First, I'm perfectly aware that this is absolutely not what can be done for audio mixing in Final Cut Pro. 😊 But here's the thing.
I got a problem of silent audio recorded by a simple boring lavalier mic. I tried to do the thing by the book using Limiter and Multipressor (one at a time, of course) — I get the audio level almost 0 dB, but the volume is still not as high as I'd like to get. From different videos I can hear that people somehow make their voices loud even at lower audio levels — I really have no idea how they do that.
Clicking here and there, I noticed that when I apply any surround panning mode to my mono sound, the wave form gets much lower — but the sound get drastically boosted with no overloading. I know, that's not what anyone should do with the sound, but I'd like to understand why I get such an effect? I really would like to know how to get the same effect, but with proper tools.
My guess is that it's doubling the signal, and Final Cut prevents clipping by virtue of its 32-bit floating point processing (which pretty much every NLE and DAW has these days).
As for boosting the audio, I assume you first tried selecting the audio clip and either using the volume slider on the clip itself or the volume slider in the inspector? That would be one way of recovering volume from a clip where the gain was set way too low. Another way is to go to the effects browser and look in the "Levels" section for the Gain effect and drop that on the clip. You can then use that to raise the gain. No need to use a limiter or compressor to do this.
What recorder was used for this lav mic? The new generation of Sound Devices MixPre recorders can record 32-bit floating point wav files (not to be confused with the 32-bit floating point architecture of Final Cut itself), which means you can set the gain to a very low level so you never have to worry about clipping. The files may sound silent but if you raise the gain you can bring them to appropriate levels.
Note that "0 db" on the clip slider scale is simply a relative indicator (if it's less than 0 db it indicates that you've adjusted the volume downward from its original level). To set levels, you should use the sound meters; each individual clip should normally average -12 to -18 dBFs on the meter scale. That may sound too quiet, but when you have lots of audio tracks superimposed on each other their volume sums and your master track can end up clipping.
For boosting audio I use only Limiter of Multipressor (haven't decided yet which one is more appropriate), I don't touch the volume slider, it's not adaptive to an unbalanced audio recording. I meant that the surround panning somehow normalised the sound enriching it at the same time.
"I meant that the surround panning somehow normalised the sound enriching it at the same time."
Right, and as I said I think what's happening is that the surround panning is doubling the signal (or perhaps multiplying by 5 if it's 5.1 surround), which will make it sound louder and fuller.
I still think using the Gain plugin is the right way to go.
In a digital audio workstation (DAW) and even in an analogue console, there are several ways to control volume. The optimal workflow is called gain staging, and it starts with adjusting the gain controls: you use the gain knob or slider to set the track's average signal to your desired target level (as noted above, typically -12 to -18 dBFS).
Then, once you have all your tracks set to the desired level, you use the mixer's faders to adjust the tracks relative to each other, bringing up the dialogue, bringing down the music, and so forth.
Final Cut doesn't have a mixer. As far as I can tell, the volume slider is analogous to a fader. To boost the volume of a clip that was originally recorded with too little gain, adding the Gain effect will increase the gain.
I would a limiter and compressor to help me meet loudness targets (which have requirements for peaks and LUFs values) and to help things like dialogue or voiceover stand out better in the mix, but I wouldn't use them as the primary way to increase gain. That's what the gain plugin/effect is for.