Understanding mix buses
When you have two or more Mixer channels routed into a single Mixer channel, we call the combined audio a bus, also known as a mix bus. Buses are useful for combining sounds together and making them appear related to one another.
The master channel is a type of bus that collects audio from all the other Mixer channels. Most of the time when we talk about a bus, we aren't referring to the master channel, though.
You can generally think of a bus as a checkpoint along the way to the master channel. Does the audio coming out of the bus sound good so far up to this point? In most songs, you will have a bus for your drums, a bus combining the layering of your instruments, and a bus for your vocals.
Mix bus best practices
Compression effects should be applied to the bus to gain a sense of cohesion among the instruments. For example, drum instruments are often grouped together in a bus and then given compression. Note that compressor is going to change...