I’m having some issues with compressor, I have the default one that comes with Audacity, and I have installed the Plugin Pack from download
In short, SC1-4 and Compressor can’t catch very fast spikes/transients even though I set attack times to as fast as possible. No, I didn’t put too short decays, and yes, I’m trying to get those down with a strong comp. ratios.
The only thing that kinda works is Hard Limiter, but that one is very sensitive…
So we have a “it is just me, or, no, it’s not you, go to KVR and download something” situation.
Personally I like the SC-4 compressor, but you’re right, it does not catch the transients (fast attack peaks). In that way it is much like “old school” hardware compressors and has a lot of creative potential, but not good if you want to stop the peaks going too high.
The built in compressor effect works pretty well, but it is quirky and takes a bit of getting used to. This compressor is a “look ahead” compressor - it looks ahead for peaks before they happen, and adjusts the gain in advance. Thus it does catch all of the peaks, no matter how fast they occur. Details can be found in the manual: http://manual.audacityteam.org/o/man/compressor.html
The “Hard Limiter” can be useful, but you need to be careful with it. How it works is to “chop off” the tops of the highest peaks, and then has an option to add back a reduced version (the “residual”) so as to reduce the amount of distortion. It can work quite well on snare drum and similar types of sound. Like “tape saturation” it will limit the peaks and add a controlled amount of distortion. If you want to avoid distortion, don’t use this effect.
The difference between a “compressor” and a “limiter” is the time scale. A compressor will adjust the “gain” (amplification) gradually so as to reduce the dynamic range, whereas a limiter will adjust the gain very rapidly. In extreme cases (such as the “Hard limiter” and the “Leveler” effect, the gain is adjusted instantaneously. The down side of “instantaneous” gain adjustment is that it creates distortion. (I strongly don’t recommend using the “Leveler” as a compressor effect - it creates far too much distortion - can be useful as a distortion effect though.)
Both compressors and limiters tend to have “character”. Although they are all doing pretty much the same thing, the “sound” of the effect can vary greatly from one effect to another. Compressors and limiters are quite a long way on the “arty” side of the art/science divide
There are lots of effects that could be included with Audacity, but the current list of effects is already quite long. Personally I think that a proper Limiter effect should be included as standard. Feel free to make a request here: http://forum.audacityteam.org/viewforum.php?f=20
The appropriate “speed” of a compressor depends on what sort of compression effect you are trying to achieve. You’ve not said what “the job” is.