If you’ve got a track in your mix that just isn’t coming through, or perhaps there’s a section of a song which is really crowded and it’s hard to make out any individual instruments, a fairly standard suggestion will be to “make some space” or “carve an EQ hole” for the parts that are getting lost. Which is great, if you know what that means. If, on the other hand, it has you reaching for a hammer and chisel, then perhaps you need a different metaphor.
Imagine you have a bunch of shapes cut out from various colours of paper, and you want to stick them up on a yellow board.
The red, blue and purple ones are fairly easy to see, but the light green one is a bit underwhelming, and the yellow one is almost invisible. This is what we refer to as masking.
One way to fix this would be to replace the troublesome shapes with ones cut from a different colour. But you’re quite attached to the yellow and green. So you take some black paper and cut out pieces to go behind the original shapes, with a bit of a margin.
Now you perceive a border around the yellow shape and it becomes instantly visible. Essentially, you made a hole in the background, unmasking the shape.
And that’s all that “carving an EQ hole” really is. In the same way that objects of similar colour are hard to visually distinguish, sounds that overlap each other’s frequency spectrum can become masked too. So when you are layering one sound on top of another, which consists of similar frequencies, you need to create a border. The simplest way to do this is by making an EQ cut in the background at a frequency that is particularly important in the foreground. Of course, the “background” may actually be another foreground track that is playing at the same time, but the principle stands.
One thing to be aware of: if you put the black pentagon in place above, you may find it counterproductive. What is happening there? In fact, a lot of the time we don’t so much perceive things directly, but by the contrast with whatever they’re next to. The purple on yellow is already fairly high contrast, much more so than purple on black, so the pentagon actually gets harder to see. The metaphor doesn’t hold perfectly in this case, but it serves as a reminder to always make EQ moves in context, and with a purpose. Blindly cutting frequencies is pointless and potentially harmful to your mix. Carve holes in places you’ve identified as overlapping, and use your ears, not your eyes, to decide whether it has made things better or worse.