By Justin Joseph Hall
Compressors and Limiters are audio effects that control volume or amplitude of sound. They are used to create equilibrium in what you hear. They are often used in the mixing and mastering stages of music. They are also used when mixing dialogue for film, video, and radio.
Let’s start with the compressor. The compressor is created to lessen the ratio of dynamic range in a recorded sound. For example, if you are recording guitar input and someone accidentally bumps the pickup (which is like the microphone for a guitar that “picks up” the sounds of the strings to amplify them), there will be a spike in the waveform that is much louder than the rest of the recording. The compressor dampens a spike in volume so it doesn’t stick out as much. It diminishes the amplitude of the sound wave by “compressing” any sound that registers above a certain threshold at the compression ratio you set it at.
When using a compressor there are a few key terms.
The threshold refers to the amplitude that the compressor kicks in. So if you set the threshold at -16dBs then anything that goes over -16 decibels will be affected by the compressor.
The ratio refers to the amount of compression. If you have a 3:1 ratio in your compressor then the sound above the threshold will become ⅓ as loud as the original.
The attack or attack time is how fast the compressor will kick in after the amplitude gets over the threshold. Often too fast of an attack time may compress small peaks that barely get over the threshold and can sound odd unless it’s sustained for at least a short period of time. The length it takes to turn on the attack time is usually measured in milliseconds. This is an adjustment you can alter in order to make sure the compressor isn’t turned off and on too often which can be noticeably irritating.
The release or release time is the delay that the compressor should shut off if the amplitude goes under the threshold. This is to prevent the compressor from turning off and on if the amplitude wavers on the threshold and sometimes drops below it. The release and attack times are adjusted to make the transition into the effect smoother and less noticeable. Often the presets work well from the factory, but play around with the setting to see if you can make it sound smoother, especially if the effect is sounding choppy.
The knee is a gradual curve in how the compressor affects the amplitude. So if your compressor is set at -16 decibels at a 3:1 ratio, the knee may prevent the compressor from compressing the sound to 3:1 ratio directly at -16dBs. Instead, it will be applied on a gradient that is adjusted by the knee. It makes the compressor effect more gradual and less noticeable.
Compressors limit the range of the amplitude of a sound. This is key for making recorded sound easier to listen to on speakers. There are as many different types of speakers as there are flavors of ice cream. Compressing sound to a smaller range makes it so listeners won’t hurt their ears if they turn up the volume during a quiet moment in a film that’s followed by a loud explosion scene. Compressing the sound should make it so that people don’t have to turn the volume up or down at all.
Often, the more a sound is compressed the more pleasant the listening experience. Many podcasts, audiobooks, and radio shows use compressors on the vocals so that voices sound similar throughout the program and listeners don’t have to fiddle with their knobs.
In music, pop songs are highly compressed. This is pleasant and makes all songs have a similar dynamic range. Some of the least compressed music is classically recorded music where the range of instruments will not be heard without a large dynamic range. In a recording of an orchestral concert, you want to hear the comparison of the piccolo solo, strings section, and the full orchestra playing without sacrificing the uniqueness of each sound.
Photo of author.
Limiters
Compressors are often used in conjunction with Limiters. The two audio effects work very well in tandem. As the Compressor makes the dynamic range smaller, it softens the loudest sounds. This is because audio begins from no sound and increases like a bar graph in amplitude. If we have a sound that is -10 decibels, the peak of that soundwave is at -10 decibels. When we compress it, the mountain moves to a lower peak.
A Limiter is often applied after a compressor. This is because once you have the desired ratio of your sound, all peaks are compressed to a certain height of amplitude. A limiter brings those peaks higher or lower equally across your sound’s amplitude without going over.
The Limiter earned its name because even though it may increase the amplitude, it limits the amplitude to a Limiter’s threshold. Like the Compressor’s threshold, a Limiter’s threshold is a cutoff point that says no sound peaks will go over this amount. This is to prevent peaking, which is when a recorded sound is too loud for a mic and distorts. For digital audio this is 0 decibels. For analog audio it can vary and depends on what you’re working with, but it is often seen from +6 to +12 decibels.
Limiters also have release and attack inputs which work the same as a compressor’s. They are measured in milliseconds and are when the Limiter kicks in (attack) and when the effect drops out (release).
Why use a Limiter?
So that the sound you are creating is loud enough to hear after a compressor is applied. It would be very annoying for someone to compress a song to be very quiet and then listen to a song that is really loud afterward because you’d have to keep adjusting the volume.
For example, if you listen to a classical song that barely uses a compressor and has a huge dynamic range from -60 decibels all the way to -3 decibels, and follow it with a pop song, you want the loudest part of each song to be at the exact same peak. This is so if you set the music at a house party at a certain volume, that volume is never exceeded and you don’t suddenly scare your neighbors with O Fortuna blasting right after listening to a compressed pop song. Most music players have a setting that levels the peaks of songs so you may already be familiar with this automated process in those kinds of digital music players.
Pop songs actually use Limiters a lot. This trend of mixing music slowly became popular with the rise of Rock N’ Roll wanting loud music, and the Metal of the 1970’s pushing that trend even more. Sound mixers kept making mixes louder by compressing them to a small dynamic range and then raising the peaks to the maximum volume. This puts songs near the top of the possible amplitude without going over 0 decibels and distorting. Eventually, the radio ads also started competing with this music to get the listener's attention and radio ads started being mixed even louder than the music. This became known as the “Loudness Wars,” which really peaked (pardon the pun) in the 1990s and 2000s. You can really hear the dynamic range lessening, especially in pop music during that time.
Compression and limiting are powerful tools that help you create the listening experience you want whether it’s for a song, radio show, or even a movie mix. Compressors and Limiters are used in virtually every professional sound mix of any sort. Learn the ins and outs and what sounds good to your own ear, practice a lot, and you’ll have mastered one of the basics of sound mixing.
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