When working with audio, whether it is music production, composition, engineering or any other sub-field within the industry, sometimes you find yourself running out of physical inputs/outputs on your trusty audio interface and wish you had bought that shiny, fancy one with more than enough channels than you will ever need.
Although this could be true and, if your work increasingly demands for more physical channels, you should probably think about upgrading your gear, sometimes you just need a quick solution. Perhaps you simply need an additional headphone channel, or you might have a second audio interface laying around that you wish to integrate to your current setup, or maybe you are collaborating with someone and you just want to use their and your interface simultaneously.
A quick and easy fix for this is to use a long time feature of OSX known as "Aggregate Audio Device".
Whichever DAW you use on your system, you are probably not able to select multiple interfaces within its preferences/settings window.
Creating an Aggregate Audio Device simply sums up all the inputs and outputs of the devices you wish to combine, creating a new virtual audio interface that has them all.
Here's how you can do that following a couple of simple steps.
If you would like to simply use the integrated headphone socket of your computer as an additional stereo input/output, you need to:
1 - Connect your interface
Simply plug in your interface to the computer.
2 - Open the Audio MIDI setup window
This can be found in the Utility folder within the Application folder, or simply typing "Audio MIDI Setup" in the Spotlight research feature (cmd + space, or top right corner of the finder, selecting the little magnifying glass). Open the Application.
3 - Open the Audio Setup Window
Opening Audio MIDI Setup might automatically show just the MIDI setup window, depending on which windows were open the last time you closed the application. Select Window > Show Audio Devices
4 - Create your combined audio Interface (Aggregate Device)
Click on the plus icon (bottom left of the window), and select "Create Aggregate Device".
It will automatically create a new Audio Interface Instance and on the main part of the window you can select which I/O of each interface you would like to include in your new combined device.
Select your connected audio interface (in the shown picture it's Universal Audio Thunderbolt) and built-in microphone/output.
You can then drag and drop your devices to order the various I/O, labelling them as well. This will become the order in which each Input/Output will appear in your DAW.
It is always best to have the most powerful interface with highest clock quality to manage the synchronisation of the audio interfaces and have the others follow it. When creating your combined interface, be sure to select the best of the aggregated devices as the master or clock source.
5 - That's it!
You can rename your new device simply double clicking on its name on the left side of the window, and then you can close Audio Devices.
In your DAW Audio settings/Preferences you should find your new aggregate device with the name you gave it.
This simply sums the I/O of your device with the built-in I/O of your computer, and you will find all of the inputs and outputs listed together.
If you would like to combine multiple interfaces, the drill is exactly the same, but when creating your aggregate device, simply select whichever interface you would like to combine instead of the built-in device.
Even though this could save the day and be a great solution when in a hurry, it is always best to use a single interface with the number of inputs you need.
This is to avoid synching issues between the combined interfaces when they are recording audio.
That's it! You are set to go and ready make some awesome music/sound design/mixing/mastering/anything-that-I'm-sure-will-sound-great!