Aggregate audio devices on Mac: what they are and when you need one

An aggregate device on a Mac combines two or more physical audio interfaces into a single device that macOS treats as one, with all their input and output channels added together. You build it in Audio MIDI Setup, and it exists for one main job: recording several sources at once, in sync, from gear that would otherwise be separate devices. It does not set per-app volume, and it does not send different apps to different speakers — so if that's what you're after, an aggregate device is the wrong tool.

"Aggregate device" is one of those macOS terms people find while chasing an audio problem, latch onto, and then can't tell whether it's the answer. Often it isn't. The name sounds like it might route or mix your apps; it does neither. This guide explains exactly what an aggregate device is, how it differs from the similarly named Multi-Output Device, the handful of cases where you genuinely need one, and the far more common case where what you actually want is per-app control.

What an aggregate device actually does

Core Audio can talk to one hardware interface at a time as a device. An aggregate device is a virtual wrapper that stacks several real interfaces so the system sees a single device with the sum of their channels. Join an 8-channel interface and a 2-channel interface and you get one 10-channel device that any app can open. That's the whole idea: more channels than any single box you own provides.

Because the interfaces each have their own clock, one of them is chosen as the sync (clock) source, and macOS applies drift correction to the others so they stay lined up. Ideally every device runs at the same sample rate; if they can't, the aggregate resamples the stragglers to match the primary. This is the fiddly part, and it's the reason aggregate devices belong to recording setups rather than everyday listening.

You create one in Audio MIDI Setup (Applications → Utilities): click the + at the bottom-left, choose "Create Aggregate Device," then tick the interfaces you want to combine and pick a clock source. It's a native macOS feature — no third-party software, no cost.

Aggregate device vs Multi-Output Device

Audio MIDI Setup offers two combine-style options with confusingly close names, and they do opposite things. An aggregate device is mainly about inputs: more channels in, recorded together. A Multi-Output Device is about output: it fans one playback stream out to several devices at once, so they all play the same audio simultaneously — handy for sending the same music to two sets of speakers.

One practical gotcha with a Multi-Output Device: while it's your output, the keyboard volume keys often stop working, because macOS can't set a single master level across two independent devices. You end up setting each device's volume separately. That surprises a lot of people who set one up just to play music in two rooms.

When you genuinely need an aggregate device

There are real cases, and they're almost all about capture:

• You're recording more inputs than any one interface has — say a band tracked across two interfaces at once.
• You want to record from a microphone interface and capture computer audio through a virtual audio device in the same session, as one device your DAW can open.
• A specific app demands a single device that exposes both the inputs and outputs you need, and no single piece of hardware provides all of them.

If none of that describes you, you probably don't need an aggregate device — and forcing one into a plain listening or calls setup usually creates the sample-rate and drift headaches described above for no benefit. A virtual audio device (like BlackHole) is the piece that captures app audio; the aggregate is just the glue that lets a DAW see it alongside your mic. If that distinction is fuzzy, start with what a virtual audio device is and whether you need one.

The problem people actually have

Most searches that land on "aggregate device" are really about something simpler: I want different apps going to different places, or at different volumes. Music to the desk speakers, a call to the headset, alerts kept quiet. An aggregate device does none of that. It combines hardware into one bucket; it has no concept of "this app here, that app there." Neither does a Multi-Output Device, which forces the same audio everywhere.

That's per-app routing and per-app volume, and macOS has no built-in way to do it — which is exactly the gap third-party mixers fill. It's a different job from what Audio MIDI Setup is for, and reaching for an aggregate device to solve it is the classic wrong turn.

Where Fader fits

Fader is a keyboard-driven per-app volume mixer and audio router. It doesn't build aggregate devices or replace Audio MIDI Setup — it does the job those devices can't. You pick an output device per app (music to speakers, Zoom to a headset), set each app's level, and reach any of it from your keyboard without hunting through windows. Aggregate and Multi-Output devices still exist for the recording and same-audio-everywhere cases; Fader handles the everyday "each app, its own volume and its own output" case those devices were never meant to touch.

If your real goal is playing sound on more than one output but with control over which app goes where, that's routing, not aggregation — see how to use two audio outputs at the same time, per app.

Bottom line

An aggregate device combines several audio interfaces into one device with more channels, and it earns its place in recording setups where you need to capture more sources than a single interface allows. A Multi-Output Device is its playback-side cousin, pushing one stream to several outputs at once. Neither sets per-app volume or routes individual apps to individual devices — that's a separate problem macOS doesn't solve on its own. Know which one you're actually trying to fix before you open Audio MIDI Setup, and you'll save yourself a lot of drift-correction guesswork.