How To Use a Compressor: Understanding Dynamics

One of the hardest audio processors to understand is the compressor.  Even after several years of using compressors many of my students and readers still have lots of questions about how to dial in the sound that they are trying to get.  Compressors are in the Dynamics Processors family which also includes limiters, expanders, gates and noise reduction.  Dynamics processors work in the Amplitude Domain.  Compressors work on the amplitude of an audio signal, which is basically the loudness of the signal.  Look at a waveform view of an audio signal:

Graph of a Sine Wave with Amplitude and Frequency

Graph of a Sine Wave with Amplitude and Frequency

The vertical axis shows Amplitude, which in analog (electrical)  audio refers to the amount of voltage in an analog signal. When the wave is above the center line, then the voltage is positive and when the wave is below the line the voltage is negative. Audio (in the electrical analog sense)  is AC or Alternating Current which means the voltage goes from positive to negative and then back again. The further away from the center line, the higher the voltage and the louder the wave will sound.

The Dynamics of music is generally thought to be the differences between the loud parts of music and the quiet parts of the music.  The dynamics of audio includes all of the differences in amplitude along the waveform.  In most pop music, for instance, the loudest parts of the music are the snare drum hits, followed by the lead vocal, then the background music. Notice in the following image the red dots above the waveform.  They are marking the locations of the snare and kick drum hits in the music.

The red dots mark the locations of the snare and kick drum hits.

The red dots mark the locations of the snare and kick drum hits.

Notice that there is audio in between the loud hits as well, but that it just has a lower amplitude. Compressors and all dynamics-based effects work on the amplitude of the audio, to adjust and change the differences in voltage.  The loudest level in digital audio is 0 dB Full Scale or (0 dB FS) which means that anything above that level will be distorted or simply just an error.  We can’t change the loudest possible level, but we can change everything that is below that level.

What a compressor does:

A compressor attenuates (decreases amplitude) audio that is above a threshold by a ratio.  The attack time is how quickly the compressor starts to attenuate the signal after the threshold is exceeded and the release time is how quickly the compressor stops attenuating the signal when the audio drops below the threshold level.

Probably the most common use of a compressor is to make an audio signal sound louder without peaking out the signal and causing clipping and distortion.  In a nutshell, the loudest parts of the audio signal (the peaks) are made a little bit quieter so that all of the signal can be boosted by the amount that the peaks were attenuated.

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