Compressors and limiters are invaluable tools for an audio engineer. They are used in many facets of audio processing and sound dynamics. Learning how to use them correctly can make a big difference in the quality of your recordings and mixes.
To compress or limit an audio signal means to reduce its dynamic range, which is the difference in decibels, between its quietest and loudest parts. In doing so, the louder parts of a performance are reduced/compressed and the quieter parts are inherently amplified.
Limiters are just compressors with really high ratios and fast attacks. Any compressor with a ratio of 20:1 or more is considered to be a limiter. You will not use a limiter nearly as much as you will a compressor, but it’s still useful in many situations.
Despite the importance of compression and limiting they are often misunderstood. To fully understand their roles in music production lets start by breaking up the process into three phases - the Recording phase, the Mixing phase, and the Mastering phase.
The Recording Phase
The Recording Phase is exactly what you think it is. It’s the point in which you are recording some sort of audio signal. By using a hardware compressor, at this time, you can apply compression on the incoming audio signal before it travels through an audio interface into the digital realm.
A great example of why compression is so useful at this stage can be seen in the process of recording vocals. Lets say you are recording an RnB singer who is really quiet on more intimate passages but much louder in other parts of the performance. Most of the time, this results in the louder parts clipping and the quieter parts not being loud enough.
You combat this by applying some mild to moderate compression on the incoming signal, reducing its dynamic range so the louder parts don’t clip and the softer parts are amplified enough to stand out. This would also be effective for a live performance.
Note: Any clipping during the recording process will result in distortion and cause damage to your recordings that you can’t fix – by professional standards this is unacceptable, unless of course, it’s the sound you want.
Here are some audio examples of a vocal performance with and without compression in the Recording Phase.
(Vocals courtesy of Slik Nixin)
You should have noticed that the example with compression has a much more consistent loudness level than its counterpart. The performance without compression starts off really loud and then gets much quieter towards the end, which will present some problems further along in the mixing process.
We can verify this by taking a look at the visual difference in the actual wave files of these two audio signals.
Try getting your hands on a good preamplifier/ compressor combo. For vocals and acoustic instruments I really like the Universal Audio LA-610. It comes with a built in T4 Optical Compressor and will give your recordings a warm, lush feel.
UA LA-610 Mk II preamp/compressor combo
On to the Mixing Phase…
The Mixing Phase comes after you record an audio signal and start processing it in the digital world (your DAW). This is when software compressors get to work and help you get the most of out your recording. Once you reach this stage you have a lot creative freedom to tailor the dynamics of a sound to your liking.
If you are looking for some great software compressors check out I.K. Multimedia’s T-Racks bundle or some WAVES products. Both of these companies produce hi –end plug-ins that often emulate legendary hardware - just be prepared to drop some serious cash if you choose to go with WAVES processors.
Now, lets listen to the same vocal example after using some software compression to fine-tune the dynamics of this performance.
The vox sounds intelligible, clear, and will sit well in the overall mix.
Here’s a screenshot of the compressor and the settings used on this vocal take.
Add some Equalisation, and a little reverb to polish it off and we get this:
The Mastering Phase
In the Mastering Phase you polish off the final mix of your musical arrangement.
Most modern mastering techniques use some form of compression and or limiting to dictate the overall loudness of all the audio signals combined. Typically, I like using a combination of T-Racks and Waves processing plug-ins in the Mastering Phase of my recordings.
I use the SSL Master Buss Compressor to get a smooth reduction of high levels and then add a Brick Wall Limiter as a final safeguard against any strong peaks by applying these processors on the master output of my arrangement.
Note: You can achieve good results using these plug-ins, but don’t be fooled into thinking they can truly compete with the high-end outboard gear of a major studio.
With that being said, here is a short snippet of a song by the same artist. Pay attention to the difference in loudness of the two examples.
The beat was taken from the Epic Dubstep sample pack. Be sure to get your hands on it!
As you can see that second example is much louder. By using the two processors mentioned above I was able to get a much higher loudness level out of the overall mix.
For more information on simple, yet effective mastering techniques read through an article I wrote titled: How to Master Your Tracks Using T-Racks 3 Deluxe, the article contains some great insight to help get you going in a hurry.
Now that we know when and where to use compressors and limiters lets look at how to use them.
Most compressors, unless Optical, use the same parameters to manipulate the dynamic range of an audio signal. Here are those parameters and what they do.
The ‘attack ‘ determines how fast the compressor reacts to an audio signal beyond its set threshold. It dictates how long the signal will sustain itself above threshold before the compression kicks in and reduces it.
Fast attack times cause the compressor to respond very quickly compressing any signal above threshold right away. Slower attack times allow the signals initial transients to escape through before the compression takes place.
The ‘release’ setting controls the speed at which the compressor releases the audio signal and allows it to return to its original gain level. Use slower release times for audio with long sustained notes and quicker release times for busier, percussive passages.
All of the compressors other parameters revolve around the threshold. The threshold is the level at which exceeding audio signals are to be compressed. Set in dB, signals above threshold hold get reduced once the compressor reacts.
Ratio determines the amount of compression applied to signals registering above threshold. Common ratio settings range from 2:1 to 8:1. Here is an example of how these ratios work. If the ratio is set to 2:1 any incoming signal 2 dB over threshold will be reduced on the output signal 1 dB.
Here is how I simplify these terms to clarify their roles.
Attack – how fast the compressor kicks in.
Release - how fast the compressor lets go.
Threshold - the loudness level at which compression is applied.
Ratio - the amount of compression applied.
Learn how to use compressors and limiters well, especially compressors. Compression should be a major tool in every audio production you do, but with it comes a lot of responsibility. Do not get reckless! You will end up with what I like to call a “hot mess” and literally stomp the dynamic life out of your music. Be smart, be strategic, and remember – a lot of times less is more.
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