AUDIO COMPRESSION 101: What is Compression?
There are many illusive terms floating around the audio world and “compression” is no exception. The trouble with compression is that, for such a useful tool, the parameters to control it are rather cryptic if you don’t know what you’re looking at. Coupled with urban legends about what you “should” or “shouldn’t” use compression for, it really begs the question, 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.”
This is the thermostat of your AC. Whenever the volume gets louder than the threshold, the compressor kicks in to bring it back down.
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.
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.
This controls how fast the compressor stops compressing once the source signal has dropped below the threshold.
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 discovered his love of theater and passion for creating dynamic environments early on as a volunteer at his home church in Pennsylvania. His degree in architecture and years of experience as a volunteer prepared him for his calling to full time ministry where he led teams in the areas of lighting & scenic design, audio production, graphic design, branding, and interior design. He and his wife Emily have recently moved to New York City, where he has come full circle into architectural lighting. Rob enjoys blurring the line between theatrical, experiential, and architectural design; keeping one foot in ministry as often as possible.