How to Use the macOS Audio MIDI Setup Utility to Combine Multiple Audio Interfaces

Podcasters and music producers often face a frustrating hardware limitation: standard recording software usually only allows you to select a single audio input device at a time. If you have two USB microphones plugged into your Mac, you cannot record both simultaneously in apps like GarageBand. Fortunately, macOS includes a powerful hidden tool called “Audio MIDI Setup” that allows you to virtually bind multiple audio interfaces together into a single, unified “Aggregate Device.”

Why Use an Aggregate Device?

An Aggregate Device tricks your Mac into seeing multiple separate physical microphones or audio interfaces as one giant piece of hardware with multiple channels. This means you can plug in three different USB microphones, bind them together, and record all three simultaneously onto separate tracks in your digital audio workstation (DAW), entirely bypassing the software’s single-device limitation.

Step 1: Open the Audio MIDI Setup App

This utility is pre-installed on every Mac but is hidden inside the Utilities folder.

1. Press Command + Space to launch Spotlight Search.
2. Type Audio MIDI Setup and press Enter.
3. Alternatively, open Finder, navigate to the Applications folder, open the Utilities folder, and double-click the app icon.

Step 2: Create a New Aggregate Device

Ensure all your USB microphones or audio interfaces are plugged into the Mac before proceeding.

1. In the Audio MIDI Setup window, look at the bottom-left corner and click the small Plus (+) button.
2. Select Create Aggregate Device from the drop-down menu.
3. A new item named “Aggregate Device” will appear in the left sidebar. Double-click the name to rename it to something recognizable (e.g., “Podcast Setup”).

Step 3: Assign Audio Interfaces to the Group

Now you must select which specific hardware devices belong to this new virtual group.

1. Click on your newly created Aggregate Device in the left sidebar.
2. In the main right-hand panel, you will see a list of every audio device currently connected to your Mac.
3. Check the Use box next to each microphone or interface you want to combine.
4. Ensure the Sample Rate at the top of the window matches the highest quality supported by your devices (typically 44.1 kHz or 48.0 kHz).
5. Look for the Clock Source drop-down menu. Select your most reliable audio interface as the primary clock source to prevent audio drifting.
6. Check the Drift Correction box for all *other* devices in the list. This ensures their audio stays perfectly synchronized with the master clock source.

Step 4: Select the Device in Your Software

The virtual device is now ready to use.

1. Open your recording software (GarageBand, Logic Pro, Audacity, etc.).
2. Navigate to the software’s audio input settings.
3. Instead of selecting a specific microphone, select your new Aggregate Device (e.g., “Podcast Setup”).
4. You can now route the individual channels from that virtual device to separate recording tracks.

By mastering the Audio MIDI Setup utility, you can build complex, multi-microphone recording setups on a Mac without having to purchase an expensive hardware mixer.