What is a high-pass filter?
A high-pass filter is an EQ curve that is used to remove low-frequency sounds from an audio signal. It is called a high-pass filter because it allows high-frequency signals to pass through, while attenuating (reducing the amplitude of) lower-frequency signals.
High-pass filters are fantastic when used correctly to clean up woofy signals and tighten up arrangements. When used incorrectly, they can cause more problems than they solve.
Recklessly making cuts just because you can is inadvisable. You can strip the life and groove right out of your music. To help you master the high pass filter, here are six ways to use them while mixing and how to listen to the effects of over-filtering.
In this piece you’ll learn:
How to use a high-pass filter intelligently
Problems to avoid when using a high-pass filter
When to use a shelf over using a high-pass filter
Use these mixing tips instantly
How to use a high-pass filter
1. Remove low end rumble
Almost all audio sources, especially of the live variety, include unwanted sounds that lurk below the most important frequency information.
While consistent wind and humming are usually pretty easy to spot, this is not always the case. Many unwanted sounds are hidden by the louder core of the signal and can be overlooked, especially when working against a deadline.
As we get to the lowest parts of the spectrum around 20–40 Hz, we might not even hear rumble due to the limits of our hearing, making it something of a silent headroom killer in large quantities.
Take the following screenshot of a spoken-word vocal, shown off in Neutron:
I don’t want that low end bump. It is a rumble from the narrator stepping on the mic stand.
Now, the noise doesn’t come up so clearly on a pair of earbuds. But this screenshot comes from a movie premiering at the Maryland film festival, where the speakers presumably can reproduce the low end. Thus, I have to get it rid of it, like so:
Take your time to scope out where these sneaky sounds exist, and as you remove them, consider that where you set the filter cutoff will depend on the signal at hand. What worked on one instrument might grab too much of one or not enough of another.
Pro tip: If you’re using Neutron and have trouble determining just how high you should cut, hit command (or control for PC) and select the lowest node, which will single out the band of frequencies it covers. Sweep up until you start to reach the meat of the instrument and pull back slightly, then release the click to hear the entire signal. You want the noise to be gone without compromising the intelligibility or power of the audio.
2. Don't remove a lead instrument's fundamental
In most cases, the lead element in a song ought to be in your face. Take the vocal, for example: as the central element of a song, vocals need to sound clear. Most of the time, everything below roughly 100 Hz gets in the way, triggering unnatural compression, and fighting with other elements of the mix.
Of course, you must exercise caution as you approach the fundamental of the vocal—the lowest frequency that determines the musical note. The weight of the performance sits at the fundamental, and even a slight intrusion is enough to squash the energy. A loud voice without oomph will struggle to carry a tune from start to finish.
Let’s listen to how a high-pass alters a vocal in the audio file below—the first clip is unprocessed, the second has a high-pass at a sensible position, the third high-pass cuts through the fundamental, and the fourth is noticeably thinned by an aggressive cut.
Unprocessed Vocal /No High-Pass Filter
Vocal with Some High-Pass Filtering
Vocal with More High-Pass Filtering
Vocal with Most High-Pass Filtering
We can hear a reduction in low end noise from the first to the second clip, which has the inverse effect of brightening the vocal (it’s already somewhat harsh to start with). The third clip is where a lot of new mixers will end up—in an honest attempt to clean the vocal, the cutoff is placed too high in the spectrum, sucking out important frequency content. If you really don’t pay attention at this stage, you’ll end up with a paper thin vocal, like the fourth clip.
If you’re dealing with a deep voice buried in the unwanted mud, try a pass of RX with the De-hum, De-rustle, or even the Spectral Repair (in attenuate) modules to better isolate the vocal. Then apply a more gradual filter slope, plus an EQ bell curve, to attenuate buildups in the more mid-high bass range if necessary. There’s no exact science here, so a bit of experimentation is necessary to get it right.
3. Unmask the low end
If the kick and bass elements in your mix sound good on their own but struggle to make an impact in the context of the mix, this can be a sign you need to take away the lows present in other instruments. If unecessary low end exists, you'll get frequency masking that can muddy up the low end.
This issue often appears in the low mid range, between 150–350 Hz, which is what gives music its heft and power. It's one of the more problematic areas in a mix, since the higher harmonics of kicks and lower harmonics of synths, guitars, and much more all converge here, which leaves you with a mix that sounds bloated and tired.
Pro Tip: It might be tempting to apply a drastic global filter on an entire submix to keep unwanted frequencies from afflicting the low mids—but this approach carries two huge problems.
First, doing so takes away too much energy in too blunt a manner—like hammering a screw into a wall instead of using a drill. Instead, try shaping individual instruments with subtle cuts to make space for the kick and bass, as these two instruments have arguably the most important role in the low mids, and as such, should be heard with clarity. Reining in conflicting frequencies from less important elements will greatly help.
The second reason brings us to an issue worth highlighting:
High-pass filters can introduce unwanted phase shifting in transient material. They can alter the timing of the affected information. The changes are subtle, but depending on the material, they can be noticeable and/or harmful, creating a feeling of blurriness and smeariness. I can demonstrate this in Ozone by turning on the secondary curve option, which shows the time shift:
The solution here is to work on the individual instrument level, and to use low shelves to subtract low mid content on sources that are highly rhythmic.
Need to take out some low mid range from a muddy pad? High-pass it! Want to get out some low mid girth from some chugging metal guitars? Try a shelf first, so as not to smear the timing.
Look at how much more gentle that is in the time domain!
In the video tutorial below, learn how to shape the low end with the Masking feature in Neutron and Relay.
4. Tighten up the low end
A stuffy low end can drag down a mix and make it feel stuck. The kicks don’t come through and the bass is more floppy than punchy. If your low mids are in order, an excessive buildup of mud somewhere in the 20–35 Hz range is worth investigating.
By high-passing these frequencies on the drum mix, or even on the master output, you remove unnecessary noise and tighten up the low end. This also has the bonus effect of making the mids and highs seem more present and loud.
Now, here come the drawbacks to this approach:
It is easy to overdo it here and take away too much bass, making other parts of the spectrum brittle in comparison. So, don’t do that: use the frequency analyzer and solo mode in each EQ to help you find the fundamental information of the signal.
That phase issue we mentioned earlier also comes into play. Looking for the fundamental and staying well clear of it with the curve can sidestep that problem. On the master bus, you can experiment with the linear phase mode of Ozone to avoid phase-smearing (though sometimes this cure can sound worse than the disease, due to pre-ringing, a necessary byproduct of linear-phase EQ).
Lastly, If you find yourself doing this time and time again, your room might be the issue, rather than the mix. Untreated bedroom studios are notorious for over-representing low end and this might be what you keep hearing! So be sure to test this move out on multiple playback systems and A/B judiciously before sending the mix back to a client. There should be a feeling the mix has opened up without losing its edge.
5. Try dynamic EQ instead of static EQ
Across most of these examples, the types of audio mentioned are dynamic in nature. Vocals, basslines, and percussion change over the course of a song, sometimes very quickly, and the high pass filter you set should be able to reflect and follow that movement.
A dynamic EQ allows for just that. While most filter cuts are static, in that they don’t change according to the incoming signal, dynamic EQs are able to ‘track’ signal content, and change cutoff points to better shape sounds as they change throughout a mix. This is particularly helpful when working with signals that, no matter where you set the cutoff, sound either too thin or too muddy.
A dynamic EQ cut offers the perfect middle ground. It will let in some less desirable frequencies so you can feel a lower-range presence, but clamp down as soon as they become too active. This allows for the natural character and space of a recording to come through without it becoming overwhelming.
A dynamic cut can come in handy with just about all instruments and especially vocals. Geoff Manchester expertly shows off how to use a high pass filter in Follow Mode in the following video, about six minutes in:
6. Avoid removing the mix's body and punch
High-pass filters are one of the most overused tools due to the obvious and immediate effect they have on a sound. With a strict hand, one might nix all lows that poke out from mid and high-range instruments, and in the process, unintentionally remove what actually gives the mix depth and groove. Even worse, steep curves can introduce a ringing or distortion around the cutoff point, especially when placed partway through busy material.
All this to say—If you’re mixing hi-hats or another upper range instrument that has something interesting going on in the lows that provides punch to your music, you don’t have to severely filter it out just because these elements typically favor higher frequencies. Again, you can use a shelf to cut frequencies as well.
When we use a high-pass filter, we are striving for clarity. But clarity in a mix doesn’t come solely from EQ and filtering. Pan position and level are equally useful to help separate elements in a mix. So when balancing tracks, be sure to explore these options before defaulting to a high-pass.
Start using high-pass filters in your mix
Despite the simple function of a high-pass filter—to remove low frequencies from a signal—it is a versatile tool that can be used in dozens of scenarios for a greater sense of tonal balance.
Like all audio tools, however, it has the potential to be overused with disastrous results. When I first started producing and mixing music, I’d go overboard with my filters and EQ cuts and end up with music that sounded unnatural and flat. Over time, I’ve learned that subtle, intentional moves are what gets the job done more effectively.
So, high-pass only what needs to be high passed—what I mean by this is to cut the frequencies that are stopping your mix from communicating clearly to the listeners, and not those you think should be removed based on a preset or an instrument range chart. And a friendly reminder—you can experiment to your heart’s content with high-pass filters starting a free trial of iZotope’s Music Production Suite Pro membership and firing up Neutron, Nectar, or Ozone. Happy mixing!