Production

AUDIO COMPRESSION 101: What is Compression?

Compression in the audio sense is simply reducing the dynamic range in volume.

Think of it as a heating & cooling system – without it your house gets very cold in the winter and very hot in the summer.

With it, your house may only be a bit warm in the summer and a bit chilly in the winter. This is reduced dynamic range – a smaller range of extreme high and low points.

In actuality, compressors work more like AC than heat. Compressors reduce the volume once the level rises above the threshold. But what if you want to make the soft parts louder instead? Then you first reduce the loud parts, then boost the volume of the entire signal afterward. So essentially, the loud parts are back where they started, but you’ve “made the quiet parts louder.”

Threshold

This is the thermostat of your AC. Whenever the volume gets louder than the threshold, the compressor kicks in to bring it back down.

Ratio

This is where the metaphor starts to break down: Compressors don’t try and get sound level to a specific volume. Since the input sound is always changing levels, a compressor works to reduce the range between the loud parts and the quiet parts. The ratio controls how much the input signal is reduced. At 1:1, the compressor isn’t doing anything – what comes in goes out. At ∞:1, no matter how loud the source is it will not come out above the threshold.

Soft Knee/ Hard Knee

The “soft knee” setting on some compressors smoothes out the ratio so that sounds just barely louder than the threshold aren’t reduced by the full ratio.

Attack Speed

This controls how fast the compressor reduces the volume. At a fast speed, the reduction is made nearly instantly. At slower speeds, the volume takes time to reduce.

Release Speed

This controls how fast the compressor stops compressing once the source signal has dropped below the threshold.

Gain

Remember how the compressor can technically only make the loud parts quieter? Once it’s done that, the gain adjusts the level of the compressed signal, allowing you to trick it into giving you the effect of boosting the soft parts.

In a future post we can look into some of those urban legends regarding when to use or not use a compressor. But in the meantime, an understanding of what compression actually is goes a long way toward helping you decide for yourself when it may be the right tool for the job.

ROB MEROW
Lighting Designer
OneLux Studio | New York, NY
oneluxstudio.com