If you’ve been mixing for any length of time, you know how valuable the high-pass filter (HPF) can be.
It removes excess low end from your non-bass-heavy tracks, allowing you to clean up the low frequencies, making room for the kick and bass.
But then there’s this thing called a low frequency shelf. What’s that all about? In the picture below you can see both a high-pass filter and a low frequency shelf.
A high-pass filter actually filters out the low frequencies entirely. The curve slopes downward at a specific “steepness.”
As you move further to the left in the frequency spectrum, the signal gets progressively lower and lower.
If you set the HPF at 150 Hz, for example, 120 Hz will still be audible, but will be turned down. 80 Hz will be much quieter than 120. 40 Hz will likely be completely unheard. The high-pass filter essentially REMOVES those lower frequencies.
(click to enlarge)
But let’s say you don’t want to completely remove the low frequencies, but you’d like them to be turned down a bit. That’s where a low shelf can come in handy.
If you look at the graphic again, a low shelf looks just like it sounds. It’s a horizontal line. Any boost or cut applied to the shelf also applies evenly to every frequency below it.
This can be extremely helpful on a bass track that has just a little too much low end. You can turn everything below 150 Hz down by 3 dB. You’re not getting rid of the good stuff down at 80 Hz, you’re just turning it down.
Where I use this a lot is on a bass track, where I use a regular EQ curve to cut out some of the “muddyness” around 120 Hz, but then I’ll also use a low shelf to turn the bass frequencies down a bit…if needed.
This lets me shape the tone of the low end with the bell-curve, and then increase or decrease the loudness of the low end with the shelf. Here’s what that looks like:
As you might have guessed, the same can be applied on the other end of the spectrum, when dealing with the high frequencies. I tend to use this on the lows more than the highs, though.
Joe Gilder is a Nashville-based engineer, musician, and producer who also provides training and advice at the Home Studio Corner.Note that Joe also offers highly effective training courses, including Understanding Compression and Understanding EQ.