Compression reduces the distance between the loudest and quietest parts of an audio signal. It compresses the “dynamic range” by making loud sounds less loud and quiet sounds less quiet.
The main reason to use compression is to make the levels of loudness – the dynamics of a song – more consistent.
If you have a musical background, this can seem puzzling. In band and piano lessons you’re constantly told that it’s good to have louder and quieter parts of a song because it makes them more interesting to listen to. Why is there a tool that deliberately takes this away?
Good question. While mixing you will often need to control for variance in an instrument’s loudness.
For example, most singers are unable to consistently sing at the same volume. You can almost expect a vocalist to hit high notes far louder than low ones while recording. This doesn’t give a very consistent sound and as such you should compress the vocal track so the highs aren’t as loud.
Reasons to Use Compression
There are several reasons to use compression in your tracks…
To ensure consistency of volume: Compression reduces the dynamic range of an audio signal to provide more uniform level of loudness in the waveform.
To reduce overly loud parts of an audio signal: Is the attack of the snare hitting too hard? The vocalist hitting high notes too loudly? Compression can help solve these problems.
To give your mix more power: Providing consistent volume across multiple instruments gives your overall mix more power. In a song where a vocalist’s low notes are overly soft, you’ll repeatedly run into issues where the mix sounds weak – not what you want.
To blend multiple tracks together: By ensuring no one instrument is hitting harder than the others you get a more blended sound in your mixes.
As an effect: Compression can also be used to generate new sounds, but that’s a topic for another time.
Compression is most commonly applied to vocals and drums, though it needs to be used periodically on other instruments as well.
How Compressors Work
An audio signal flows into the compressor, has its dynamic range reduced, and an output signal comes out. Setting this up is easy but does require you to understand a few settings. We’ll walk you through these and then show you how to apply compression to your tracks.
Here are the basic settings you need to know:
We need to set a point that says “any sound over this level of loudness will be compressed.” That point is the threshold.
Once the signal is louder than the threshold compression is able to kick in. No compression occurs when the signal falls below the threshold.
Use the compression ratio to determine how much the dynamic range will be reduced once the audio signal crosses the threshold. The greater the ratio, the more the sound is compressed.
You’re probably wondering what this means and how you can apply it in your mixes. Say you set a ratio of 3:1, which is a pretty light amount of compression. This means that for every 3 dB the audio signal is over the threshold, the output signal will be reduced to 1 dB, effectively reducing output by 2 dB.
At a 2:1 ratio, an audio signal 8 dB over the threshold will have its output reduced to 4 dB.
At a 1:1 ratio, an audio signal 5 dB over the threshold will have no output reduction. Your ratio needs to be set greater than 1:1.
You have total control over the amount of compression applied, but it’s common for a ratio to range from 2:1 to 20:1.
Limiting occurs when compression stops any sound from crossing the threshold. To achieve this, set your compressor to a ratio of 60:1 or infinite (or use a limiter plugin). Use limiting to prevent clipping and to put a hard ceiling on the volume of an instrument or the overall mix.
When applying compression, the knee control determines the rate at which compression is applied. A “hard knee” means the effect is applied very quickly, whereas a “soft knee” is more gradual.
It’s common to use a soft knee in situations where you’re using a high compression ratio. This prevents the listener from being able to hear the difference between compressed and uncompressed parts of the signal. If your ratio is above 8:1, consider using a soft knee.
When compressing an instrument you reduce the louder noises in an audio signal, which can often leave things quieter than desired. Use the gain control on your compressor to boost the output signal to a satisfactory level.
How to Use a Compressor
Now that you understand the basics, it’s time to show you how to use a compressor. Before we dive in, we want to note that as a new producer you will find it particularly helpful to use a compressor with a visualizer. This will eliminate your reliance on hearing subtle changes since you’ll be able to see when compression kicks in. FL Studio’s Fruity Limiter and FabFilter’s Pro-C 2 are two compressors with visualization, though there are many other options available.
To use a compressor, first lower the threshold below the signal’s input peaks. Then set your ratio. You should hear compression taking effect. If not, adjust the knee. Listen carefully for changes in loudness and tweak the settings to until you’re happy with the sound.
Finally, boost the output signal with the gain control to ensure the track is playing at a desired level of loudness.
Here’s a quick example of what the process looks like using the compressor in FL Studio’s Fruity Limiter:
Now you’ve got the basics down!
Other Compressor Settings
These two additional controls will help you get the most out of compression.
This setting controls how fast compression begins. If you’re compressing something like drums you’ll need a fast attack time, but with other instruments a fast attack may reduce sound quality and introduce distortion.
Release is the amount of time it takes to stop compressing after the signal falls below the threshold. If this is set too low an instrument’s sound may be distorted.
Both attack and release are measured in milliseconds. If your goal is to aggressively compress an instrument you should set a lower attack and release. For more mild compression the opposite is true.
Where to go from here…
You’re ready to begin adding this important element to your mixes! While compressors do have more advanced settings, you can worry about those in the future.
Start by experimenting with tracks that have substantial dynamic range. If you’re new to compression it can be challenging to hear subtle differences in volume, so make sure you’re using good monitors or headphones and give yourself some time to get used to listening for volume changes.
Be warned: There is such a thing as overcompression, which isn’t something you want in your songs. To hear what this sounds like, push your ratio to the max and set your threshold way too deep. Make sure you avoid this unfortunate effect.