Each measurement
- Rest the phone at the midpoint between the two speakers, mic facing up. Set it down rather than holding it; a hand is a large reflector.
- Tap Enable microphone and grant permission when Safari asks. (You only need to do this once per browser session — the button becomes "✓ Microphone live".)
- In Home Assistant, play synctest.wav to the grouped speakers. You'll hear click-left, click-right, click-both, pause — repeating.
- Tap Measure any time while the track is playing. The tool finds a full cycle on its own. Take 5–6 readings and use the session average.
The track spaces Left and Right exactly 300 ms apart. Anything other than 300 ms in the recording is the offset, and which click arrives early tells you which channel is late.
To tell whether a delay belongs to a particular unit: swap the channel assignment in Music Assistant without moving anything, then measure again. If the sign flips, the same physical box is still the late one — the delay is intrinsic to that unit. If the sign stays put, the delay lives in the channel routing rather than the hardware.
