> has the effect of making the quiet parts louder and and loud parts quieter
Not quite. The ceiling of the signal is the same. The quiet parts have gain added but the louder parts (over the threshold/above the knee) receive no modification at all.
Once compression is complete you might even do a normalization pass to ensure that the loudest impulse in your audio achieves 0dBFS.
Put the two together and you have a "wall of noise" effect.
I wrote “roughly” on purpose, but I think your description is wrong. A compressor triggers when the signal goes above a threshold, this applies a compression factor to the loud signal, which reduces the overall gain of the loud sounds (they become quieter). That is what reduces the overall dynamic range. Making the loud sounds quieter.
Most compressors then have a ‘make up’ gain control to recover the lost volume. That process makes the quieter sounds louder.
Not quite. The ceiling of the signal is the same. The quiet parts have gain added but the louder parts (over the threshold/above the knee) receive no modification at all.
Once compression is complete you might even do a normalization pass to ensure that the loudest impulse in your audio achieves 0dBFS.
Put the two together and you have a "wall of noise" effect.