Auxilliary sends are simple processors in a bus or track channel strip. They tap the signal at a specific point in the signal flow (pre-fader, post-fader, before or after EQs and other plugins, etc.) and send a copy of that signal to a bus, without affecting the normal signal flow downwards to the channel fader.
Aux sends from several tracks are collectively sent to a bus in Ardour, to create a monitor mix for a musician, or to feed an effect unit. A bus used in this way is considered an auxilliary bus or Aux bus even though it is the same as any other bus. The output of such a bus might be routed to separate hardware outputs (in the case of headphone or monitor wedge mixes), or returned to the main mix (in the case of an effect).
Aux sends are not JACK ports, use External Sends to send audio to Jack ports. External Sends can send the tapped signal somewhere else directly, which is not usually possible on hardware mixers.
Adding a new aux bus
Choose Session > Add New Track or Bus. In the New Track & Bus dialog, select "Busses" in the Track/Bus selector at the upper right.
Adding a send to an aux bus
Context-click on the processor box for the track you want to send to the bus, and choose New Aux Send. From the submenu, choose the bus you want to send to. A send will be added (and will be visible in the processor box). Note that the submenu may be empty if you have not created a bus yet.
Pre-fader and Post-fader Aux Sends
Depending on whether you context-click above or below the fader in the processor box, the new aux send can be placed before or after the fader in the channel strip. Post-fader aux sends are typically used when using an aux for shared signal processing (FX), so that the amount of effect is always proportional to the main mix fader. Pre-fader sends ensure that the level sent to the bus is controlled only by the send, not the main fader—this is typical when constructing headphone and monitor wedge mixes.
Adding a new aux bus and sending a Track Group to it
You can add aux sends to all members of a group and connect them to a new aux bus with a single click. After creating the track group (and adding tracks to it), context-click on the group tab and choose either Add New Aux Bus (pre-fader) or Add New Aux Bus (post-fader). A new aux bus will be created, and a new aux send added to every member of the track group that connects to this aux bus.
Altering Send Levels
You can alter the amount of the signal received by a send that it delivers to the bus it connects to. There are three approaches to this:
Use the Send Fader
Every send processor has a small horizontal fader that can be adjusted in the usual way. It is not very big and so this can be a little unsatisfactory if you want very fine control over the send level.
Mapping the Main Fader
Double-clicking on the send in the processor box will allow you to use the big fader of the mixer strip to control the send. The visual appearance of the mixer strip will change to reflect this. Double-click the send again to revert back to normal function for the strip.
Map Aux Sends To Main Faders
Pressing the button marked Aux Sends on a aux bus will alter the channel strip for every track or bus that feeds the aux bus. Many aspects of the strip will become insensitive and/or change their visual appearance. More importantly, the main fader of the affected channel strips will now control the send level and not the track gain. This gives a larger, more configurable control to alter the level. Click the Aux Sends button of the aux bus again to revert the channel strips to their normal use.
Clicking on the small "LED" in the send display in the processor box of the channel strip will enable/disable the send. When disabled, only silence will be delivered to the aux bus by this track. When enabled, the signal arriving at the send will be delivered to the aux bus.
Send panners can be configured to either be independent of the main panner, or to follow it. The latter could be useful for Reverb effects, or for in-ear monitor mixes delivered in stereo.