Understanding compression
When mixing, we call the range in volume from loud to quiet the dynamic range. Compression is an effect to be applied to a sound to reduce the dynamic range. After compression is applied to a sound, the loudest parts of the sound become quieter relative to the quieter parts. The volume of the whole sound is then raised. Reducing the dynamic range means that you have less change between the quietest and the loudest parts of your sound.
Why would you want to use compression? Imagine you were having a conversation with someone and wanted someone else to hear a recording of the dialog. In the recording, some parts of the dialog might be really loud while others might be quiet. You might whisper in some parts and yell in others; you might move close to or further away from the microphone. All of these factors will affect the volume of the end result of the recording. For someone listening to the recording, you don’t want them to be struggling to hear...
