In this article we’ll discuss the essential properties of mixing and mastering EQs and explore the science behind a little-known mastering secret: if you apply a cut with a mixing EQ such as a high, low, shelving or peaking cut, it can unexpectedly increase your peak levels whereas a mastering EQ guarantees your peak levels will never increase when cutting. This is crucial if you want to safely apply subtractive EQ to an existing master, or even apply an EQ cut after the peak limiter on your master bus.
Mixing is a creative process. You need tools you can push hard without worrying about whether they will introduce problems. You also need instant feedback as you audition the sound you are sculpting. A good mixing EQ should be extremely forgiving when applied creatively; it should not introduce artefacts such as pre-ringing or distort at extreme frequencies. Also, to avoid interrupting your creative process, a mixing EQ should be geared towards having a very small (or zero) amount of latency.
In exchange for their desirable qualities, mixing EQs change the phase of your audio. These phase changes are largest around the corner/center frequencies of the applied EQ filters. Phase changes from equalization are normally not a problem especially on individual tracks. When you turn your hand to certain mastering tasks however, introducing phase changes can lead to unexpected problems.
A mastering grade equalizer has perfectly linear phase. Linear phase EQ is transparent but adds significant latency and, if not carefully applied, can cause pre-ringing artefacts. The care needed to apply linear phase EQ and the delay caused by the latency make it a poor choice for mixing, but it has some essential properties for mastering.
If you're trying to use a mixing EQ for mastering, phase changes can lead to trouble. Mixing EQs introduce phase changes which means even if you apply a cut, whether it’s a high, low, shelving or peaking cut, the phase change around the corner/center frequency can increase the peak levels of your audio, even if the change in phase is small. This behaviour is counter-intuitive, but very useful to know. Any phase change causes the frequency components at the corner/center frequencies to be shifted relative to each other. The frequency components can unexpectedly sum together to increase your peaks (constructive interference). Perfectly linear phase EQ on the other hand will never increase your peak levels if you apply a cut.
Phase Critical Mastering
Certain mastering tasks are extremely phase sensitive. You may be in the situation where you want to apply an EQ to an already mastered track or you want to put an EQ after the limiter on your master bus. Just as long as you're applying subtractive EQ using a mastering grade linear phase equalizer, you won't run into problems. If you don't use a linear phase EQ, you could increase your peak gain and cause clipping due to phase changes.
To understand visually how phase changes can increase peak gain, we can use the fact that audio can be deconstructed into a sum of sine waves. Let's examine two frequency components of our audio: a 50Hz and 150Hz sine wave.
The following diagram shows a 50Hz and 150Hz sine wave combined together. The peak gain of the combined waveform is 0.75.
The following diagram shows a 50Hz and 150Hz sine wave combined together with a different relative phase to the first diagram. The peak gain of the combined waveform is now 1.00. Keep in mind, this is an exaggerated phase shift to clearly highlight the impact of phase changes on your peak levels. In practice changes would be much smaller.
For a mixing EQ, phase changes can occur at the corner/center frequencies of the filters. Even applying subtractive EQ is no guarantee that your peak levels won't increase due to these phase changes. The lesson: never use a mixing EQ for phase sensitive mastering tasks.