Group busses are a feature of many mixers (physical and virtual). They’re often just like regular track channels, except they don’t receive their audio from a disk file, tape channel or live instrument. Instead, they receive their audio from one or more other channels within the mixer. Group busses are sometimes also referred to as submixes.
Group busses are useful if you want to apply any processing (including gain) to several instruments as if they’re one. The most common types of processing to use on group busses are similar to what you’d use on a single channel:
- Gain. More common known as the volume fader. This allows you to adjust the volume of a group of instruments all together. This is most useful when the grouped instruments all serve a similar function in the mix, or are even perceived as a single sound source. Stacked backing vocals or synth pads are good examples of this. Even though you might have a complex pad sound that is made up of four or five layers, the end result is that they combine to form a single sound source. Adjusting their volume as a whole (rather than each individual track individually) is faster and maintains the relative balance of all the layers.
- EQ. This allows you to adjust the tone of a group of instruments. Because EQ is (theoretically) a linear process, applying an EQ change to a group bus is the same as applying the same change individually to each of the individual channels. This can save a lot of mixing time when you have a stack of instruments that have a similar tone. Stacked backing vocals are a good example of this. You might have as many as a dozen different backing vocal parts all recorded by the same singer in the same room with the same mic. If all the tracks need a 3dB dip at 2.5kHz, it’s much easier to apply this tonal change once (at the group bus) instead of a dozen times.
- Compression. This is where it starts to get tricky. Group bus compression is spoken about a lot but often misunderstood. It’s often used to ‘gel’ several instruments together. When one instrument in the bus triggers the compressor, the subsequent gain reduction will apply to the whole group. That means the other instruments will be compressed even though they didn’t trigger the compressor. This can be as subtle as some gentle gain riding through to dramatic and deliberate ducking. This can be effective in helping instruments to ‘gel’ because the uniform gain reduction tells our brains that the sounds are behaving in sync. They’re acting as one, thus should be listened to as one. Keep in mind, however, that this works best when the instruments are quite dynamic and work the compressor. The more the compressor is working, the more audible the effect will be.
With this in mind, you should be able to make effective decisions around when and how to use group busses.