Compressors are one of the most widely used tools in the studio. In this post I will explain the controls of a Compressor, as well as what it’s job is.
A compressor is a device that controls the dynamic range of audio. Simply put, it turns the signal down when it gets too loud. Compression has been used for decades to control the dynamics of speech for broadcast. It didn’t take long for recording engineers to see the value of compression on instruments as well.
Compression works to make a performance sound more confident and consistent, helping it cut through the rest of a dense mix. Here I will explain some basic controls that just about every compressor has.
Basic Compressor Controls
Input Threshold
This knob tells the compressor what level to start compressing. When the signal rises above this level the compressor will begin to attenuate the signal.
Ratio
The Ratio controls how much compression will be applied when the signal passes the threshold.
A ratio of 2:1 means for every 2dB over the threshold, 1dB will pass through. This is considered light compression.
A ratio of 10:1 means for every 10dB over the threshold, 1dB will pass through. This is heavy compression.
Attack
The Attack sets how long the compressor waits after hearing a signal that passes the threshold before it starts compressing.
Very handy for allowing important transients to pass through on drums.
Release
The release tells the compressor when to let go of the signal and how long it takes for it to return to its original level.
This knob can control the ‘pulse’ of the compressor, which can be used to create a rhythmic pumping effect.
Makeup Gain (Output Level)
The output level is used to balance the signal and make up the gain that was lost when compressing.
When audio is compressed we are turning down the peaks of the signal to bring them closer to the quieter sounds. Boosting the signal after compression will result in a more consistent, confident performance that sounds louder.
Perceived Loudness
The human ear is a curious thing. It takes more energy for us to hear very low or very high frequencies at the same volume as midrange frequencies. The most sensitive part of our ears is between 2-4kHz. This is the range of speech intelligibility. We hear and understand these midrange frequencies much easier at lower volumes.
The Fletcher Munson Curve’s, a hearing test developed by Bell labs, describes this phenomenon. The Fletcher Munson curve shows us that human ears are most sensitive around the area of speech 2-4kHz and least sensitive to frequencies at a higher or low extreme. This test measured the ‘equal loudness’ of test tones at different frequencies. The results found that much more energy was required to hear low frequencies at the same perceived loudness as midrange frequencies.
Therefore many Stereo Systems often have a ‘Loudness’ button, which is sometimes labeled as bass boost. This boosts bass frequencies to make the audio appear to be louder without changing the overall volume.
Compression can make use of this understanding to make a track a louder perceived volume even though it is at a lower actual volume.
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