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| Text | For many years, I looked for an explanation of how audio compression makes quiet sounds louder. There isn't one, because it doesn't. However, it is true that compressors (usually) make loud sounds quieter. What's the difference? Compression is the process of taking sounds that are louder than a given threshold, and making them somewhat quieter. Compressors - that is, effects devices with the word "compressor" written on the box - usually contain two circuits. The first one is a compression stage, which is followed by a separate signal-boosting stage. That second part is what makes the quiet sounds louder, but it does that by making the entire compressed signal louder. It's referred to by various terms, but the most common is "make-up gain", because it applies some amount of gain (amplification) to the signal to "make up for" the overall drop in volume caused by the compression. And that's why explanations of how "loud sound gets quieter" is never followed by "and this is how the quiet sounds are made louder." There actually is a related technology that dynamically makes the quiet bits louder. It's called "upward expansion", and it's a crucial part of how Dolby noise-reduction works. Yes, downward expansion is also a thing; that's how noise-gates work. |