eq fx tips Home Studio Mixing

FX Tips – EQ: Mixing with high pass filters

A high pass filter is a type of equalizer that eliminates frequencies below a specified cutoff point. Since it is low frequencies that are removed, only high frequencies may pass (thus the name). You might also hear high pass filters (HPF) referred to as low cut filters which is a bit ambiguous. High pass filters have a few very important roles to play in the mixing process. Your home studio recordings can benefit greatly from wise use of the powerful eq techniques outlined below.

Sweeping a high pass filter

Sweeping a high pass filter is very similar to the techniques outlined in my eq sweeping guide. The main differences are in the Q (bandwidth) settings used and what you listen for as you sweep. Not all equalizers offer adjustable bandwidth on their HPF. Some offer a highly adjustable bandwidth while others offer preset slopes (6, 12, 24dB per octave are common). A higher Q, a higher slope value, and a lower bandwidth value all result in a steeper HPF. Steeper high pass filters roll low frequencies off more aggressively. Set your high pass filter up initially at a frequency of 20Hz and Q of 1.4 (about 1 octave bandwidth). If your HPF only offers preset slopes then set it to the steepest setting. If your HPF doesn’t allow you to set the slope then don’t worry about it. Your HPF may also not let you go all the way down to 20Hz. If that is the case then just go as low as possible. You should end up with something like Figure 1.

initial hpf setup
Figure 1

Now we are ready to begin the sweep. With your whole mix playing start to sweep the HPF upward. Identifying the sweet spot and what you do with the eq after you have found it is largely dependent on how you are using the high pass filter. These cases are covered in the next few sections.

High pass filter for clarity

Once I finish the tracking phase of my project I will usually start the mixing phase by adding a high pass filter to almost every track. The number of tracks that get the treatment is proportional to the number of tracks in the mix. A dense 30 track rock mix will have a lot more HPF action than a two track songwriter demo (which probably won’t have any). We are using the eq in this case to bring out extra clarity in our mix. The low end energy on multiple audio tracks can really start to build up and cause a lot of masking in the bass region of your mix. Using eq like this on many of your tracks can really open up the low end and make extra space for that fat bass guitar line or solid kick drum (or both).

Sweep to find the point where the track has lost its low end muddiness but retained its low end character. The slope is then backed off to a more gentle Q of around 1.0 (bandwidth 1-1/3 octaves).

High pass filter for solid low end

In the previous section I mentioned that my mixes will typically have a HPF on almost every track. Even kick drums and basses? Especially kick drums and basses! When you boost a low frequency based track you also have the potential to boost a lot of rumble. Kick drum mics are especially vulnerable to extra resonance and rumble from inside the shell or standing waves in the room. Sometimes the HPF on these instruments will be fairly low, around 30-40Hz, just to eliminate the low end rumble as described in the section on clarity. You can also use the high pass filter to allow a more aggressive boost in the lower frequencies.

As an example, we will consider a kick drum that gets a nice, fat low end boosted by 6dB at 100Hz. You will probably notice a lot of extra low end rumble with a loss of clarity in your mix. The problem will be even more evident if you have a subwoofer hooked up with your home studio monitors. A quick look at the frequency response graph in Figure 2 will give us some clues about why this has happened.

kick drum boost with no hpf
Figure 2

It is immediately apparent that our 100Hz boost has affected frequencies all the way down to 20Hz and below. There is still about a 0.5dB boost at the 20Hz mark (orange dot). I don’t know of any audio engineers who would boost a kick drum by any amount at 20Hz. The extra boost in the low frequencies is a side effect of the 100Hz boost and is what causes us to lose low end definition. We can turn on the HPF band and adjust it to get rid of the problem frequencies in our kick drum. Engage the HPF and sweep to find the point at which the rumble is fixed. You could end up with a setup like that depicted in Figure 3.

kick drum boost with hpf
Figure 3

The HPF has eliminated all the extra and unwanted low frequencies caused by the desired 100Hz boost. We are left with a solid and clean low end on the kick drum. Here are some audio files to demonstrate this principle.

  • [sound clip A]: This is a two bar clip from a TouchyFeeliacs recording. Most of the tracks are muted to allow you to focus in on the sound of the kick drum. The kick drum has its natural low end here with no doctoring of the eq.
  • [sound clip B]: This is the same two bars but this time the kick drum’s low end has been boosted by 6dB at 60Hz. The low end has gotten a lot more robust but it also has a dirty character.
  • [sound clip C]: A high pass filter has been applied to the kick drum eq at 50Hz. Focus on listening to just the kick drum’s low end and you will hear the difference. This frequency range now has a very refined and focused sound.

Note: All three of the sound clips exhibit a kick drum sound that could work in a mix. You have to decide what kind of sound you are looking for and apply the techniques that will get you that type of sound. You will likely not notice a difference unless you line the tracks up in your DAW and listen critically to each one.

High pass filter for correction

Microphones are notorious for picking up the slightest bit of rumble. This can be caused by handling noise or vibrations stand vibrations. A home studio often lacks the resources to float the floor for soundproofing and isolation from ground vibrations (caused by street traffic or perhaps the washing mashine downstairs). These vibrations are passed from the ground and floor through your microphone stand and straight into the microphone. A shockmount for your mic can go a long way toward eliminating these problems but shockmounts can be expensive and not all mics fit very well in them. If you find your track suffering from this or any other type of rumble or low frequency problems you can use a high pass filter to correct it. I will typically set the eq up with a steeper slope for this type of work. This will allow you to filter out more of the problem without attenuating too much of the wanted low frequencies above it. A high pass filter attenuates by 3dB at the frequency it is actually set to. If set at 20Hz you can still have slight attenuation all the way up to 30Hz or higher (depending on the slope, see Figure 1). The steeper your slope, the less frequencies above the cutoff point will be affected.

High pass filter for ambient mics

Ambient mics are most commonly known as drum room mics in contemporary music. They may also be used to pic up a room sound on the guitar, backup vocals, piano, or anything else you can think of. The ambient mics are used to enhance the spaciousness of the track, not for its main sonic character. You will often find that your ambient mic makes your guitar track sound terrible when the faders are set to the same level. Turning down the ambient track can help but you will still get plenty of coloration in the main track’s content. We can use a high pass eq to help the ambient track work for the mix since our desired spaciousness it typically associated with the high frequencies.

HPF used for this purpose are typically a lot more extreme in their settings. It would not be unusual to have your room mic for guitar or drums with the HPF set all the way up at 3-6kHz (or higher). I like to use a gentler Q of 0.7 (bandwidth around 2 octaves) to give a more natural falloff on the ambient track. Finding the cutoff point can be accomplished by setting your main and ambient tracks at the same level. Then sweep the HPF until you can hear the fundemental frequencies of the main track unaffected by the ambient track.

Recommended eq plugins

Waves Musicians 2 Native Software
price check:
Musician’s Friend
Guitar Center
Pro Sound
Waves Musicians Bundle 2
This is my most recommended way for home recording engineers to get into the quality of Waves plugins. The bundle includes three of my most used plugins: Renaissance EQ, Renaissance Compressor, and Renaissance Vox. Coupled with the Musician’s Bundle 1 you would have just about all the plugins you will ever need in your home studio.The high pass filter on this EQ is fully adjustable for bandwidth and frequency. Of all the equalizers listed here, this is the only one that lets you set the cutoff frequency all the way up to (and above) 20kHz.

Waves SSL 4000 Collection Native
price check:
Musician’s Friend
Guitar Center
Waves SSL 4000 Collection
This package from Waves is a bit more expensive and only includes three plugins. Since the eq is the exact same eq in the included channel strip it is actually more like getting only two plugins. I’m not a big fan of the bus compressor which brings it down to just one cool plugin, the channel strip. Oh what a plugin it is though. The higher price and one recommended plugin might make it seem like less bang for the buck, but if I could justify spending the cash on this I would be using the SSL eq on everything instead of the Renaissance Eq. These are very cool sounding plugins.The high pass filter on this EQ does not have a fixed bandwidth of 18dB per octave.

Wave Arts Power Suite 5
price check:
Musician’s Friend
Wave Arts Power Suite
Wave Arts are making some great plugins. This bundle does not include a standalone eq but an integrated channel strip. Though the channel strip may be overkill for simple eq tasks the plugin makes up for this shortcoming by including the excellent Wave Arts MasterVerb. This is the only bundle listed here to include a reverb. It would be a good choice if you want to upgrade your general plugin library but have a limited budget.The high pass filter on this EQ does not have an adjustable slope.

Sonalksis SV-517 Equalizer Plug-In Hybrid CD Win/Mac*
price check:
Musician’s Friend
Sonalksis SV-517 Equalizer
This is a basic package that includes only the equalizer. It is kind of pricey to get just and equalizer but Sonalkiss make great products and have their die hard fans. If you have expensive tastes this could be your eq.The high pass filter on this EQ has preset slopes at 0dB (off), 6dB, 12dB, and 24dB per octave).

Your home studio is your playground, have fun!