Often beginning mix engineers are told to use the ‘boost and sweep’ method to find and remove problem frequencies.
Essentially, this method consists of:
1) Making a sharp narrow boost with a band of parametric EQ
2) Sweeping the frequency of the band (this sounds a bit like a wah wah), listening for any spots that are particularly unpleasant
3) Changing the gain of the band from positive (boost) to negative (cut).
Theoretically, this is a way to improve the sound – especially when you can hear something ‘wrong’ with the sound, but can’t identify it with your ears alone. This technique on its own is not necessarily bad. I find it useful sometimes too. Of course, its becomes less necessary as you hone your listening skills. Even less so if you’re using well-recorded audio or samples.
The problem is when people make the assumption that since one band of ‘boost and sweep’ is good, more bands must be better. And so the fourth step is often added:
4) Repeat until you run out of EQ bands.
This is a really great way to butcher a sound! This approach will inevitably produce an EQ curve that looks like the sonic equivalent of Swiss Cheese – full of holes. This is because looking for ‘bad frequencies’ by boosting and sweeping will almost ensure you find something to cut. Strong boosts with a narrow bandwidth will make anything sound bad. There’s no point at which boosting and sweeping will stop finding ‘bad frequencies’.
The end result will sound very unnatural because the tonal shape of the sound is so warped. In addition, this kind of EQ curve can create resonances – dramatic tonal features that are constant and don’t change with pitch. These kinds of tonal features are also common in recordings made in small untreated rooms. It’s ugly! Don’t do it!
Boosting and sweeping is one of several techniques that a mix engineer can use to identify a trouble spot. But it won’t tell you if there is a trouble spot. For that, you have to use your ears and listen to the sound in context – in the mix. Sometimes the problem really is an ugly resonance that should be reducing using a notch EQ… but more often it’s something to do with the way several sounds are interacting together. And these problems are most often caused by inadequate monitoring or a lack of direction and focus.