Types of audio compressors

There are many different types of audio compressors. In case you didn’t know, audio compressors were originally (and some still are!) hardware devices.

Like with many hardware devices, there are many ways to achieve the desired outcome. With audio compressors that outcome was to squash the audio range by turning down the volume peaks. The different ways that this could be done produced different effects that not only became noticeable but also desirable.

They are not alone in this respect. Tape recording produced hiss, tonal changes and wow and flutter (the audio effect of the wobble from the tape going over rollers). Digital audio recording doesn’t have any of these side effects, however, you can get plug-ins to produce these sound “inaccuracies”. Tape echoes, spring and plate reverb all had side effects that are now desirable and part of “the sound”.

Anyway back to compressors, the way they worked produced different types of compression effects. If you want more information about using effects for beginners check out my guide.

If you are unfamiliar with the basics of audio compression you can introduce yourself to using them with my audio compression settings chart which covers the basics.

More specifically you can learn about using compression on vocals and once you have more experience you’ll want to know about using sidechaining to control compression.

Optical Audio compressor

One of the ways an audio compressor worked was by using light.

If you use the audio signal to drive a light-producing device, like a bulb, the louder the signal, the brighter the bulb. Position a light-dependent resistor next to the bulb and you have a resistor that varies its resistance depending on the volume. Use the resistor to vary the gain by inverting it and you have the basics of a compressor.

As I mentioned earlier each type of compressor has its own sound and optical compressors have a particular sound. There will be a delay in the optical device producing light and it won’t instantly dim either. Also, they won’t be linear. That means that a doubling in volume won’t necessarily double the light produced by the bulb. This all transfers to how the device sounds.

Most of the earlier optical compressors didn’t have many controls due to the optical way they worked. This meant that a lot of the controls that you get with modern compressors were “fixed” on optical compressors. This was another reason why they had a typical sound.

Audio compression ratio

One of the classic electro-optical compressors was the Teletronix LA-2A. There are some plug-in emulations and if you take a closer look you won’t find a ratio control. As previously mentioned this is fixed. It’s actually about 3.1 but again this is one of the reasons it has a “sound”.

As always you should try to listen to these devices and learn how they sound. Words will only get you so far, but the words I’d use would be smooth and non-aggressive.

Tube audio compressors

Obviously, before we had transistors, electronics was all about tubes.

Transistors replaced them in almost everything electronic. They performed better, were cheaper and smaller so it was hardly unexpected. The thing was though that although technically better, with a better audio quality (less distortion etc.) some people missed the sound that valves or tubes imprinted on the sound.

Therefore, it’s no surprise that valve compressors are desirable for their “sound”. If I had to describe the sound in words it would be “warm” and “coloured”. Again the only way to get to know this is to listen to some examples.

FET

After vacuum tubes, the next technology was the transistor.

FET or field effect transistors have become popular in the design of compressors. Coming from an electronics background (electronic technician) I have to be careful how I explain how each device works as this would bore most people rigid and this guide is about different types of compressors, not how to intricately build one.

Transistors were easier to manufacture than vacuum tubes if you take a look at them both it’s not hard to see why. They also perform audio tasks like processing and amplifying with more linearity and less distortion. FET transistors became popular for compressors because of their speed. They were ideal for compressors.

VCA compressors

If you know anything about analogue synthesisers you will probably be aware of what a VCA is (or what it stands for at at least) a VCA is a voltage controlled amplifier.

You may also be aware of envelope followers (they have nothing to do with the royal mail.) An envelope follower produces a control voltage dependent on the audio input. It accurately “follows” the ups and downs of the audio signal.

Now if you think couldn’t you use these two modules to produce a compressor the answer is yes. That’s pretty much what a VCA compressor does (again a bit of a simplification without getting too much into the technicalities.)

Glue compressors

Originally glue compressors became popular because of the SSL desk having bus compressors built into it.

When used on a group of sounds it had the effect of “glueing” the mix together. As this became a desirable effect, several glue compressors have become available. All trying to achieve that exact sound. One of my favourites is the Native Instruments one which has similar controls.

If you want to know more then check out my guide about glue compressors.

Types of audio compressors: Native Instruments Komplete 12 FX bundle

While on the subject of Native Instruments, if you are looking to experiment with different types of compressors you can get the Native Instruments Komplete 12 FX bundle.

This contains lots of fantastic effects all in one package. I’ve mentioned the Raum reverb in my vocal reverb round-up. Also, the Choral chorus is fantastic and mentioned in my chorus round-up. This bundle comes with 7 different compressors so if you want to experiment and listen to the different sounds and compare them, you can’t do much better than to get this package.

These cover all the types of compressors I’ve mentioned above.

Vari comp

This is a simulation of a tube compressor. It also has a sidechain input and wet-dry control for easy parallel compression.

VC 2A

This is a simulation of an optical compressor based on a real hardware model however this simulation also has a sidechain input and a wet-dry control so you can do parallel compression without additional routing.

VC 76

In keeping with the other Native Instrument compressors in the series the VC 76 is a simulation, this time a FET simulation. The strength of this model is its speed. So you’ll easily be able to get punchy sounds from this.

VC 160

This is the VCA model and is particularly easy for beginners to get going with as there are no attack and release controls. These are automatic. One less thing to worry about if you’re starting out with compression.

Solid Dynamics

Another Native Instruments compressor. This one has a gate expander on the end of a fully functional compressor for responsive, clean and works on anything sound. It also features a hard or soft knee and the ratio goes all the way up to infinite (limiting).

Solid Bus compressor

This is the simulation of the SSL Bus compressor. Famous for “that” sound that became synonymous with glue compressors.

Phew! See what I mean? If you want to experiment with different types of compressors and how they sound there is more than enough to get your teeth into, all in one package and that’s before you even consider the other stuff you get.

If you are into recording (of course you are if you’ve got this far) you get loads of other brilliant plug-ins as well.

Types of audio compressors: Use two compressors

Once you start to notice the sound differences and the strengths of each type of compressor you’ll start to see how each one will benefit your recordings.

Once you have a bit of experience you’ll probably find you need to use two. That’s right, two compressors on the same track. That’s because you can get better control by using two compressors each providing a little compression rather than relying on one to do it all.

Don’t worry record producers have been doing it for years. It’s a common practice for many. You can choose each compressor for its benefits. And yes it does make a difference which one you have first and which is second. It’s just the same as other effects an echoed chorus sounds different to a chorused echo.

Generally, when using two compressors it’s sometimes known as serial compression because you put one after another. Notice this is completely different to parallel compression which is mixing some dry signal back.

On vocals, you would use the first compressor to tame the peaks making sure none of the fast transients got through. The second compressor would then even out the dynamics of the performance and perhaps add some warmth and colour to the recording.

I say generally but of course, feel free to experiment and listen. Don’t just take what others say as the be-all and end-all. Recordings are different and what sounds good on one might not on another.

Conclusion: Types of audio compressors

Theoretically, a perfect compressor will just reduce the gain exactly as required without any other audio effects.

Ironically as the technology got better, the side effects became less and the audio quality of modern compressors became better but the older models became more sought after because of their distinctive sound.

You probably won’t be able to hear much of a difference between compressors if you’re just starting out. When I first started using a compressor, it was one that I’d built from a magazine and I didn’t even think it was working because I couldn’t hear what it was doing.

The sounds they produce aren’t as noticeable or “in-your-face” as reverb or echo for instance, but they are as if not more important to the overall sound. Give yourself time and be patient you’ll soon start to notice subtle differences. Follow my guide below about audio subtleties.