USB audio interface shows intermittent crackling and dropouts only when macOS app uses CoreAudio aggregate device

Environment

MacBook Pro (macOS 13.5) using a Focus-brand USB audio interface. Users create a CoreAudio aggregate device combining the USB interface and the internal microphone to allow simultaneous routing. Problem appears in Logic Pro and Zoom: intermittent crackling/pops and brief audio dropouts (~50–200 ms) during recordings or calls.

Symptoms

Audio has periodic crackles and short dropouts only when the aggregate device is selected as input/output. Using the USB interface alone (not aggregated) is stable. Dropouts correlate with increased GUI activity in unrelated apps (browsers) and occur more frequently when the aggregate device includes devices with differing sample rates or clock sources.

Observations

CoreAudio logs show 'Underflow' and 'ring buffer overrun' messages when aggregate device active. Clock drift warnings appear in Audio MIDI Setup if sample rates are not identical. Sample rate for interface forced to 48 kHz; internal mic at 44.1 kHz; aggregate configured with drift correction enabled. Increasing buffer size in DAW reduces dropouts but introduces latency. No USB disconnect events in system logs. Activity monitor shows occasional spikes in WindowServer and kernel_task when dropouts occur.

Already attempted

Created aggregate device with drift correction on/off; enabling drift correction reduces but does not eliminate crackles. Forced all devices to 48 kHz where possible; still saw occasional glitches. Increased CoreAudio buffer in Logic Pro to maximum — glitches decreased. Reinstalled audio interface driver (latest vendor release) and tested on separate macOS user account (same result).

Question

Why does creating a CoreAudio aggregate device introduce intermittent crackles and dropouts even when devices appear healthy individually, and how can I configure the system to avoid audio underruns without unacceptable latency?

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AI DIAGNOSTICS

Incident analysis

Updated 2d ago• Trouble ID

Most likely causes

Aggregate device resampling paired with host CPU spikes (WindowServer/kernel_task) causing processing delays and audio underruns. 90%

Sample rate mismatch and drift between internal mic and USB interface requiring continuous resampling (CPU overhead) that leads to ring buffer underruns. 80%

Driver-level interaction or bug in the audio interface driver when used in aggregate mode under macOS 13.x. 35%

Summary

Aggregate devices combine multiple hardware clocks into a virtual device. If one device's clock drifts or sample rates differ, CoreAudio uses drift correction, which resamples to keep streams in sync. Resampling and clock adjustments increase CPU load; under heavy GUI or kernel activity this can cause buffer underruns and dropouts. The fact single-device operation is stable and issues reduce with larger buffers indicates buffer underruns caused by increased CPU/IO from resampling and drift correction, or by mismatched USB isochronous scheduling.

Suggested Fixes

Diagnostic step 1

Avoid aggregate devices when possible: use the USB interface alone as both input and output or route internal mic through software if required (eliminates clock drift).

Diagnostic step 2

If aggregation is needed, ensure all member devices are set to identical sample rate and bit depth (e.g., 48 kHz, 24-bit) at the OS and device firmware level to reduce resampling work.

Diagnostic step 3

Increase CoreAudio buffer size where acceptable; test the smallest buffer that eliminates underruns. Use dedicated audio performance mode: close background apps and disable energy-saving features.