Two-Device Sync Test: Audio Latency Test for Two Devices
Overview
Two-Device Sync Test. Load this page on both devices, click Start Click Pattern, and the Two-Device Sync Test plays a repeating click alongside a visual metronome so you can judge by ear whether the two devices land together or drift apart. Drag the how out of sync did it sound slider to log what you heard — there's no way to measure sync between two separate devices directly from a browser, so your ears are the actual instrument here, which matters for multi-room setups and a phone paired to a TV. Try the gaming headset test tool for a quick, no-install way to check this yourself.
Got a phone and a laptop playing the same song, but one sounds late? The two-device sync test plays one click on both and tells you the audio delay between them in milliseconds, so you know exactly how far to shift one source. Like other audio sync tests, it works with speakers or Bluetooth headphones.
How the Two-Device Sync Test Works
Each device shows a flash on its screen and plays a click on a repeating one-second cycle. Because the audio arrives after the picture, you drag a slider to hold the picture back until the two feel simultaneous. The value is a relative latency: it includes the display and browser timing, so you compare devices, not absolute numbers. The free skype mic test gives you a clear answer instead of guessing.
Reaction Time and the Trailing Average
Tap-based testers ask you to tap instead. A short countdown leads into the first beat, then each tap is compared to the beats; first you calibrate your own reaction time against a visual flash, and that baseline is subtracted. A calibration over several rounds, with outliers dropped, keeps accuracy high, and a trailing average of the last taps smooths out one slow press. At 96 bpm there are 625 ms between beats, so an offset between your devices longer than 625 ms can't be read: 760 ms shows a modulo of 135 ms. Keep the screen in peripheral vision and test in a quiet room.
Reading the Flash Delay in Milliseconds
Run the sync test once per device. The flash delay each one settles on is its end-to-end audio output latency. Subtract the faster device from the slower one:
$$\Delta t = L_B - L_A$$
Say a laptop on wired cans reads 11 ms and a tablet driving a Bluetooth speaker 2.6 m away reads 214 ms. The offset is 203 ms, and about 7.6 ms of it is just sound crossing the room. A tap-based latency tester gives the same kind of figure.
What Audio Latency Each Device Adds
Delay builds up through the whole playback chain: the app, the operating system audio stack, the DAC or radio link, and finally the speaker driver. Every piece of hardware adds its share, which is why two devices rarely match.
Wired Headphones vs Bluetooth Latency
Approximate ranges by connection:
| Connection | Approx. delay | Comfortable for |
|---|---|---|
| Wired headphones or speakers | 2-15 ms | Live instruments, everything else |
| aptX Low Latency | about 40 ms | Gaming, video |
| aptX Adaptive / LE Audio | 50-90 ms | Video, casual gaming |
| AAC (most earbuds) | 120-220 ms | Music, most video |
| SBC (older Bluetooth) | 170-270 ms | Music only |
Two wired devices usually sit close; add one Bluetooth link and its latency dominates the offset.
Why Codec Choice Drives Bluetooth Audio Latency
Wireless audio has to be encoded, sent in packets, buffered against radio interference and decoded again. The codec sets most of that wireless delay, and both the transmitter and the receiver must support it, so a phone that only speaks SBC never gets aptX Low Latency even from capable earbuds.
How Much Latency Is Acceptable
Judge your two-device gap by the job. For background music, even 250 ms rarely matters. Video calls and movies start to feel off near 90 ms. Competitive gaming needs sound cues such as footsteps under roughly 50 ms, and music production or playing through monitors needs under 15 ms.
Syncing a Hall PA and a Bluetooth Speaker with the Latency Test
I'm setting up a Saturday spin class in a 14.2 m community hall. The laptop runs the playlist into the hall's wired PA at the front; the back rows can't hear the beat, so a phone streams the same playlist to an AAC Bluetooth speaker on the back wall. The question: how much delay does the laptop need so riders hear one beat, not a slap-back echo? The answer turns out to be 164 ms, and it is only right for the middle of the room.
I open the test on each device and stand 1.5 m from each speaker in turn, nudging the slider until flash and click line up. The PA reads 23 ms, the Bluetooth speaker 187 ms. The audio delay between the two devices is 187 - 23 = 164 ms. Because both readings include the same 1.5 m of air (about 4.4 ms each), that part cancels in the subtraction.
I enter 164 ms as an output delay on the laptop's playback, then check where riders actually sit, using 343 m/s for sound in air:
| Laptop delay | Front row gap | Middle gap | Back row gap |
|---|---|---|---|
| 0 ms | 196.7 ms | 164.0 ms | 131.3 ms |
| 120 ms | 76.7 ms | 44.0 ms | 11.3 ms |
| 164 ms | 32.7 ms | 0 ms | 32.7 ms |
| 200 ms | 3.3 ms | 36.0 ms | 68.7 ms |
The reference point I judge against is the precedence (Haas) effect: two copies of a sound arriving within roughly 30-40 ms of each other fuse into one, and the louder, nearer source sets the direction. At 164 ms, no seat is more than 32.7 ms out, so the room hears one beat. At 200 ms the back row sits at 68.7 ms, a clear echo.
The catch nobody mentions: the subtraction only aligns the point equidistant from both speakers, because each metre of room adds about 2.9 ms. A hall much longer than about 17 m can't be fixed with one delay setting at all, since the ends drift past the fusion window whatever you choose. There, the better move is turning the back speaker down so it fills rather than competes. The input I'd re-measure first is the Bluetooth reading: AAC latency can change after a reconnect, so I rerun that half of the test before every class.
Audio/Video Mismatch: Lip-Sync Errors by Frame Rate
Picture and soundtrack are two devices too: in live production, an HDMI camera outputs video slightly behind real time while the microphone feed stays live, so an audio/video mismatch appears at the switcher. A mixing board or streaming hardware can delay the audio to match, once an AV sync test tells you by how much. Lip-sync errors are usually counted in frames, so the frame rate matters.
Find the Offset on the Audio Waveform
Play an audio video sync test on a screen at full volume, film it, and load the clip into an NLE. Park the playhead on the beep in the audio waveform and count the frames to the matching visual. Converting our 203 ms example:
$$\text{frames} = \frac{\Delta t \times \text{fps}}{1000}$$
- 23.98 fps: about 4.9
- 29.97 fps: about 6.1
- 59.94 fps: about 12.2
The same check calibrates a TV, soundbar or projector; raise the volume enough for the microphone to catch every beep.
Reported Output Latency in the Browser
Browsers expose two estimates through the Web Audio API: baseLatency (the internal buffer) and outputLatency (the path to the hardware). This reported output latency can't see inside wireless earbuds, so the measured audio latency usually reads higher.
How to Reduce Headphone Delay Between Two Devices
- Turn on gaming mode or low latency mode in the headphone app.
- Choose aptX Adaptive or LE Audio when both ends support it.
- Keep devices charged and close to the source.
- Re-run a Bluetooth latency test after each change; repeat sync tests and compare.
When nothing else helps, delay the faster device by the measured offset or switch it to wired; then run the latency test again to confirm.
Who Uses an Audio Latency Test
Gamers check their gear, video editing and podcast teams line up monitoring, and anyone running a second speaker for a party or a home cinema uses an audio latency test to make two devices sound like one.
Frequently Asked Questions
- What does a two-device sync test measure?
- It measures how much later the sound from one device reaches you than the sound from another. You take a flash delay reading on each device, and the difference between the two readings is the offset you need to correct.
- What is audio latency?
- Audio latency is the time between a device starting a sound and you hearing it. It builds up through the app, the operating system, the DAC or Bluetooth link and the speaker driver, so two devices playing the same track rarely line up.
- Why is Bluetooth so much slower than a wired connection?
- Bluetooth audio is encoded, sent in packets, buffered against radio interference and decoded again. Wired outputs usually add only a few milliseconds, while common Bluetooth codecs such as AAC and SBC often add well over 100 ms.
- How large an offset between two devices is acceptable?
- Under about 15 ms suits music production and live instruments, under about 50 ms is fine for gaming and two speakers in one room, and offsets near 90 ms or more make video look out of sync and speakers echo.
- How accurate is a flash-and-click reading?
- It is a perceptual estimate, not a lab measurement, and it includes your display and browser. Because both devices carry similar overhead, the difference between them is more reliable than either reading alone. Repeat each reading two or three times.
- Why does the browser's reported output latency differ from my reading?
- Browsers report baseLatency and outputLatency through the Web Audio API, but those values cannot see inside Bluetooth headphones or speakers, so a measured reading on a wireless device usually comes out higher.
- How do I fix the offset once I know it?
- Add the offset as a delay to the faster device in your player, mixer or AV receiver, turn on a low latency or gaming mode, use aptX Adaptive or LE Audio where both ends support it, or switch the slower device to a wired connection.