How To Use Multiband Compression For Audio Mastering

Audio mastering is an essential step in music production. When the mix for a track is done, you need to master it, or send it to an audio mastering engineer if you're not too clued up on the process. He or she will check it for errors, enhance it, and master it to a commercial level.

One of the most important steps in the enhancement stage of audio mastering is applying multiband compression. Creative but subtle fine tuning of this audio tool can really make the difference between a ‘suitable for internet' and a ‘commercial sounding' mix. With that in mind, today we're going to look at what multiband compression is, and how to use it when mastering your songs.

Note: This guide was written by audio engineer Marcel van Ling at track-mastering.com.

What Is An Audio Compressor?

Before you work with a multi-band compressor, you first you need to know the principles of audio compression. When the volume of an audio signal exceeds a predefined threshold, it will be attenuated from that point on with a certain ratio. This allows you to control the dynamics of a song. If you lower the threshold, you'll reduce the dynamics of the audio.

Almost all audio compressors have the following controls:

Threshold

When the audio signal exceeds the threshold the compressor will start to compress the sound.

Ratio

This is the amount of compression. Choosing a ratio of 2:1 will reduce a 4 dB peak above the threshold with 2 dB. Choosing a ratio of 4:1 will reduce this peak with 3 dB. If you set the compressor ratio to an infinite number it is called a limiter (Figure 1).

Attack

This is the speed at which the compressor must work. With a very fast attack, the audio above the threshold is very rapidly compressed (For example, with a percussive sound such as a snare drum, it'll lose its transient sound).

Release

This is the speed at which the compressor stops compressing. Experiment with the release time and you will notice the difference in the character of the sound. If you want an instrument to have more sustain you should use a fast release time.

Make Up Gain

Because the overall sound level is reduced in audio compression, you can correct this with the make up gain. If you use a ratio of 3:1 with an average sound level above the threshold of 9 dB, then you should set the make up gain to 6 dB.

What Is The Difference Between A Single And Multiband Compressor?

Why do we want to use a multiband compressor in audio mastering? Because it'll give you more control over the dynamics of the audio signal. With this you can process the low end of the mix without even touching the cymbals. This is why it is a very useful tool.

A single band compressor applies dynamic processing to the entire range of frequencies, while a multiband compressor works on individual bands of frequency ranges.

In audio mastering, normally the mix is divided into four frequency bands:

1. 20 – 120 Hz
2. 120 – 2000 Hz
3. 2000 – 10000 Hz
4. 10000 – 20000 Hz

Each of these bands have a single compressor with a separate threshold, attack time, release time and make up gain.

How To Set A Multiband Compressor

Ok, so let's look at how to set up a multiband compressor. Before you start compressing your mix, you should keep in mind that audio mastering is all about fine tuning the dynamics. So here's a tip: Don't compress the signal with 10 dB unless it is really necessary. And if you find you have to do that, it's probably a bad mix and should be sent back to the mixing engineer for another go.

For all four bands of the compressor, first apply the following settings:

• Set the compressor ratio to 1.5 : 1.
• Bring down the threshold to the point where it will attenuate the audio signal with 1 to 3 dB.
• Set the make up gain to the same amount of average compression.

Now you can play with the settings of each frequency band. When you hear edgy and sharp highs in the mix, you may want to work on the third band.

Set the ratio to 2:1 with a fast attack and slow release. What this does is flatten out the sharp transients in the mix. Be careful with the attack time tough. When you apply very fast attack times (like 1 ms or so) there is a chance of audio distortion. If this happens, bring back the attack time to the point where these artifacts aren't audible.

When using a multiband compressor, you need to know the principles to apply the correct changes. Unfortunately, there are no predefined attack and release times in audio mastering. This is because very mix and every music genre needs different settings. That said, I suggest you start with the attack at around 20 to 30 milliseconds. Bring it down to soften the attacks of the instruments, or up to let more of the transients through.

Final Multiband Compression Tips For Better Audio Mastering

Ok, so here are some final multiband compression tips for better audio mastering:

• A fast attack and / or release time can create distortion, so avoid.
• A fast attack time will flatten transients.
• A slow attack time lets the transients of the instruments through.
• A slow release time gives instruments more sustain.
• When you hear a pumping sound, you probably want to use a longer release time or a higher threshold.
• Instead of an equalizer, use a multiband compressor for the low-end.

Important

Remember, if you are working on a good mix, you probably don't want to apply too much of multiband compression. In audio mastering, every action has its own disadvantage. Sometimes, less is more.

About The Author

Marcel van Ling is an audio engineer and the owner of the online mastering service track-mastering.com. You can contact him by writing him an email via support(at)track-msatering.com.