What does a compressor actually do? Threshold and ratio explained

A compressor turns down the parts of a sound that go above a set level, so the loud and quiet parts sit closer together. The threshold is the level where it starts working. The ratio is how hard it squeezes: at 4:1, every 4 dB above the threshold comes out as 1 dB.

How to do it

1. Start with a ratio around 3:1 or 4:1 for vocals and instruments.
2. Lower the threshold until the loudest notes get 3 to 6 dB of gain reduction.
3. Use a fast attack to catch peaks, or a slower one (10 to 30 ms) to let the punch through.
4. Set the release so the meter falls back between notes or hits.
5. Turn the output (makeup gain) up until it's as loud as before, then compare.

Why it works

An uneven vocal or bass jumps out on one note and disappears on the next. Compression evens that out, so every note sits in place and you can turn the whole track up without the peaks poking out.