When you start creating mixes, you quickly realize that the low and low-mid frequencies are problematic. Without care and attention, they can sound muddy, boomy, and unclear. Mid- and high frequencies are much easier to manage—you can have multiple instruments and voices taking up the same sonic bandwidth and still hear everything clearly. Try this in the low frequencies and it’s a mess.
We feel low frequencies in our bodies. It’s where the punch, the groove, and the drive of a track lives. It’s critical to your mix that instruments in the low frequencies be clear, full, and bold.
As with many mixing decisions, it’s not about increasing the level or power of bass elements; rather, it’s about eliminating the stuff that gets in their way.
Enter the ubiquitous high-pass filter. The name is fairly self-explanatory; it filters out sound so that only high-frequencies pass through. The high-pass filter is sometimes called a low-cut filter. It’s easy to see how it works with a diagram. The horizontal represents frequencies, and the vertical represents amplitude, or volume.
In this example, the filter is set at 80 Hz, which means everything under 80Hz will be reduced in volume—just follow the slope of the line. For reference, 80 Hz is about the same as the low E string on a guitar. The low E string on a bass is one octave lower, about 40 Hz (that’s E2 and E1 on the piano).
For most instruments, including the human voice, there’s very little of value below 80 Hz. The bass guitar and thud of the kick drum usually live between 40 Hz and 250 Hz. So, the general wisdom is to high-pass everything except the bass and kick drum to 80 Hz or higher. The kick and bass will then have room to be heard clearly, which usually adds punch and groove to your mix.
Every single digital audio workstation (DAW, the software you use to record audio) has a high-pass filter. Usually, it’s a feature of your EQ (equalization) plug-in.
How high is your high pass?
The next question, then, is how high should you set your high-pass filters? That depends on your material. The rule of thumb is to dial up the filter during playback on a track until it starts sounding thin, then back down a bit. If you high-pass too much on guitars, pianos and vocals, you could rob the mix of warmth and body. If something sounds thin in solo, it could be just right in the mix; never judge your settings when listening to a track in solo—it only matters if the mix sounds good.
High-pass filters can also be used on bass and kick drum, but normally they are set very low. For dance music, you may want to include and emphasize the sub-bass (below 40 Hz). For most rock, pop, country and folk tracks, I recommend minimal high-pass filters on the bass and kick drum. Again, what matters is the kind of sound you’re after. Mix with purpose and you’ll get what to where you want to be much faster.
Microphones with high-pass filters
Some microphones have a switch for a high-pass filter. Usually it looks like this, where the crooked line indicates the “on” position for the filter. Usually they’re fixed at 80 Hz.
If you’re recording vocals or guitar (where there will be a bass in the arrangement), it’s advisable to use the high-pass filter on your microphone. Eliminating low frequencies you know you’re not going to need during the recording phase allows you to record a more consistent, louder signal.
Always mix with purpose
Finally, it’s easy to get carried away and high-pass everything judiciously. If you mix with purpose and subtlety, the overall effect can be dramatic. In other words, a subtle high-pass filter on 8 tracks, when they are all mixed together, can make a big difference.
I’ve personally found using high-pass filters to be the one technique that is universally effective on just about any mix, from a simple voice-over narration to a full band. Next to volume, it’s the move that makes the biggest difference to your mixes—in a good way.
If you’ve never mixed with using a high-pass filter, try it out on an old mix and see if it doesn’t clear out the muddiness and open up the sound. I’d love to hear your thoughts—leave a comment below.