“IIR” stands for Infinite Impulse Response. This is one of possible ways to implement digital EQ. This way is very CPU lightweight. But it’s impossible to make linear-phase EQ using IIR. So the most digital min-phase equalizers are IIR.
Let’s get cookbook peaking filter formula and make simple EQ. We’ve got such a nice curve:
Next, move frequency up to 10 kHz (we’re on 44.1 sample rate) and see that:
You can see now, the perfect symmetric shape of “bell” is distorted and not symmetric now. How does it sound? Such non-symmetric curve has harsh, sharp, “digital” (in bad meaning of word) sound. But bell shape of true analog EQ doesn’t distort here. I think, symmetric curves sound more natural to our brain and thus leads to better sound perception.
Read more of this post