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Stereo Channel Tester

Check that your left and right channels are correct — not swapped, mono, or dead. Play a tone on each side (a low tone on the left, a high tone on the right), take a quick guided quiz that catches swapped or mono output, or run an in-phase vs out-of-phase check. No microphone needed.

🎧 Works with headphones or speakers. With headphones the left/right split is obvious; with speakers, sit centered so you can tell the sides apart.

Manual test

Volume 40 %

Phase / mono check

Both play the same 440 Hz tone on both channels. Out-of-phase inverts the right channel: on headphones it sounds wide/hollow and "in your head everywhere"; if you mono-sum it (or on a mono speaker) it nearly disappears. If in-phase and out-of-phase sound identical, you’re probably hearing mono.

Guided channel check

Press Run, listen, and tell the tool which side you heard. It detects swapped, mono, or dead channels.

Run the check to verify your channels

How to Use the Channel Tester

Stereo only works if the left channel reaches your left ear/speaker and the right reaches the right. Surprisingly often they’re swapped (reversed cables or a flipped connector), collapsed to mono (a bad adapter, a mono output, or an accessibility setting), or one side is dead. This tool makes those problems obvious.

The manual test plays a distinct tone per side — a lower 440 Hz on the left, a higher 880 Hz on the right — so you can immediately tell which channel is which. The guided check plays each side in turn and asks where you heard it, then compares your answers to diagnose swapped/mono/dead channels automatically. The phase check reveals whether your channels are wired with matching polarity and whether you’re really getting stereo.

What the results mean

Correct — left tone on the left, right tone on the right. All good.
Swapped — you heard each tone on the opposite side. Reverse your speaker cables, or fix the channel mapping in your software/OS.
Mono / both sides same — both tones came from the same place (or dead-center). Your output is summed to mono, an adapter is wrong, or one channel feeds both.
Dead channel — you heard nothing on one side. That speaker/earcup is disconnected, muted, or broken.

About the phase check

In-phase plays the same tone on both channels normally; out-of-phase inverts one channel. With correctly-wired stereo, out-of-phase sounds noticeably wider/hollower and largely cancels when summed to mono. If in-phase and out-of-phase sound the same, you’re almost certainly hearing a mono signal. Out-of-phase that sounds normal while in-phase sounds thin can indicate a reversed polarity on one channel.

Frequently Asked Questions

Do I need a microphone?

No. This is an output test — it only plays sound and asks what you hear. There’s no microphone access at all, and nothing is recorded.

Why is the left tone lower than the right?

Using two different pitches (440 Hz left, 880 Hz right) makes the channels instantly distinguishable by ear, so you can tell at a glance — or rather, by ear — which side is playing and catch a swap even without watching the screen.

My channels are swapped — how do I fix it?

For speakers, swap the two cables at the amp/interface (or physically swap the speakers). For headphones, check you’re wearing them the right way round. In software, look for a left/right swap or channel-mapping option in your OS sound settings or your app’s audio routing.

It says "mono" but I have stereo gear — why?

Common causes: a mono (TRS vs TRRS) adapter or splitter, a "mono audio" accessibility setting enabled in your OS, a Bluetooth device in a mono/hands-free profile, or an app summing to mono. Disable mono audio in accessibility settings and check your adapters and Bluetooth profile.

What does out-of-phase tell me?

It inverts one channel’s polarity. On real stereo it sounds wide and hollow and cancels when summed to mono. If it sounds identical to in-phase, you’re hearing mono. If out-of-phase sounds full while in-phase sounds thin, one of your channels likely has reversed polarity (a miswired cable).

Why can’t I pick which output device plays?

Browsers route Web Audio to your system default output, so set the device you want to test as the default first. The tool then plays the L/R tones to whatever your default output is.

The "Both" tone sounds centered — is that right?

Yes. Playing the same tone equally on both channels creates a "phantom center" that appears to come from straight ahead (or the middle of your head on headphones). If "Both" sounds off to one side, that side is louder or the other is weak.

Is anything recorded or sent anywhere?

No. The tool only generates tones in your browser and reads which answer button you press. No audio is captured, recorded, or transmitted.

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