079 – Dynamics Processors 101: Compressors
Compression can help you build a well balanced mix.
Written by Scott Adamson
In a recent blog post about gates, we began our discussion on dynamic processors.
Another type of dynamic processor is a compressor. This is a really big topic, but it’s a tool we use all the time in live sound, so understanding the fundamentals is important for achieving a well-balanced mix.
When we talk about dynamics, we’re referencing the difference between the loudest and softest parts of an audio signal. Dynamics are a huge part of music, but it’s part of our job as live sound engineers to make sure they aren’t out of control. If the loud parts of a signal are piercing or the quieter parts are inaudible, we can use a compressor to find a good dynamic balance.
In a nutshell, compressors reduce the peaks of a signal, which provides a sort of automatic level control. The louder parts will be quieter, which lets you turn the overall signal up. Then you should be able to hear the quieter parts better without those louder peaks taking over the mix.
Just like gates, compressors have a threshold, attack, and release. We also have the ratio and knee settings.
Once the signal crosses the threshold, the ratio determines how much it’s compressed, or how much the volume is reduced. A ratio could be anywhere from 1:1, which is no compression, all the way up to infinity to one.
With a 1:1 ratio, you’ll get 1 dB for every 1 dB of signal above the threshold. With a 2:1 ratio, for every 2 dB of signal above the threshold, you only get 1 dB output. This is pretty gentle compression that could work nicely for a lot of vocalists.
If we change the ratio to 4:1, for every 4 dB the signal that goes above the threshold, you still only get 1 dB output. This can be somewhat intense, especially with a low threshold. But using heavier compression can be effective if you keep the threshold high enough that you’re only compressing the loudest parts of the signal.
A ratio like 20:1 or higher would have a sort of brick wall effect. After the signal hits a threshold with that kind of ratio, it won’t really get any louder. This can also be called a limiter.
When you’re setting the threshold and ratio, it’s crucial to watch the gain reduction meter which shows you how many decibels you’re actually compressing. Remember — the compressor is inserted into the signal chain and will affect the entire signal. If you’re compressing more than about 6dB for any input, especially with vocals, you might have trouble getting it to the right place in the mix.
It’s also important to note that different compressors have different sonic qualities. In the studio, we’re able to use them to really crush a sound — I especially enjoyed doing this to inputs like drum overheads or room mics. That can be a cool effect, but it’s a lot more difficult in live sound.
Since any microphone will be fed back through the speaker system, there’s a higher possibility for feedback when using compression. You can create a feedback loop if you’re compressing the mic, then have to boost it way up in a speaker. This goes for both front of house and monitors, so be careful about going too hard on any mic.
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Attack, release, and knee individually have the potential to really change the sound, especially with drums. Just like with gates, the attack is how quickly the compressor kicks in after the signal crosses the threshold and the release is how quickly it returns to normal once the signal drops down below it. The knee simply lets the compressor kick in a little before the signal actually hits the threshold, kind of like a soft threshold. This can help make the compressor sound more natural.
In a recent webinar, veteran FOH engineer Harley Zinker gave a great piece of advice for drum compression: Start with slow attack and work backwards until find the sweet spot. Ask yourself - what is the compressor doing to the transient?
Overall, compression is tricky! It’s easy to overuse it, but it’s also a necessary part of getting a solid mix.
The amount of compression used varies from genre to genre. Sometimes quieter jazz or acoustic shows will need little to no compression at all, while rock, pop and hip hop often rely on a pretty compressed sound on almost every channel. As with all things in live sound, the more you practice and experiment, the more confident you’ll feel making decisions that help you achieve your best mix.
