Left/right stereo test.
Confirm your left and right channels play on the correct sides — and only their own side. Tap Left, Both, or Right, auto-cycle the channels hands-free, then check balance and speaker polarity. Everything runs in your browser; nothing is uploaded.
Press Left, Both, or Right — or hit Space to auto-cycle.
Which side is which.
Left plays a 440 Hz tone; right plays 660 Hz — two pitches so you can tell the sides apart with your eyes closed.
Check these before you start
- Face the speakers correctlySit between the two speakers with the left one on your left. A test is meaningless if the speakers themselves are placed on the wrong sides.
- Centre the OS balance sliderWindows and macOS both have a left/right balance control that can drift after an update — set it dead centre before judging a channel as weak.
- Wear headphones the right wayMost headphones mark L and R inside the cups or on the stems. Swapped-feeling earbuds are usually just worn on the wrong ears.
- Start near half volumeYou only need enough level to clearly place each channel. Raise it after the first tone if you need to.
How to run the left/right test
- 1Play one channel at a time. Press Left (or the L key) and confirm the sound comes only from your left. Repeat with Right (R). The two use different pitches so you never have to guess which side is playing.
- 2Cross-check with Both. Both plays both pitches together — you should hear the low tone on the left and the high tone on the right at once. If they land on the wrong sides, your channels are swapped.
- 3Auto-cycle hands-free. Hit Space to loop Left → Both → Right with an on-screen caption, so you can walk to the speakers and listen without touching the keyboard.
- 4Check balance and phase. Sweep the balance slider to hear the image move smoothly across the middle, then A/B the in-phase and out-of-phase tones. Deeper problems? Try the headphone test or the full sound test.
How the stereo test works
Each channel button routes a synthesized tone through a stereo panner set hard to one side — pan −1 for left, +1 for right — so exactly one speaker or earpiece should make sound. Using 440 Hz on the left and 660 Hz on the right (a perfect fifth apart) means you can identify a channel by pitch alone, which matters when you're standing behind the speakers or the room is echoey. Everything is generated live with the Web Audio API; no files are downloaded and nothing is recorded.
The phase test plays the same tone through both channels, but the "out of phase" version inverts the polarity of one channel (its waveform is flipped, gain ×−1). When two correctly-wired speakers play inverted signals, the pressure waves partly cancel in the air between them, so the sound goes thin and the centre image collapses. If in-phase and out-of-phase sound the same, your output is being summed to mono somewhere — a Bluetooth codec, an accessibility "mono audio" setting, or a mixer — and true stereo isn't reaching your ears.
When a channel test helps
New speaker setup
After wiring bookshelf speakers to an amp, confirm the left cable really feeds the left speaker before you trust the soundstage.
Gaming and positional audio
Footstep direction in shooters depends on correct channels. Swapped sides make every audio cue point the wrong way.
Films feel "off"
Dialogue or pans landing on the wrong side usually means crossed speaker wires or a reversed cable, not a bad mix.
Second-hand earbuds
Cheap or well-worn wireless buds sometimes reconnect with the channels swapped. A ten-second check catches it.
Mixing and mastering
Before you pan anything, prove your monitors are correct and in phase — a polarity flip ruins every stereo decision after it.
TV or soundbar install
Optical, HDMI, and cheap RCA runs are easy to cross. Verify sides before mounting anything to the wall.
What the result tells you
Match what you heard to the likely cause and the fix:
A channel is completely silent
A half-seated plug, a broken conductor in the cable, a dead driver, or the OS balance pushed fully to the other side.
→ Reseat the plug, swap in another cable, centre the OS balance, then retest — if the same side stays dead across cables and sources, the driver has failed.
Left and right are swapped
Speaker wires crossed at the amp, a reversed cable, or headphones/earbuds worn on the wrong ears.
→ Swap the wires at the amp or flip the plugs. For buds, drop them in the case and reconnect; check the L/R markings on cups.
One side is noticeably quieter
A drifted balance slider, a partially inserted jack, or an aging driver losing output on one side.
→ Centre the balance control first, fully seat the jack, and compare on another device to isolate hardware from software.
Out of phase sounds the same as in phase
Your audio is being summed to mono — a hands-free Bluetooth profile, a "mono audio" accessibility toggle, or a mono soundbar input.
→ Disable mono audio in accessibility settings, reconnect Bluetooth in stereo (A2DP), and confirm the source is a true stereo output.
The centre image drifts to one side
Unequal channel levels, asymmetric speaker placement, or one speaker further from your ears.
→ Match speaker distances, centre the balance, and re-run the balance sweep — the tone should pass cleanly through dead centre.
Both tones come from one speaker
A mono cable (TRS used where TRRS is needed) or a shorted plug bridging the channels.
→ Use the correct cable for the jack and inspect the connector — a bent or dirty contact can bridge left and right.
Stereo test glossary
- Channel
- One independent audio stream. Stereo carries two — left and right — each meant for its own speaker.
- Panning
- Placing a sound between the speakers by changing its relative volume in the left and right channels.
- Balance
- The overall left/right level offset for everything you play. Centre means both channels are equally loud.
- Phase / polarity
- Whether a channel's waveform is upright or flipped. Two speakers playing opposite polarity cancel in the air and sound thin.
- Mono summing
- Collapsing left and right into one identical signal. It hides stereo faults — and any real stereo effect.
- Stereo image
- The sense of where instruments sit across the space in front of you, from hard left through centre to hard right.
Frequently asked questions
Why is one side quieter than the other?
Start with software: the OS balance slider drifts off-centre surprisingly often, especially after driver updates, so set it dead centre first. Then check hardware — a jack that isn't pushed all the way in commonly drops or weakens one channel, and a tired driver can lose output on one side. Compare the same test on a second device: if the imbalance follows the headphones or speakers, it's the hardware; if it stays with the computer, it's a setting.
My earbuds swap left and right after I reconnect them. Why?
Some true-wireless earbuds negotiate which bud is the "primary" each time they connect, and occasionally they pick the wrong one, flipping the channels. The reliable fix is to put both buds back in the case, close it for a few seconds, then take them out together so they re-pair cleanly. If a specific bud is always the primary, connecting that one first before the other can keep the sides consistent.
What does "out of phase" actually mean here?
In phase, both channels push and pull the air in step. Out of phase, one channel's waveform is inverted, so while one speaker pushes air the other pulls — the two waves partly cancel in the space between them. On correctly wired speakers this makes bass and the centre image sound thin and hollow. That thin sound is the point: it proves your two channels are genuinely separate. If in-phase and out-of-phase sound identical, something is summing your audio to mono.
Does turning on "mono audio" fix a swapped channel?
No — it hides the problem instead of fixing it. Mono audio sends the same mix to both sides, so you'll stop noticing that they're swapped, but you also lose all stereo separation: no positional cues in games, no width in music. Use it only as an accessibility aid for single-sided hearing. To actually fix swapped channels, correct the wiring, cable, or how you're wearing the headphones.
How do I fix speakers that are wired backwards?
For passive speakers on an amp or receiver, the left and right cables are crossed — swap them at the back of the amp so the left terminal feeds the physical left speaker. For a cable with a reversed connector, replace it or use the correct adapter. Software "swap channels" options exist in some OSes and drivers, but fixing the physical cause is far more reliable and won't silently un-swap after an update.
Why do both tones come out of my TV or soundbar's centre?
Many soundbars and TVs downmix stereo to a single front array, or default to a "mono"/"standard" mode that collapses the channels. Check the audio menu for a stereo or "direct" mode, and make sure the source (streaming box, console) is set to output stereo or surround rather than mono. If you're testing a full 5.1/7.1 layout, use the surround sound test instead — it checks each speaker individually.
The balance sweep jumps or dips in the middle. Is that bad?
A smooth sweep should glide through dead centre with no gap or volume dip. A dropout in the middle points to a phase or wiring problem, or a comb-filtering room reflection between the speakers. First rule out the room by testing on headphones — if the sweep is smooth there, the issue is speaker placement or polarity, not the channels themselves.
Is this stereo test accurate on Bluetooth headphones?
Yes, with one caveat: Bluetooth must be connected in a stereo profile (A2DP), not the hands-free/headset profile that apps like Teams or Discord force for the mic. In headset mode audio collapses toward mono and quality drops, which can mask channel problems. Close any app holding the microphone, reconnect the headphones, and retest. The small Bluetooth delay is normal and doesn't affect which side plays.