Compression 101: What’s a Compressor?

I just started my most recent build a couple of days ago. It’s a stereo 1176 Rev. D style compressor from Hairball Audio/ Mnats. I’ll post more about the build itself in a separate posting, but for now, I wanted to get into more the practical uses of compressors if you’re unfamiliar with them.

See, just the other day, I had come up from my basement lair after working on my stereo 1176 for a few hours. My dad asked what I was doing, so I told him I was working on my newest project and that it was a compressor. He asked me why I needed more than one compressor, and I gave him a pretty short answer in order to avoid talking his ear off for the next hour.

It’s understandable though. Even to people who have been recording for a little while, compressors can be quite esoteric in nature. I come across the problem all the time whenever this hobby happens to be brought up. Mic preamps and equalizers are pretty easy, since most people use them fairly often in their day to day lives, but compressors are a different story.

So, here’s my best shot at explaining what exactly a compressor does and how it works. I’ll probably do a follow up posting in a couple days about the different types of compression as well. So without further ado…

What is a compressor?

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This GSSL that I built is one example

According to the dictionary, a compressor is a device that reduces the dynamic range of an audio signal. While that’s an accurate representation, it certainly leaves a bit to be desired. For instance, if you don’t come from a music/recording background, some of those terms might not make any sense to you. So let’s just start there.

Dynamic Range refers to the difference in volume between the loudest and softest parts of an audio signal. For example, a song with a high dynamic range will have very soft parts as well as very loud parts. Think about some classical music that you’ve heard. In general, classical works have a very high dynamic range. To contrast, a song that seems to stay relatively level in volume throughout would be said to have a very low dynamic range. Metallica’s “Death Magnetic” album is the most notorious example I can think of of songs with very low dynamic range.

Dynamic range (or dynamics) are very important for a song to have. It’s one of the many ways to create contrast in musical works, and in my opinion, one of the most effective. In that case, it might seem counterintuitive to use compression, but there are a lot of very good reasons for it.

So if dynamics are so important to music, why do we use compressors at all? The answer is tricky to convey, but suffice to say that without it, a lot of recordings wouldn’t sound very good at all. Compression’s main purpose in music is to make sure that something’s heard above something else. Let’s use a typical rock song as an example.

In our rock band, we have two guitars, bass, drums, and a singer. I’ll go over how and why compression is applied to each element of this group in a little detail.

First up is guitars. Guitars, by their very nature, tend to have a very sharp attack, a quick decay, and (depending on the guitar) a moderate amount of sustain. What this means is that the initial pluck of the note is much louder than the residual ringing of the string. This can be a big problem in a mix because a guitar recorded straight through a microphone into your DAW will have more of a percussive than a melodic effect. All you’d end up hearing would be the sound of the pick striking the string. Sometimes, that’s exactly the sound you want, but more often than not, you’re going to want to better hear the notes the instrument is playing. By tuning a compressor to to trigger on the attack of each note, you can essentially raise the level of those sustained pitches while also lowering the volume of the plucked sound. In the case of heavily distorted guitars, the compression is supplied ahead of time by the amplifier or stomp box.

Bass guitar has a lot of the same issues as a normal guitar, although it tends to not have as much of a decay as a guitar would. So in most cases, compression would be applied to bass in much the same way. In addition to letting us hear the notes more clearly, compression on guitars also has the added benefit of slightly smoothing out a performance.

Drums are a much different animal when it comes to compressing. Because of the nature of percussion instruments, it’s possible to drastically change their character with compression. For the sake of comparison to the guitars, drums have an extremely sharp attack, a very quick decay, and virtually no sustain. Depending on your needs, you may want the drums to be very ‘in your face’ or more subdued.

Vocals are the most obvious candidate for compression, as the human voice has one of the largest dynamic ranges of all instruments. It can be a tricky task to get a vocal track to sit nicely overtop of all of those other loud instruments, but compression makes it more than possible. Without it, vocal recordings usually dip in and out of audibility, and it can become very difficult to understand what the singer is saying.

Hopefully I didn’t ramble along too much there. I didn’t get into anything too technical because I really haven’t explained at all HOW compressors do what they do, but I’ll get to that right now.

How Do Compressors Work?

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The blue and silver unit is an example of an 1176 style compressor

Compressors work by analyzing the amplitude of incoming audio signals in real time. How they do this is largely dependent on the type of compressor, which I’ll talk about more in the next post. We can understand how they work a little bit better though by taking a look at the controls that are common to just about every compressor.

Threshold

Threshold, although not marked control on many compressors, is still something that is crucial to their functionality. Simply put, the threshold is the limit when the compressor starts compressing. For example, if your threshold is set to -1dB, the compressor will not effect any part of the signal that is quieter that -1dB. Some compressors have a variable threshold that allows you to set the value in accordance to the audio signal you’re processing. A lot of compressors, like the 1176 style, have a fixed threshold, which in order to trigger, you need to increase the volume of the signal going into the unit.

Attack

Attack is simply how quickly the compressor responds to signals that cross its threshold. For sources such as drums, a faster attack time is desired, whereas with vocals, you usually look for slower attack times. Attack is very important because it really gives you a lot of control over the envelope of your recording. Think back to the example of the rock band from earlier. Despite its usefulness, the attack control is often left off of a lot of compressors in the place of a fixed or dynamic attack time. Some notable examples are the LA style comps.

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Don’t let the other knobs fool you; there’s really only one that does anything.

Release

Release is how long the compressor remains triggered after the signal falls back below the threshold. At slower release times, the compressor will continue to compress a signal long after the threshold has been crossed. At faster times, it will recover much more quickly. Release is sometimes referred to as ‘Recovery’ on certain kind of compressors.

Ratio

Ratio lets you know just how much you’re compressing. Some common ratios are 2:1, 4:1, 8:1, etc. For example, at a 2:1 ratio, for every two decibels of signal that crosses the threshold, the compressor will only allow one to pass through. So if your signal passes 8 decibels over the threshold, the compressor will reduce the amplitude by 4dB. Any ratio over 10:1 is considered limiting.

Makeup Gain

Once you’ve sent your audio through a compressor, it will inevitably come back quieter than what you sent in. Makeup gain is basically just another volume knob right before the compressor’s output that lets you raise the volume of the signal back up. It’s worth noting that if you compare a signal that has not been compressed to one that has been compressed at the same volume, the compressed signal will seem louder due to the decreased dynamic range.

This post has already gone on long enough, so I’ll save the rest of the info for the next post that deals with types of compressors. I hope that this has helped shed some light on the topic for those of you who are unfamiliar with compression. If all of this still makes no sense to you, don’t worry. It really didn’t make any sense to me either when I first started learning about them.

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