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To Bring Out the Bass

When you cut high frequencies, it allows the lower frequencies to be more prominent. Remember, a low-pass filter lets the low frequencies pass through the filter better.
So if you have an instrument that needs more low end (say, a bass guitar), you can apply an LPF.
To Tame High Frequencies
You can also use an LPF to reel in an instrument with harsh high frequencies, like a hi-hat or even a vocal.
In this context, you may not want to cut too many high frequencies, as this can drain the life out of the instrument. But cutting just the super high frequencies can help.
To Add a Muffled Effect
If you apply a drastic LPF, you can create a cool muffled effect, as if the instrument is underwater.
This isn’t a typical mixing technique but rather something you might do to a supplemental track.
Low-Pass Filter Mixing Techniques
Now let’s go over some low-pass filter mixing methods you can start using in your next mix…
Cut Unwanted Sounds

Sometimes you might hear a sibilance or hiss hanging out around 10 kHz.
A low-pass filter can cut those out without altering the quality of the frequency spectrum. Just don’t go extreme, or you could suck the energy from the mix.
Create Depth

Depth in your mix means the listener perceives your track as having a front and back. Some instruments will sound further back and others will be more up front.
You do this mainly with the gain knobs and even panning. But using an LPF can make an instrument sound less present.
By applying an LPF, you can move elements to the back of your mix, creating depth.
Make Space Between Similar Instruments

Filtering is all about making space.
A low-pass filter removes high frequencies. So you can make room for high frequencies in other instruments.
For example, if you have a mix with lots of vocal takes, things can sound cluttered. By applying an LPF on the vocal takes that don’t need it, you can end up with a less muddy mix overall.
You could argue that mixing is just as much about removing as it is adding.
Balance Vocals

One of the most common clashes that can happen in a mix is between the lead vocals and background vocals.
To leave space for the lead vocals, you can apply an LPF to the background vocals.
You want them to be, you know, in the background. Rather than competing for the spotlight.
Add Edginess
Remember how you can peak your low-pass filter before it starts to cut frequencies?
If you do this, you can remove unwanted higher frequencies. All while giving the energetic spots a boost right before the energy fades with your cut.
Just narrow the bandwidth (octave) and you’ll see the peak come in. But don’t be too drastic, or it can start to sound boxy.