As I mentioned before (here)
my favourite DAW is Reaper and in this post I will describe how I use stock plugins in Reaper for reducing sibilance or de-essing vocals.
In a perfect world, if you have a vocal recording with pronounced sibilance, you would simply re-record the vocals, and get rid of the problem that way. But this option is not always open, and you sometimes have to work with what you have. For those occations, this is one way to do it;
What you are trying to achieve is to remove unwanted high pitched harsh sounds, typically in the 5 - 8 kHz range. Those sounds are often linked to the letter s ( hence de-essing) or heavy breathing close to the microphone. If you apply heavy compression, they can become even more pronounced.
So what you want to do is to reduce the volume at this frequency range, but only when these hissing sounds are present. Your first thought might be to apply EQ and a notch filter. But since the eq would always reduce the volume of those frequencies, it would make the audio sound strange and odd, rather than to reduce the unwanted hissing sounds.
The goal is only to reduce this frequence band when the hissing is present. The tool for that is a compressor, only set to act on a certain frequency range. It will only become active when the threshold is passed in this range of frequencies, resulting in no compression in other parts of the audio spectrum. Of course you can get special plugins that will do this. But here I will show you that the stock ReaComp plugin can do it just as well. No need to spend a lot of money on a dedicated plugin.
On your audio track load the ReaComp plugin and open the menu. On the bottom part there are two input fields restricting the frequency range for the compressor to be active. Adjust this to cover the 5-8 kHz range and start looping over a part of your track that is particularily full of hisses and esses. Pull down the dB Threshold until you see the compression kick in. How much this is will depend on the volume of the track, and how pronounced the sibilance is. Once you hear an effect, start to play around with the attack time and ratio. Leave the release time until your satisfied with the others. Note that if you dial up the ratio high it effectively becomes a limiter and no sound above the dB Threshold will be send to the output (in that frequency range that is).
I prefer to not click the 'Auto make-up' check box. If you click this, the volume of the track will automatically be increased to match the reduction applied by the compressor. I prefer to manually adjust the level of the track if necesarry after de-essing.
There is one important setting more that you need to be aware of, the 'RMS'. This is the time window that the plugin use to determine whether the audio signal is above the threshold (to be compressed) or not. This is not to be confused with the attack time. The 'RMS' need to be much shorter than the attack time. The default is 5ms, but it can be dailed down to 1ms or even 0.1ms.
Now you are set up. I next adjust three parameters (dB Threshold, attack time and ratio) until the audio comes out enhanced. Typically I use a short attack time 10ms and a ratio of at least 4. But don't forget to adjust the dB threshold as well. I leave the release time until I have adjusted the other three. Then play around with the settings for a while until you are satisfied with the result.
Good luck!