Just about every mixing tutorial on the interwebs sings the praises of the highpass filter — indeed, it’s a useful tool. Inexperienced hands, highpass filters can shape your low end, add clarity to your tracks, eliminate low-frequency mud, and restore precious headroom.
The highpass filter’s unassuming cousin — the humble lowpass filter — doesn’t seem to get the same amount of respect.
Admittedly, a lowpass filter won’t produce as dramatic a result as a highpass filter. That said, well-placed lowpass filters can tame unruly hiss and sibilance, while adding polish and dimensionality to your mixes.
In this blog, we'll discuss what a lowpass filter is, and we'll outline several common ways that you can implement this essential tool in your own mixes.
What is a Lowpass Filter?
A lowpass filter removes frequencies above a defined cutoff frequency, while allowing those below to pass. This enables you to attenuate (lower the level of) high frequencies.
Most commonly used parametric equalizer plug-ins — FabFilter Pro-Q, iZotope Neutron, Waves Q10, and even the one that comes bundled with your DAW — include this type of filter, as well as highpass, bell, shelf, notch, and other shapes.
The plethora of different EQ plug-ins out there each contain a unique control set. The two most common controls that you will encounter on a lowpass filter are:
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Frequency defines the cutoff frequency of your filter, meaning that all frequencies above it will be attenuated. This is indicated in hertz (Hz or kHz).
Higher frequency settings will yield less aggressive filtering. For example, a 15kHz cutoff frequency will attenuate fewer frequencies than a 5kHz cutoff.
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Slope determines the steepness or severity of your filter. This is measured in dB-per-octave increments.
The higher the slope is set, the more aggressive the filter will attenuate high-frequency sounds. For example, a filter with a 6dB-per-octave slope would offer a more subtle result than a filter with a 24dB-per-octave slope.
Common Uses for Lowpass Filters
Eliminate Unwanted Elements
Listen to your mix. Do you hear any undesirable or excessive high frequencies? If that’s the case, a lowpass filter may be the perfect solution
Eliminating a track's high-frequency content can really benefit your mixes, especially if the offending frequencies aren't adding anything compelling.
Say you have a kick drum with a piercing "click" sound or a bass guitar that's drenched in fuzz or distortion. These are both low-frequency instruments — their high-frequency content is oftentimes not essential.
Whenever you increase the level of these tracks, their high-frequency content will get louder as well. This can have the side effect of burying other instruments that contain lots of high frequencies within your mix.
Deploying a lowpass filer in the 10kHz–15kHz range is an easy and effective way to tame out-of-control high frequencies, while simultaneously bringing out a track's lower frequencies. But be gentle — filter too aggressively and your track will turn into mud.
Solve Frequency Collisions
Mixing is all about carving out sonic space. Just like you'd use a highpass filter to create space for low-frequency instruments, you can use a lowpass filter to make room for high-frequency instruments.
A common rookie mistake that novice engineers make is to add desirable frequencies instead of removing undesirable ones. If your lead vocal track is being masked by layers of synthesizers or walls of high-gain guitars, resist the urge to crank the vocals — instantiate a lowpass filter on the competing track instead.
Again, there's no reason to be heavy-handed with this. Use just enough filtering to unveil the buried track. Too much filtering, and your track will lose its definition and clarity.
Not sure which frequencies are the problem frequencies? No problem. Our MIXROOM plug-in will guide you through the process every step of the way, supplying you with what you need to improve your sound and achieve a pro-sounding mix.
Uncover Lead Vocals
One of the most infuriating frequency collisions you'll encounter while mixing is between lead and background vocals. And the more vocal tracks you add, the more pronounced the problem becomes.
It's all about competition. After all, there's only so much frequency bandwidth to go around. And because they all contain similar-sounding material, your vocal tracks are going to clash with — and mask — one another.
To resolve this, place a lowpass filter on your background vocal tracks with a cutoff around 5kHz–12kHz. This will decrease the number of conflicting frequencies between the tracks, enabling your lead vocals to sit at the forefront of your mix.
Improve Dimensionality
While mixing, we tend to focus on the stereo field — panning from left to right. That said, front to back is equally as important.
After all, this is how we perceive space in the real world. If something is close to us, it's loud and bright. If it's further away, it’s quieter and less bright.
In a mix, we can re-create this sort of dimensionality by lowering a track's volume and rolling off some of its high end.
Keep your mix's focal points, such as vocals, drums, guitars, and solo instruments, up front. Gently push pads, auxiliary percussion, and other decorative elements back — just don't push them back too far.
Maximize Reverb Effects
If an inexperienced mix engineer applies too much reverb to a vocal, it obscures the track's transients, reducing the performance to an inarticulate mess. On the other hand, a pro-level mix engineer can drown a vocal in reverb, while maintaining every ounce of the track's clarity and intelligibility.
Ever wonder how they do it? One technique is to apply a lowpass filter set around 10kHz–12kHz to just the reverb effect.
Like we discussed earlier, rolling off a track's top end pushes it back into the mix. The same applies to reverb. Rolling off its upper frequencies tucks it out of the way, leaving room for your vocal to shine — even in the midst of long, lush ambience.
Add Bite and Clarity
Lowpass filters can also be used to add edge to an anemic-sounding track. Instead of cranking the track's upper midrange frequencies for extra bite, use a resonant lowpass filter instead.
What's a resonant lowpass filter, you might ask? In a nutshell, it creates a boost at your specified cutoff frequency prior to attenuating it.
If you place a resonant lowpass filter at just the right frequency in your flat-sounding track's upper midrange, it will inject it with bite, clarity, and energy, while gently attenuating all of the competing frequencies above it.
There’s no reason to do anything drastic — push this technique too far and you'll end up with a harsh, boxy-sounding track.
Eradicate Distracting Hiss
Ever been handed a mix with a track that contains a steady stream of high-frequency hiss? If you haven't, you will — it happens to everybody sooner or later.
While there are a lot of advanced noise-reduction tools out there, they can be overkill in some situations. In the case of high-frequency hiss, deploying a lowpass filter set around 8kHz might do the trick with a lot less effort than deploying a more complex solution.
Just adjust the slope until you achieve a natural-sounding result. You definitely don't want to get carried away with this technique — hyper-aggressive lowpass filtering will yield a dull, muffled character that may be more objectionable than the hiss.
Conclusion
While they're not a tool that you'll want to use aggressively on every track in your mix, lowpass filters are an important part of every seasoned mix engineer's arsenal. These indispensable sound shapers give you a lot of creative and corrective control over your mix and are an easy-to-use problem solver in a myriad of situations.
We want to remind you again, however — be gentle! Ham-fisted use of lowpass filters — and equalization in general — can create more problems than it solves.
Nonetheless, learning to deploy lowpass filters properly will elevate your mixes, imbuing them with depth, balance, and clarity.
