When recording both electric and acoustic guitars, it's surprising how much 'mud' is captured in the bottom of the frequency range. For the sake of this article, we'll call 'mud' frequencies from 200 Hz and below. While sometimes its ok to have those frequencies on the guitars in your mix, more often than not, they get in the way of the bass, kick and anything else down there. Let's take a look at the use of filtering tighten up those guitars and clean up the mud.
First Things First
Before removing any of the bottom frequencies in your guitar, give the tracks a listen. Solo up the drums and bass and get a tight rhythm section going. Then, bring in the guitars and just focus on what is happening. Did you step on the bass and kick? Or can you still hear everything clearly? Once you've determined that indeed, you can make the bottom of the mix tighter, its time to attack the guitars and remove some bottom.
One of the first plugs-ins I turn to when mixing guitars is the Waves PAZ Analyzer. What this useful tool does is give you a real-time visual representation of frequencies, peak/RMS levels, stereo spread and phase. If you don't have this plug in, simply find any one that gives you a visual representation of the frequencies in your mix. This way, your ears and eyes can confirm with each other exactly how much low-end information is in each track.
I place the PAZ as the last insert on my Pro Tools Master Fader, and then by soloing up the guitars, I can see just how they sit in the mix. This help gives me the information I need to make educated mix decisions.
Clean It Up With The Filter
Most times when EQ'ing guitar, I will insert a full function plug-in that features multiple bands of EQ and filters. I tend to use a variety of plug-ins depending on the needs of the track, but the Sonnox Oxford EQ is one of my favorites. The full version features 5 bands of parametric EQ as well as high- and low-pass filters, which is what we'll use to clean up the mix.
High-pass filters will do just that, let the high frequencies pass through. Confusingly, they are also referred to as low-cut filters (which do just that, cut lows). Either way, they do the same thing—clean up your mud.
On the Oxford, I simply click on the LF filter and select a frequency around 100 Hz. Then, by turning it on and inserting it in the signal path, the EQ will filter out frequencies below that selected value, called a 'cutoff frequency'. Immediately, you'll hear the bottom end clean up on your guitar track.
On the Slopes
The numeric value above the sets the slope of the filter. The slope, measured in dB per octave, is simply how sharp the cutoff is below the selected cutoff frequency. At higher values (such as 36 dB/octave), the removal of frequencies below 100 Hz (in our example) will be aggressive. At lower slope values (such as -6 dB/octave) will be more gentle, allowing a bit more low end information below the cutoff frequency to get through.
Acoustic vs. Electric
With acoustic guitars, I tend to use a sharper slope values to remove the bottom frequencies more aggressively. They tend not to be necessary in a mix (because often there are other guitars in there as well). Also, I often use a higher cutoff frequency, starting at around 120 Hz.
Of course, if the acoustic guitar is a featured instrument in the track, you may not want to filter out the frequencies as much. But as always, use your ears. You may be surprised how much low end of an acoustic guitar can be cut out without missing it. Again, the purpose of removing these unwanted frequencies is to make a cleaner mix where all the instruments have their own space.
Once I have a basic value set for my filter, I will sweep the values. That simply means turning the knob for the cutoff frequency up or down in value, to hear the removal of bass information. I will first do that with the acoustic guitar solo'ed to understand what I'm working with. There will always be fluctuations with the sound depending on the type of guitar, mic(s) used, or DI.
But once I hear it solo'ed, I then put it back in the track and listen to how it sits. I'll then sweep the cutoff frequency as high as I can until I hear too much bottom being removed. Then, I'll knock it back down until the guitar sounds just right.
With electric guitars, I tend to use a softer slope, and a lower value (say around 140 Hz). This way, some of that low end still gets through, but the guitars don't sound too light on the bottom, where the beef is. It may not seem like you're taking out too much, but remember, across multiple tracks, the effects are cumulative.
Most often, when I record guitars, there are multiple microphones used. So just like the acoustic guitar above, every instance and track is different, depending on the guitar, amp, mics and positioning. But 99 percent of the time, I will be filtering all the microphones in some way.
Once again, I will solo up each electric guitar track, and listen to the sound. I'll insert the filter and sweep it around to hear how much information can be cut. Then, I'll put it back in the mix and do the same thing, sweep the cutoff frequency around until the guitar track is just right.
At that point, I will often also click through a variety of slope values, to see how sharply or lightly the bottom can be removed. Many time, the guitar tracks will be doubled and panned left and right. If that's the case, I'll use different slope values and cutoff frequencies on each mic and each left/right part.
Using low cut/high pass filters with acoustic and electric guitars is an easy and quick way to clean up your tracks. As always, use your ears and don't cut information out just to do it. Make sure it has a purpose and that the end result is a cleaner, clearer mix.