cool, doesn’t it?

I should mention that frequency overlap can be a good thing. I find when I go overboard with frequency slotting and get a little too precious about things, the mix can start to feel a bit anemic. It’s sometimes those overlaps that help create new sounds you didn’t realize was holding the mix together or making things sound big. I think of this much in the way that crosstalk (think old consoles) or bleed (think live band tracking) can help glue a mix together in a certain way.

The above mentioned are mostly utilitarian uses of filters, but there are so many ways to use them in a mix: from filter sweeps or just creating stark contrast between different mix elements, the creative applications are infinite.

Like everything else in audio (and life?) keep it simple and use your ears. It you found any morsel of this helpful- share with your friends/colleagues/students/partners/peers.

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