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| Compressors / limiters were originally introduced into the studio to stop loud volume peaks from distorting or saturating. |
When the volume is below the threshold, the compressor / limiter does nothing (unless broken or cheap). The difference between compressors and limiters is explained later in the section on "Ratio Settings"
Compressor / limiter Functions
Compressors / limiters have two main functions (and three other minor ones.) The first function is to get a better signal-to-noise ration, which means less tape hiss. The second function is to stabilize the image of the sound between the speakers, which means more presence.
Better Signal-to-noise Ratio : Less Hiss
Recording extremely dynamic sounds, with a wide variation from soft to loud, requires turning the volume down so that the loud sounds don't overload and cause distortion. Distortion is against the law. Get distortion go to jail. But when you turn the volume down, the soft portions of the sound barely move the needles on the tape player. And if the needles are hardly moving on the multitrack, you hear as much tape hiss as you do signal. This condition is known as bad signal-to-noise ration and sounds very similar to an ocean: "shhhhhhhhhhhhhh."
By using a compressor to turn down the volume when the signal gets too loud, you can then raise the overall volume above the tape noise. By turning down the peaks, you can record the signal hotter on tape. Then the softer portions are loud enough so that you don't hear the tape noise.
When recording digitally, there isn't much noise to worry about. However, if you record too softly the quality of the recording is not as good. You are actually recording at a lower bit rate.
Therefore, compressors / limiters are also good to use when recording digitally. In addition, limiters can be used to keep very quick sound peaks that you might not even hear from going into the red and distorting.
Stabilizing the image of Sounds : More presence
After years of using compression to get rid of hiss, people realized that the sounds often appeared more present when compressed. By evening out the volume peaks on a sound, a compressor/limiter stabilizes the image of the sound between the speakers. A sound naturally bounces up and down in volume, as shown by the bouncing VU meter. When a large number of sounds fluctuate naturally, their bouncing up and down can become extremely chaotic - similar to trying to watch a 24 VU meters at once. A compressor / limiter stabilizes or smoothes out, the movements of sounds that result from these moment-to-moment fluctuations in volume. Once compressed, the sound no longer bounces around much, so the mind can focus on it better. Therefore, the sound seems clearer and more present in a mix.
The busier the mix (the more instruments and the more notes per instrument), the more sounds in the mix are normally compressed. This is because the more sounds and notes, the more chaos. It is difficult to keep track of a number of instruments in a busy mix in the first place. By stabilizing the sounds, the entire mix becomes clearer. Most "acoustic" sounds are compressed, although different engineers disagree about whether to compress the drums or not. Often the kick drum is compressed, and then the entire drum set is compressed with an stereo compressor.
Once the sound has been stabilized, you can then turn up the overall volume and put the whole sound right in your face. This is commonly done in radio and TV commercials to make them sound louder, so that they jump out and grab your attention. This might be annoying in radio and TV commercials, but it's great for a lead guitar or any other instrument you want extremely present in the mix.
This also works when putting sounds in the background. The problem with low volume sounds is that they can easily be lost (masked by other sounds) in the mix, especially if the volume of the sound fluctuates very much. Therefore, it is common to seriously stabilize sounds that are going to be placed low in the mix with compression. They can then be placed extremely low in a mix without fear of losing them.
A better signal-to-noise ratio is obtained by compressing the signals on its way to the multitrack. However many engineers will also compress the signal on its way back from the multitrack during mix down to stabilize the sound even more.
Sharper or Slower Attack
Besides less hiss and more presence, a compresor / limiter also makes the attack of a sound sharper. Once you turn down the louder part of a signal, a sound reaches its maximum volume much quicker.
With a shorter and sharper attack, sounds are much tighter, punchier, more distinct, and more precise, which makes them easier to dance to. On the other hand, a higher quality, fast compressor will actually hep to remove sharp "spikes" on the attack of a sound - softening the sound. A good compressor can mellow out the sound of a guitar with a sharp edge on the attack.
More Sustain
A compressor / limiter is also used to create more "sustain." This is commonly used on a guitar sound. Just as a compressor is used to turn down the volume peaks to raise a sound above the tape noise, it can also be used to turn down the louder parts of a guitar sound, so the guitar can be raised above the rest of the mix. Sustain is also specially helpful for obtaining the desired screaming feedback (when the guitar is held directly in front of a guitar amp).
Compressors are sometimes used in the same way to create more sustain on tom and cymbal sounds. The sounds seem to last longer before they fade out or are absorbed into the mix. The tradeoff is that compressing toms and cymbals will bring their level down, so that you actually hear the bleed more. However, depending on your musical values and the project you're working on, you may want to give this a try.
Less resonance
A final solution of a compressor / limiter is that it evens out resonances in a sound. Resonances occur in two places in instruments: hollow spaces and materials. When a hollow space (like the body of an acoustic guitar) has two parallel walls, it will boost the volume of particular resonante frequencies. Tap on the body of an acoustic guitar in different places, and you can hear the resonant frequencies. Materials (like the neck of a bass guitar) will also resonate at certain frequencies, boosting the volume of those frequencies. Play any guitar and you will notice that certain notes are louder and more resonant than others because they are activating the resonances in the body of the instrument.
A compressor / limiter evens out the volume of these resonances by turning down the loudest part of a sound, which just happens to be resonances.
You can read more about the most important things you need to know about mixing, and how to setup the compressors properly depending on the task you want to realize in Dave Gibson's book "The Art of Mixing 2nd Edition" A visual guide to recording engineer and production.
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