Many people don’t realize there are two types of expansion. Downward expansion is a popular choice for minimizing low-level noise like hiss and hum. It’s the opposite of a compressor: compression progressively reduces the output level above a certain threshold, while a downward expander progressively reduces the output level below a certain threshold. For example, with 2:1 compression, a 2 dB input level increase above the threshold yields a 1 dB increase at the output. With 1:2 expansion, a 1 dB input level decrease below the threshold yields 2 dB of attenuation at the output.