An envelope, in audio terms, is basically a pattern or shape. When you write automation to a track, you are shaping how your chosen parameter will change over time, i.e. creating an envelope. 

There is a type of processor called an envelope follower. What it does is change something based on the shape of something else. A compressor is, in effect, an automatic gain reducer. When the volume of your audio exceeds a certain point, the threshold, the compressor kicks in to turn that audio down according to the parameters you have designated. Gain reduction is applied based on the volume envelope, the pattern or shape of the sounds volume, over time. The attack, RMS  and release controls govern how sensitive the compressor is, that is to say, how responsive it is to your audio's envelope. The ratio control governs how much will happen based on what your audio's volume envelope does. With side-chain compression, the compressor applies changes to one piece of audio based on another piece of audio's volume envelope; drums might modulate a pad for example. So a compressor is, essentially, an envelope follower. 

Some plugins have built in envelope followers. This is particularly common with distortion and filter plugins, where the drive of the distortion or the cut-off frequency of the filter is modulated by the incoming audio's envelope. Reaper allows you to link any effects parameter to the volume of incoming audio so that it will change when the volume changes. 

One particularly popular application of envelope following is in all kinds of guitar playing. In this application, the guitar's sound is fed through a resonant low-pass filter, whose cut-off frequency is modulated by the volume of the guitar playing. The louder the guitar is played, the higher the cut-off frequency. As notes fade away, the cut-off frequency drops back down. The resonance of the filter, i.e. the steep slope to the cut-off frequency, gives the characteristic wa wa sound, which is so familiar in many genres. 

We'll use this application as an example to put together an envelope follower in Reaper. 

1. Add Reaeq to your guitar track track. 

2. From within the FX chain dialogue, change the filter type from low shelf to low pass. There is a combo box labelled "type", from which you can select the appropriate option. 

3. Decrease the low-pass filter's q, which will increase the filter's resonance. Try a q of 0.3 octaves and adjust to taste. 

4. Tab to the param button and press it, either by pressing shift space,  NVDA numpad enter or its Jaws equivalent. 

5. Press f for 'Fx Parameter List' and cursor right or enter to open the submenu, from which you need to select 'Parameter Linking and Modulation', which is also a submenu.

6. Click on 'Freq Low-pass', which should be the first option in the list and a dialogue will appear. All checkboxes within this dialogue must be toggled using NVDA numpad enter or its Jaws equivalent. Pressing space on anything that isn't an edit box will start playback. 

7. Tab to the 'Audio Control Signal Side-chain' checkbox and check it. 

8. Tab to the 'Track Audio channels' combo box and select the channels you want your envelope follower to listen to, usually channels 1 and 2. Note that, if you have a multi-channel track, you could set the envelope follower to channels 3 and 4, to which you could feed audio from another track. 

9. You now need to designate the minimum and maximum volumes to which the envelope follower will respond. To do this, we tab to the 'Audio control signal minimum' and 'Maximum' volume controls, for which there are a pair of sliders and edit boxes. When setting these, it may be helpful to have an idea of your audio's dynamic range in advance, or you can just ear-ball it. 

10. There are radio buttons that let you decide the direction of your envelope follower. By default, it's set to positive, which means that as the volume of the signal increases past the minimum point designated earlier, the effect parameter will rise from its starting point. 

You can also have the direction of the envelope follower be negative, which means that, as the volume of your incoming signal goes up, your effects parameter will go down. You can also set your envelope follower to be centered. 

11. Now, you need to set the baseline. This is the starting point for our effects parameter, i.e. where it sits when no audio is coming in, its rest state. 

At the moment, if the audio is below the designated minimum volume, nothing above 20 Hz will be allowed through the filter because the baseline it at 0. In effect, if there's any playing, it won't be heard. 

Adjust the baseline by tabbing to the checkbox that says 'Enable Parameter Modulation, Baseline, Envelope over-rides', but don't toggle it. If you press your cursor up key here, you will start moving the baseline slider up. 

Unfortunately, when moving this slider, you only get feedback as a percentage as opposed to a real-world value for the parameter, so it's just a case of listening to see where you like it. For this application, you'll probably want to keep the baseline pretty near the bottom. If you have your envelope follower direction set to negative, you will obviously need to move the baseline slider up so that there is somewhere to go when the parameter needs to move down from its rest position. 

12. The attack and release parameters, each controlled by either a slider or an edit box, affect how responsive, how sensitive the envelope follower is to changes in the incoming audio signal, much in the same way as those in a compressor. After experimentation, I believe that, although you can enter pretty much any value you like in the boxes, the maximum values are 1000 milleseconds.

The attack and release settings you choose will depend very much on the style of guitar playing you're working with. 

13. The strength control affects how significantly the effect parameter will deviate from the cut-off frequency. So, as the volume of the incoming signal crosses the minimum threshold you designated, the effect parameter will start to rise from its baseline. If strength is set to 100, by the time it reaches the maximum volume you designated, the parameter will have moved as far away from the baseline as possible. 

For this application, you probably want to set a strength value considerably less than 100. I'd recommend 20 to start, but I could be out. 

14. When you press play with this dialogue open, if you have configured things correctly, not only should you hear a change in your audio, but you may also receive feedback from your screen reader. NVDA at least treats the baseline slider as a progress bar. Much in the same way as NVDA interacts with the VU meters on Teamtalk or the meters in Virtual Audio Cable's Audio Repeaters. This can be helpful. You may not always hear a change in your audio, but hearing the parameter move via your screen reader will at least tell you something's happening. 

Once you get into the swing of it though, this feedback can get very intrusive. Toggle the feedback NVDA provides for progress bars by pressing NVDA u. You have the choice of beeps, speech, beeps and speech or no progress bar updates. 

This process sounds very technical, but once you get going, it's not hard to find a rhythm. 

There are lots of other applications for envelope following. You could create a dynamic delay, whose feedback, rate or volume is modulated by the volume of your audio. You could create a sort of gated reverb, where the reverb decay time, room size or wet was modulated by the audio's volume envelope. The possibilities are endless. 

I hope you enjoyed the article. If you've found it useful  and if you like, feel free to buy me a coffee or something. My Paypal is http://www.paypal.me/jmacleod It really encourages me to write future articles and produce demos. 
justinmacleod@hotmail.com