Drift Correction is a macOS setting that keeps two combined audio devices in sync as they play. Every device runs on its own internal clock, and no two clocks tick at exactly the same rate, so a Multi-Output Device's members slowly separate over time unless one is continuously corrected to track the other.
It's easy to assume this is something going wrong, a stutter, a bug, a bad cable. It isn't. It's the predictable, unavoidable result of asking two separate pieces of hardware, each with its own clock, to play as if they were one. Understanding why that happens, and what macOS actually does about it, makes the fix in Audio MIDI Setup much less mysterious. If you're already hearing two speakers drift apart and want the exact steps to fix it, see our guide to grouped speakers slowly drifting out of sync. This page is about the mechanism underneath that fix.
What is a clock, in this context?
Every audio device, wired or wireless, plays sound by converting a stream of digital samples into an analogue signal at a fixed rate, commonly 44,100 or 48,000 samples per second. Something inside the device has to keep that count. That something is a small crystal oscillator, the device's internal clock, ticking away independently of your Mac, independently of every other speaker in the room, and independently of any other clock involved.
When you group two devices into a Multi-Output Device, macOS doesn't merge those two physical clocks into a single shared timer. It can't. There's no wire or protocol linking a pair of consumer speakers at that level. Instead, macOS picks one device's clock as the group's reference and has the other device play against it as closely as it can.
Why two clocks never quite agree
"As closely as it can" is the whole problem. No two crystal oscillators, even two units of the exact same model from the same factory, run at precisely identical speeds. Manufacturing tolerance means each one is a little fast or a little slow compared to its nominal rate, by an amount too small to notice on its own. One device might genuinely be counting 48,001 samples in the time the other counts 48,000.
That difference doesn't cancel out or average away. It compounds, sample after sample, for as long as both devices keep playing. A gap of a few milliseconds is inaudible. A few dozen milliseconds starts to sound like a faint echo or a phasey doubling. Left uncorrected, the gap only grows the longer playback runs, because nothing about it resets on its own. This is the physical reality of combining independently clocked hardware, not a flaw in any particular device, app, or version of macOS.
Every feature free for 30 days. No account, no card details.
Download for MacWhat Drift Correction actually does
Drift Correction is macOS's answer to that compounding gap. One device in the group is marked as the primary, the clock every other device is measured against. Every other device gets its own Drift Correction setting, and turning it on tells macOS to continuously resample that device's audio stream, stretching or compressing it by a fraction of a percent, so its effective playback rate tracks the primary's rate instead of its own. The correction runs the whole time the group plays, quietly nudging the gap back towards zero before it becomes audible.
It's worth being precise about what this isn't. It doesn't rewire the two devices to share a single physical clock, and it doesn't produce sample-accurate, laboratory-grade sync. What it does is keep the drift small enough to stay below the threshold where most people can hear it, which is the realistic goal for consumer audio hardware rather than a studio word-clock setup.
Where the setting lives
Drift Correction is set per device, inside Audio MIDI Setup's view of a Multi-Output Device: select the group, and every member except the primary shows a Drift Correction checkbox next to it. It isn't turned on for every group automatically by macOS itself, which is why a freshly built Multi-Output Device can sound perfectly synced for the first few seconds and then start slipping. The setting exists, it just isn't switched on until someone ticks it. For the exact steps and which device to leave as the primary, see the walkthrough in why grouped speakers drift out of sync.
Where Chorus fits, and where it does not
Drift Correction is a macOS Core Audio feature. Chorus didn't invent it and doesn't run its own version of it, it uses the exact same system-level mechanism Audio MIDI Setup exposes. What Chorus does differently is when that setting gets configured. Building a Multi-Output Device by hand in Audio MIDI Setup leaves Drift Correction off until someone finds the checkbox and ticks it. When Chorus's Groups feature builds a group, it sets a clock master and switches Drift Correction on for every other device automatically, as part of creating the group, so there's no separate setting to remember or a silently-off checkbox to discover later.
Chorus doesn't add EQ, effects, or its own audio processing on top of this. It groups outputs using the same mechanism macOS already provides, gives the group one real volume with per-device balance, and handles the drift setting as part of building that group rather than leaving it as a manual step.
For the step-by-step on building a Multi-Output Device, see Apple's own guide to Audio MIDI Setup.
Frequently asked questions
What is Drift Correction on a Mac?
Drift Correction is a setting in macOS's Audio MIDI Setup that keeps two or more combined audio devices playing in sync. It continuously adjusts one device's stream to track another device's clock, so the two don't slowly separate the longer they play together.
Why do two audio devices need Drift Correction at all?
Because each device generates its own timing from its own internal clock, and no two clocks run at exactly the same rate. When a Multi-Output Device combines them, both start in sync, but the tiny difference between their clock speeds adds up sample by sample, so without correction the gap keeps growing.
Is audio drift a bug in macOS or in the app you're using?
Neither. It's a physical consequence of combining two pieces of hardware that were never built to share one clock. Any operating system or app that lets you group independent audio devices runs into the same limit, which is exactly why macOS ships a dedicated Drift Correction setting rather than treating it as something to patch.
Where do I turn on Drift Correction?
In Audio MIDI Setup, select the Multi-Output Device in the left-hand list, and a Drift Correction column appears next to every member device except the one set as the primary clock. For the full walkthrough, including which device to leave as primary, see our guide to grouped speakers drifting out of sync.
Does Chorus implement its own drift correction?
No. Drift Correction is a macOS Core Audio feature, not something Chorus built or does uniquely. Chorus's Groups feature configures it automatically as part of creating a group: one device is set as the clock master and the rest are switched to drift-correct against it, so there's no separate setting to find and tick by hand.
