While it might sound counterintuitive based on the name, a low pass filter creates space at the top of your mix. It can be useful for creating space for instruments which need those frequencies and creates more headroom for the mastering process. Let’s identify what is a low pass filter, and more importantly how to use one in your mix.
What is a Low Pass Filter
Let’s begin by defining what is a low pass filter.
Despite the name giving connotations of lower frequencies, a low pass filter filters out the high frequencies beyond a certain point on a track.
The name is a reference to the fact that the filter allows lower frequencies to pass through undisturbed. This is compared to a high pass filter which filters out lower frequencies and allows high frequencies to pass through undisturbed.
It’s also one of 6 EQ filter types which we can use to filter out unwanted frequencies in our tracks.
As I typically mention when talking about EQ, a big part of mixing is about getting the various frequencies which different tracks feature in your mix to fit with one another like a puzzle.
When you can’t remove conflicts with EQ or sidechaining, you can use the stereo field to keep tracks which share similar frequencies separate (see my audio panning guide to find space for similar frequency rich instruments in the stereo field).
Still, a low pass filter is very effective in creating that space by removing the higher frequencies from tracks which don’t need it so that other tracks which DO need it have it.
How Does a Low Pass Filter Work
Let’s say we want to remove the higher frequencies of a track
Setting a high pass filter at 1000Hz on a track means that everything ABOVE 1000Hz is allowed to pass through untouched whereas everything BELOW 1000Hz is filtered out and unheard on that track.
Of course the difference above and below the point of the filter isn’t instant. While we CAN make that filter an instant dropoff cliff at 1000Hz, this sounds unnatural and causes phase issues.
When creating a high pass filter, in addition to designating the frequency we want the filter at, we can set the slope which determines how steep the “roll off” of the filter is. Filter slopes are measured in decibels per octave, with a lower number of decibels being rolled off per octave being a gentler slope.
A filter slope of somewhere in the 6-24dB/oct range is generally recommended when creating a high pass filter.
We’ll still hear some of those frequencies below 1000Hz (in our example), particularly those closer to 1000Hz, but as that slope drops off those lower frequencies get harder and harder to hear the lower you go.
This sounds much more natural and creates less problems than an extreme, near cliff-like dropoff of some of the higher dB/oct options.
There’s a bit of understandable confusion when it comes to high pass filters, because when we hear “high pass” we generally think of the higher frequencies.
Based on the name it can seem counterintuitive, but high pass filters are typically used on the lower end of the frequency spectrum for a track or bus (hence the confusion).
We could just as easily call it a “low cut filter”, because it’s more about what you’re actively wanting to remove and cutting out than passively leaving untouched.
Nonetheless, that’s what it’s called, so let’s talk more practically about what does a high pass filter do and why they’re so useful in mixing.
What Does a High Pass Filter Do
As I just mentioned, a high pass filter cuts out lower frequencies at a point of your choosing while leaving the higher frequencies to pass through untouched.
But what does a high pass filter do, practically speaking?
The low end of your mix is where two of the most important instruments of your mix have their fundamental or main frequencies: the kick and bass.
These are literally the first two things you want to get sounding right in your mix as they anchor and drive your mix.
Between using microphones or recording directly in, there’s a lot of room or electronic noise, respectively, not to mention a lot of low end frequency information we don’t need in 90% of the tracks of our mix.
I’m talking computer fans, air conditioning or other appliance sounds, electronic rattle and hums, low frequencies from outside the room or building (which more easily penetrate walls as I covered in my low frequency vs high frequency comparison), etc.
All of this noise might not be noticeable on a single track, but add it up over multiple tracks and suddenly we have this mess of energy clogging up the low end which makes our mix sound muddy and amateur.
We also have low end frequencies from our instruments themselves which don’t need to be there. Distorted guitar is a great example of an instrument which has a lot of natural low end energy. The low E string on guitar is roughly 82Hz when properly tuned in standard tuning. Just the same, in my distorted guitar EQ guide I recommended using a high pass filter starting around 100Hz and even listening while sweeping higher.
There are a lot of instruments in a typical mix all fighting for similar frequency space. Stack too many tracks which utilize similar frequencies in a mix in the same area in the stereo field and the mix quickly begins to feel cluttered and messy with tracks getting lost.
In the case of EQing above the point where musical frequencies exist like in the case of EQing distorted guitar above or well above 100Hz, we’re trying to get the bass and guitar to compliment one another.
In a lot of pro rock mixes, the guitar sounds powerful and crunchy in that low end, so much so that you’d be shocked to find out that the guitar is aggressively high passed up to 300 or 400Hz.
What you’re really hearing is the product of the bass being able to freely represent the low end and compliment the guitar which has been high passed to create that space for the bass and better emphasize its own mid frequencies.
This is just one of a thousand applications for high pass filtering as I recommend beginning every single EQ on every single track in your mix with a high pass filter.
The bottom line on what does a high pass filter do is that it’s a practical way to create frequency space in the low end of your mix for the instruments which are bass heavy like the kick and bass to have sole dominion.
I explain where and how to high pass filter every single instrument in your mix in my free EQ cheat sheet, so make sure you grab it for free to instantly get your mixes sounding closer to the pros by leaps and bounds today!