Aggregate Device (In Depth)

What is an Aggregate Device?

An Aggregate Device is a virtual audio device in macOS that combines two or more audio devices into a single input. This allows applications that can normally select only one audio device to access multiple devices simultaneously.

Common Uses for Aggregate Devices

Aggregate Devices are useful in several scenarios, including:

• Recording your microphone and system audio at the same time.
• Combining multiple microphones into a single recording source.
• Using several audio interfaces simultaneously.
• Capturing audio from virtual devices such as BlackHole alongside physical audio hardware.
• Streaming or podcasting setups that require multiple audio sources to be available within a single application.
• DAWs or recording software that only allow one audio input device to be selected at a time.

Example Setup: Recording Your Microphone and System Audio into Audacity

In this example, we will record both:

• Your MacBook's built-in microphone
• Your system audio routed through BlackHole

These sources will be combined into a single Aggregate Device and recorded in Audacity.

Easy Step-by-Step Instructions for Recording with an Aggregate Device

1: Open Audio MIDI Setup.

Launch Audio MIDI Setup from Applications.

2: Create an Aggregate Device

Click the + button in the lower-left corner and select Create Aggregate Device.

3: Add Your Devices

Select the audio devices you want to combine:

• MacBook Microphone (Channel 1)
• BlackHole (Channels 2 and 3)

Enable _Drift Correction_ for BlackHole to help maintain synchronization between devices.

4: Configure Audacity

Open Audacity (or any DAW/recording app of your preference), and customize your audio setup. For this app, we will open Preferences.

5: Select the Aggregate Device

Under Recording Device (sometimes called Audio Input Device in other applications), select the Aggregate Device you just created.

6: Select the Appropriate Recording Channels

Go to: Audio Setup → Recording Channels Choose the number of channels required for your setup. Important: Aggregate Devices do not automatically mix all sources together. Instead, they expose all channels from all included devices. For example:

• Selecting Mono (1 Channel) records only the first channel.
• Selecting Stereo (2 Channels) records only the first two channels.

The correct channel count depends on how many channels are provided by each device in your Aggregate Device.

7: Create Tracks and Record

If needed, create additional tracks and assign them to the appropriate input channels.

Start recording and verify that both your microphone and system audio are receiving signals.

Potential Downsides of Aggregate Devices

While Aggregate Devices are powerful, they can introduce some challenges:

• Clock synchronization issues between devices can cause clicks, pops, or audio drift over time. Click here to learn more.
• Devices running at different sample rates may fail to work correctly together.
• Some applications may not properly recognize or map Aggregate Device channels.
• Additional configuration is often required to determine which channels belong to which device.
• Troubleshooting can be more complex than using a single audio interface.
• Latency may increase slightly depending on the devices being combined.
• If one device disconnects or changes settings, the Aggregate Device may stop functioning correctly until reconfigured.

For best results, ensure all devices are configured to use the same sample rate (for example, 48 kHz), and enable Drift Correction on secondary devices when appropriate.