Compressors and limiters are used to control the loudness of an audio signal. This purpose of which is to control the incoming level to another audio device. Have you ever heard a video or audio post where the background noise is fine but when the person speaks it is distorted? That is because the level of the persons voice is too loud for the either the next audio device in line or the recorder itself thereby causing it to overload and distort. In the movement of audio from voice to mic to mic preamp to eq to audio converter and then to computer each device has an audio range it can operate cleanly in. (The practice of maintaining an even level through all these different devices is called 'gain stageing' and requires one to oftentimes be familiar with each device in use.) While some devices can be driven past this level for a desired warming or distortion effect most often engineers are shooting for a clear and clean tone that is faithful to the original.

Proper gain staging is good, but the random world we live in is not properly gain staged. Often when recording a singer will sing a little, or a lot, louder than than anticipated in your initial level setting or the song itself will vary greatly in soft and loud parts. Bass players too often vary in their playing levels dramatically. Both of these once committed to a final recording will vary so much in level that some parts are too loud while some are not loud or clear enough. Although not a complete solution compressors and limiters can help with this problem.

Simply put a compressor will grab audio that goes over a predetermined threshold level and lessen it by a fixed ratio. This is controlled by the ratio knob. Typical ratios are: 2:1, 4:1, 8:1. After having set a threshold level with, yep, the 'threshold' knob a singer would have to sing 4 decibels above your threshold level for their voice to raise 1 decibel above the same threshold. The speed that the compressor grabs the audio is called the 'attack'. The speed of release of the compressor effect is called the release. These are important parameters especially when dealing with fast material such as drums or piano. If the attack is too fast it will cut off the initial transient of the signal and cause it to be darkened or muddied tonally. This can be heard very clearly if you put a snare sample on loop and run it thru a compressor setting about 15db of gain reduction and then vary the attack from slow to fast. You will notice that as the attack gets too fast that all the bite is lost in the first wave of sound. The release knob releases the compressor function. Typically you set release slower for vocals, bass, or anything where the notes are often sustained. This helps it sound natural. Fast release times are nearly always used for any percussive sound such as drums, piano, claps etc.

Wait! My compressor doesn't have a attack and release knob! Relax. This only means that you have what is known as an optical compressor. These have predetermined attack and release that typically vary based on how hard the unit is driven. The more compression, the more attack.

So, typically, we set the threshold to a natural amount (2-4db). Then set the ratio depending on the variance of the material in level. Then we adjust the attack and release knobs for a natural sound. The attack and release tend to affect the overall tonality of the material the most. Driving the threshold, sometimes labeled 'gain', can impart some edge (distortion) to the sound. Increasing the ratio control changes the 'knee'. Knee is how thoroughly the initial sound crossing the threshold is grabbed. A 'soft knee' is more of a smooth response while a 'hard knee' has a tighter grab. On most compressors that don't have a separate 'knee' simply increasing the ratio will harden the knee and thereby creating a tighter cut off of the sound. And this brings us to what is different about a limiter.

Where compressors are meant to control mild (and not so mild) audio peaks, a limiter is meant to limit the audio completely. And so a compressor set to a ratio of 20:1 or more is acting as a limiter. Limiters are very often used where peaks could damage other equipment: radio, the mastering of albums, live performances and their wound systems. Their effect is definitive.

Compressors are very useful tools and are indispensable in the audio world. Many are used, especially in the studio, because of their character (honestly, this is just their individual form of distortion). Knowing how to push certain models creates a certain sound that most will recognize instantly. Learning to hear the effect and 'learn' its sound is easiest if done on solo tracks with percussion (piano is a percussion instrument) being the easiest.

Anthony Saracini

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