Simply, de-essing is a process for reducing the level of sibilance in a vocal recording.
What is sibilance?
Sibilance is characterised by ‘sss’ and ‘ts’ sounds (and, to a lesser extent, ‘t’ and ‘k’ and ‘z’ sounds) in the English language. Unlike vowels, sibilant sounds have a relatively low (in volume) pitched component and a high (in volume) unpitched noise component. The unhitched noise is also focussed strongly in the higher register (unlike ‘shh’ sounds).
Why would you want to reduce it?
Sibilance is essential for intelligibility. That is, we need to hear it in order to understand the words delivered by the vocalist. Too much, however, can unbalance a mix. Some singers naturally deliver sibilant sounds loudly (this often applies to singers who aren’t classically trained). Sometimes EQ or compression can enhance the sibilance in a vocal recording (especially when the high frequencies are boosted).
When listening to the voice on its own it can be difficult to know if the sibilance is too strong. This is because our brain naturally compensates for the difference in volume between the vowels and the sibilance. In a mix, however, you’ll notice when the sibilance is too strong. You’ll raise the vocal level until the vowels are at the right level but the sibilance is too sharp and ‘sticking out’ of the mix, or your’ll reduce the vocal level until the sibilance sits well but the vowels disappear under the mix. Sometimes strong sibilance can excite the vocal reverb, making the reverb much more noticeable.
How does de-essing reduce sibilance?
De-essers are usually set up as simplified compressors with a bandpass or highpass filter in the sidechain.
Most de-essors do not have the full compliment of compression controls (attack, release, threshold, ratio, makeup). Instead, there is usually just a threshold (and sometimes a ratio control). The other controls are tuned to work with sibilance and the human voice. De-essers are among the most specialised studio tools – they don’t need a lot of controls or a wide range of operation.
The filtered sidechain changes the compressor’s behaviour so that it only reduces gain when there is sibilance in the audio. The filter is tuned quite high (usually above 5kHz) so that the compressor doesn’t respond to energy in the low or mids (where most of the vowel energy is). The compressor, however, applies gain reduction to the whole signal – not the filtered version. This means that when the sibilance is being reduced, the actual tone of the voice is not changed. It’s just made quieter.
Some de-essors use dynamic EQ instead of a compressor. They’re usually designed so that instead of reducing the gain of the whole signal, they only reduce gain to the high frequencies. Imagine a high shelf or parametric cut that only comes in when the voice is sibilant. This kinds of de-essers require more care when they’re being set up because they work by changing the tone of the voice (rather than just the level). If not configured well, they can make the vocalist sound like s/he has a lisp.
How do I set up a de-esser?
I usually wait until the mix is almost completely finished before applying a de-esser. I’ll make sure the level of the vowels in the vocal are balanced well against the other elements of the mix. I’ll then use the de-esser just enough to bring down the sibilance to an acceptable level. Usually, I aim for the lead vocal sibilance to be at a similar level to the hi-hats, snare or other prominent high-frequency sound in the mix.
I almost almost insert the de-esser after EQ and compression (but before any time-based effects such as delay or reverb, of course). This is because I use the de-esser to slightly modify a sound that I’m already happy with. Applying compression after a de-esser can actually counteract the de-essing, as the rull-range compressor can bring the sibilance level back up.
What else is a de-esser useful for?
De-essers can be very useful for backing vocals. There are some situations where backing vocals (especially stacked backing vocals) are a little messy. Most of the time it’s fine, but it’s most pronounced in the sibilance. A de-esser will bring the sibilance right down, making the backing vocals sound less messy. Use this way, a de-esser can be applied much more heavily – the intelligibility and articulation is carried by the lead vocals. Just watch out for any sections where the background vocals are exposed – heavy de-essing will make them sound weird without the lead vocal.
De-essing can sometimes be useful on drum kits – particularly on overheads when balance of the kit is right but the crash cymbals are too loud. A de-esser can sometimes be effective in reducing the level of the crash cymbals while still retaining the sense of room and space. Again – subtlety is the key here. Too much de-essing will suck the air out and make the drums sound unnatural.