Dynamic Equalizer

A dynamic equalizer is essentially a type of conventional parametric equalizer. It is "dynamic" because a dynamic control loop replaces the otherwise static gain of the filter. Parameters such as frequency and quality (Q) remain the same. Depending on the design of the control loop, a dynamic EQ can have level-dependent parameters (over/under, threshold, and range) as well as temporal parameters such as attack, release, and hold.  

How Does a Dynamic Eq Work?

A dynamic EQ makes each filter's gain control act the same way as the gain change element. Depending on the model, mimicking a compressor, an expander, or even a threshold-independent transient enhancer may be possible. Therefore, you need to decide on the filter type, frequency, and bandwidth for each bend. Then, you need to set up the band's processing type and control it. If that sounds like a lot of work, you are not wrong. However, the upside is that once you've got the hand of dynamic EQ, it can bail you out of various situations. This is because it can sometimes operate on precise frequency regions or even single frequencies.  

Why Is Traditional Equalization Not Enough?

So, the reason traditional equalization alone is sometimes not enough is that one size does not fit all. Applying 'regular' EQ during mastering and boosting some upper midrange (let's say, around 3.5 kHz) will add bite to the guitar but will also make the vocal shrill whenever it strays into that area. Apply some low-frequency shelving at 60 Hz, and it will reduce the boom of the kick drum. It will also affect the fundamentals of the bass when low notes are played. Applying EQ to individual instruments during recording or mixing does not solve the problem. A good setting for a vocalist singing softly rarely works when they begin attacking the chorus at a louder volume. Rolling off high frequencies to reduce the harshness of cymbal crashes will dull their sound even when tapped gently.  

The Common Cases Where Dynamic Eq Is Most Helpful:

1. With dynamic EQ, you can rein in the shrillness in a singer's voice when they begin belting out the chorus. But still retains the vocals' presence during the quieter verses.
2. Let's say a hi-hat is too bright in the mix, but you're happy with the snare sound. If you try to compress (or apply traditional equalization) the hats, you'll dull the snare sound. With a dynamic equalizer, you can use the snare hits as a sidechain input to the high-hat track. With this, you only roll off the top end whenever the snare isn't being hit. You can even EQ the transient of a sound one way and the sustain portion differently. You scoop out the "boxiness" of a sound without affecting its attack.
3. You can tame harsh cymbal crashes when the drummer starts getting carried away, but leave the cymbals untouched when the drummer plays with restraint.