Aux is short for auxiliary channel, a channel that receives signal sent from other channels. Instead of holding recorded material directly like a regular audio track, it collects signal arriving over a bus, processes it, and passes it to another output.
Using it as an effect return
Send signal from each track’s send to the same bus, receive it on an Aux, and apply reverb or delay there. Moving the Aux fader adjusts the level of the shared effect signal all at once.
Using it as a submix
Routing several drums or backing vocals into a single Aux lets you handle the whole group’s volume, pan, EQ, and compression with one set of controls. The advantage is that you can create cohesion while keeping the individual adjustments intact.
Difference from a bus
It helps to think of a bus as the virtual path that carries signal, and an Aux as the channel that receives that signal and lets you work on it. Depending on the DAW, it may be labeled Aux, Return, or Effect Channel.
Relationship to the track area
An Aux may exist only inside the mixer, but you can display it in the track area when you want to write automation. Even without regions, fader and plugin parameters can be changed over time.
Things to check when setting up
Confirm that the correct bus is chosen as the input and that the intended destination, such as the main output, is chosen as the output. If no signal arrives, check the send amount at the source, the bus number, the Aux mute, and the output destination in that order to find the cause.
Name them to stay organized
As Auxes multiply it becomes easy to lose track of what each one does, so give them names that describe the incoming sound or the processing, such as “Vocal Reverb” or “Drum Bus”. Matching colors and ordering as well makes it harder to pick the wrong destination in a large project.
