A very typical nugget of audio wisdom is to use those highpass filters. Use ’em everywhere! It’s even number one on my list of “3 simple tricks to a better mix” blog. But, like most things, it doesn’t come for free.

Let’s get (semi) technical.

All filters cause phase shift. Every EQ on the planet (with the exception of digital FIR filters) introduce varying amounts of phase shift depending on the amount of boost or cut.  Seeing as a highpass (and lowpass) is still a filter, the same is true here.

highpass

 

 

Here are two duplicate tracks of an electric guitar.  The bottom is the waveform after a highpass filter was applied (18dB/oct @ 80Hz) Notice how different they look? THE HORROR!

 

 

Whenever you use two or more transducers to pick up a single source and play them back at the same time, phase has a huge impact on the overall sound. In the above scenario, if I had two mics on the same guitar speaker and only highpassed one of them, there would be some serious artifacts down around the filter frequency.  The same would apply if I wanted to run these duplicates in parallel for other reasons.

Obviously, we’ve been using EQ and highpass filters since the dawn of time (ok, since the 1940’s-ish) and music still sounds awesome, so why bother to bring it up?

Just raising awareness folks. Use those filters, and then use your ears. Can you really hear those artifacts? In most cases, probably not. Mixes tend to sound overwhelmingly better with all that low-end clutter removed, so you’re unlikely to hear that subtle comb filtering as something negative. But it’s always good to keep a mental image of what’s happening in your mind when you grab an EQ.

This is why many people like to change the instrument, or the mic, or mic placement before using EQ to get the sound they’re after. Best of both worlds!