Free tool
Stereo channel test
A left and right speaker test that runs straight in your browser. Three buttons play a short tone hard-panned to the left channel, the right channel, or both, so you can confirm each side works and sits on the correct side. A fourth check plays both channels in phase and then inverted, which catches swapped speaker wires.
Stereo channel test
Left, right, both, and phase
Volume firstTurn your system volume down before the first tone. The level here is fixed and capped at about -14 dBFS, but your amp, speakers, and headphones apply their own gain on top. Start low, then raise it.
Channels · 440 Hz sine · 1.5 s
Each side plays at the same digital level, and the unused channel carries digital silence, not a quiet copy. Click a playing button to stop early.
Phase · 200 Hz sine · 4 s · both channels
Play In phase, then Inverted, from your normal listening spot. From correctly wired speakers the inverted version sounds thinner and hollower, with noticeably less bass, because the two speakers cancel where their sound meets in the room. If the inverted version sounds FULLER than the in-phase one, your speakers were already out of phase: the plus and minus wires are swapped on one speaker. Swap them back on that one speaker only.
On headphonesLeft and Right are the whole test: each ear hears only its own channel, so a swap is instantly obvious. The phase test mostly matters for speakers. On headphones the two channels never meet in the air, so nothing cancels; an inverted channel just sounds slightly odd and hard to place inside your head.
Nothing is recorded, this only plays sound. The tones are generated in your browser and never touch your microphone or leave your device.
What each result means
The tone comes out the wrong side. Something between the browser and your ears is crossed. Check the physical layer first: headphones worn backwards, a swapped pair of RCA plugs, or speaker cables that trade places behind the desk. Some audio driver control panels can also swap channels in software. The tone leaves this page digitally hard-panned, so the swap lives somewhere downstream of the browser. For a map of everything the signal passes through on the way out, read what audio routing is.
One side is silent. Start with the cheap suspects: the balance slider in your OS sound settings, a 3.5 mm plug that is not pushed all the way in, or a worn cable. A half-seated plug is a classic, it often kills exactly one channel. Reseat the plug, re-center the balance, and if the side is still dead, try the same headphones or speakers on a phone to decide whether the fault is the device or the computer.
Every button plays on both sides. Something downstream is summing the channels to mono. The usual culprits: the mono audio accessibility setting in Windows or macOS, a Bluetooth headset that dropped into its hands-free call mode (which is mono), or a genuinely single-speaker device. This test writes digital silence to the unused channel, so it cannot be the source of the leak. One honest caveat for speakers: room reflections can put a faint image on the wrong side, so lean in close to each speaker to confirm true silence.
The inverted tone sounds fuller. Your speakers were already out of phase before you loaded this page: the plus and minus wires are swapped on one of them, so the cones push while the other pulls and the bass cancels in the room. The fix is mechanical, not digital. Swap the two wires on one speaker only, at either end of that speaker's cable, and run the phase check again. In phase should now be the fuller one.
What the numbers mean
440 Hz for the channel buttons. Ears localize midrange tones easily, so a 440 Hz sine makes it obvious which side is playing. Deep bass is the opposite, it is hard to place by ear, which is the same reason subwoofer placement is so forgiving. A bass tone would work the speakers but tell you little about left versus right.
200 Hz for the phase check. Phase cancellation is most audible in the bass. At 200 Hz the wavelength is about 1.7 meters, long compared to the gap between most speaker pairs, so the two arrivals genuinely add together at your listening spot instead of combing. In phase they reinforce; inverted they cancel, and you hear the bass fall out.
Hard-panned means the tone is written to exactly one channel of the stereo stream while the other channel carries digital silence, not a quieter copy. The level is fixed at roughly -14 dBFS, well below full scale, and every tone ramps in and out over 10 ms so nothing clicks. Your system volume still applies on top.
One quirk worth knowing. If your output device reports itself as mono, the browser sums the two channels before the audio ever reaches the driver. In that case the inverted tone can cancel to near silence inside the computer, which looks spooky but is just arithmetic. To chase a specific frequency, hunt a rattle, or sweep your hearing range, use the free tone generator, and to check the other direction of your audio chain, run the free mic test next.
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