Beginners Guide to Compression
By Katie Gilchrest
Compression can seem daunting to someone just starting out in the audio production world, but after only understanding a few concepts, the process becomes easier. Use compression to make your mixes sound more professional, polished, energetic and palatable. Our beginners guide to compression should get you off to a good start.
What is compression?
One helpful way to think of compression is to picture it as an automatic fader. As the loudest peaks occur, they are lowered down to the softest level of the signal. Essentially the dynamic range is reduced, giving a sense of continuity and robustness. Compressors are very effective on instruments with an inconsistent dynamic range, such as vocalists or snare drums. There are software compressors and hardware compressors. Hardware compressors are still vital in any studio as they help get a loud signal into your DAW on recording so the signal makes use of as much as the bit rate and sampling frequency as possible, which is particularly noticeable on vocals. All-in-one channel strips are very popular.
The Input Gain is how much input level will be sent to the compressor.
As the compressor reduces dynamic range, the Threshold determines the reduction amount. Compressor Thresholds will range from 0 to -50dB. If your Threshold is set at -20dB, this means that any part of the signal that falls below that level will remain unaffected.
Input to Output Gain Ratio determines the amount of input signal to cause a decrease in the output signal. For a 4:1 Ratio, for every 4dB gain increase at the input, there will be a 1dB output increase at the output. When the Ratio reaches 20:1, it becomes a limiter. The Ratio and the Threshold work together.
The Knee is simply the slope at which the signal enters the threshold. A soft knee provides a slower, more gradual entry of the signal in the compressor, while a hard knee filters the signal quickly and more abrupt. Percussive or low frequency sounds like drums or bass work better with hard knees and material like vocals sound better with the natural soft knee.
Attack dictates the amount of time the compressor starts working on a peak in the signal that exceeds the Threshold. Fast attack times at <10 milliseconds (ms) catch the peak before it is noticeable, while slow attack times (10-100ms) allow the first transient through. Slow attack times produce a ‘punchiness’ and faster attack times are more transparent.
Opposite of Attack, Release is the amount of time before the signal returns to its normal, uncompressed level. Release times typically go from 5ms to 5 seconds. The ‘pumping’ effect can be achieved with shorter Release times.
Since a compressor reduces dynamic range, the overall signal will be reduced in gain. Makeup Gain increases the amplitude of the signal after the compressor, to ‘make up’ for the gain lost in the processing, even though the compressor gives an illusion of loudness.
With patience and practice, tuning your ears to various compressor settings on different instruments will give valuable experience and training. There is no right way to set up a compressor and each production will vary whether you are applying it to individual tracks or mastering a mix. Always trust your ears and we hope this beginners guide to compression has helped! The final judgment call should always be made on what sounds the best.
We go a lot deeper than our beginners guide to compression on our Mixing and Mastering courses. And remember that all content from our Mixing and Mastering courses is included on all of our music production diplomas, worldwide.