I understand the basic idea of compression: increase low amplitude, decrease high amplitude to some 'normalized' middle range.
I especially understand how the effect enhances electric guitar to turn the normally fast decay into a slower decay and good sustain.
But when compression is applied to a vocal signal - one where a notes are held and sung in the mid-range of dynamics - what is happening to the signal? If it's already sustained an mid-dynamic range, it would seem the compression doesn't have anything to change.
Yet when I have recorded myself singing (in the privacy of my home to save others the misery hearing me!) the uncompressed sound was 'thin' while the compressed sound was fuller. It's hard to describe the difference. Certainly just turning up the level on the uncompressed track did not sound like the compressed one. Even singing with a full, resonant voice, the uncompressed track had no 'presence', poor fidelity to what could be heard in the room. Compression made the track sound much more like what I could hear in the room.
I also noticed the same thing when recording acoustic guitar with a microphone. Even close miked, with playback level up the sound was thin and dead. Adding compression didn't really change the sustain of the acoustic guitar, but the sound brightened up and had presence. It sounded like what I heard in the room when playing.
Somehow compression made the miked tracks sound much more like the room sound, but I don't know why. It's like the dry recording just lost lots audio detail and compression magically restored it. That seems different that just compressing amplitudes to some ideal middle range.
In case it helps my mic is something like a Shure PGA48. I don't remember the exact brand and model, but it's shaped like that and the price was entry level.
The compressor was a plugin that came with PreSonus Studio One. I don't have the gear set up now (I didn't have space to keep it set up and so packed it away.) I'm not absolutely certain, but the interface image below is what I remember. I don't remember how I set it, but changing the 'knobs' changed the graph line, and I didn't know how to read the graph, so I probably left it at presets which may not be what is pictured. I wish I could be more specific, but I don't remember exactly what I did. More importantly to me, I mostly remember that just adding the effect did the job. I didn't need to mess about with the settings.