How to Create an Aggregate Audio Device on a Mac

If you are a podcaster, musician, or streamer using a Mac, you will eventually encounter a frustrating hardware limitation: macOS, like most operating systems, only allows you to select one audio input device and one audio output device at a time in most applications.

If you have two USB microphones plugged in and want to record a two-person interview into GarageBand or Logic Pro, you cannot simply select both microphones simultaneously. To solve this, Apple includes a powerful, hidden feature in a utility called Audio MIDI Setup. This feature allows you to merge multiple physical audio devices into a single, virtual “Aggregate Device” that your software treats as one massive piece of hardware.

How to Open Audio MIDI Setup

The tool required for this process is built into macOS, but it is hidden away in the Utilities folder.

1. Open a Finder window.
2. Click on Applications in the left sidebar.
3. Scroll to the bottom and open the Utilities folder.
4. Double-click on Audio MIDI Setup to launch the application.

Alternatively, you can simply press Command + Space to open Spotlight Search, type “Audio MIDI Setup”, and press Enter.

How to Create the Aggregate Device

When the application opens, you will see a list of all your currently connected audio hardware (such as your MacBook Speakers, external monitors, and USB microphones) in the left sidebar.

1. At the very bottom left corner of the window, 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 sidebar. Click on its name slowly twice to rename it to something recognizable (e.g., “Dual Podcast Mics”).

Selecting Your Hardware

Now you must tell macOS which physical devices belong inside this new virtual group.

1. With your new “Dual Podcast Mics” device selected in the sidebar, look at the main panel on the right.
2. You will see a list of all available audio devices with checkboxes next to them.
3. Check the boxes next to the devices you want to combine. For example, check “USB Mic 1” and “USB Mic 2”.

Crucial Settings: Drift Correction and Clock Source

When combining digital audio devices, their internal clocks rarely match perfectly. Over a 30-minute recording, one microphone might record slightly faster than the other, causing a frustrating echo or “drift” in the final audio file.

To prevent this, look at the Clock Source drop-down menu at the top of the panel and select your most reliable microphone as the master clock. Then, ensure the Drift Correction box is checked for every other device in the list. This forces the secondary devices to continuously sync their timing with the master clock, ensuring perfectly aligned audio.

How to Use Your New Aggregate Device

Once configured, you can close Audio MIDI Setup. The new virtual device is now permanently available to the operating system.

Open your recording software (such as GarageBand, Logic Pro, Audacity, or OBS Studio). Go into the application’s audio preferences and look at the Input Device drop-down list. Instead of choosing a single microphone, select your newly created “Dual Podcast Mics”.

Your software will now see a single device with multiple input channels (e.g., Channel 1 is Mic A, Channel 2 is Mic B), allowing you to record both microphones simultaneously onto separate tracks.