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. These problems are especially common after re-cabling a home studio, replacing a headphone cable, or changing audio output devices. This speaker test and headphone test tool makes those problems obvious without needing a separate measurement microphone or oscilloscope.
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. If you want to go further and verify that your audio balance is equal as well as correct, the left/right balance test lets you match levels by ear with precision.
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’s electrical polarity (sometimes called absolute polarity or channel polarity). 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 — the same wiring fault the phase test tool identifies with greater detail. A common cause is a speaker cable that has its positive and negative conductors reversed at one end.
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.
Does this work with Bluetooth headphones or a wireless speaker?
Yes, but with one caveat: many Bluetooth devices switch to a mono hands-free or "headset" profile when any app requests microphone access. Because this tool never asks for a mic, your device should stay in stereo A2DP mode and the left/right test should work normally. If the result shows mono, check that no other app has grabbed the mic and forced the Bluetooth device into a mono profile.
One side is quieter but not completely silent — is that a dead channel?
Not necessarily. A dead channel produces complete silence on one side; what you're describing is more likely a weak driver, a dirty or oxidised TRS jack contact, or an imbalanced channel level in your OS or app. Try the
left/right balance test to quantify the level difference. If the imbalance is large and consistent across devices, the hardware likely needs cleaning or repair.
Can I use this to verify a headphone amplifier or DAC?
Yes, within limits. The guided channel check and phase test will confirm that your DAC or headphone amp is passing a distinct stereo signal — left to left, right to right, in phase. What it cannot do is measure gain, distortion, noise floor, or frequency response. For those metrics you need a loopback test with an audio analyser. Use this tool as a quick sanity check that the device is routing stereo correctly before a more thorough measurement session.
Why does out-of-phase sound different on headphones versus speakers?
On headphones, out-of-phase creates a distinctive "inside your skull everywhere" sensation because each ear receives an inverted copy of what the other hears, making it impossible for your brain to fuse the two into a single point. On speakers, the two channels fire into the same room so the anti-phase cancellation partially occurs in the air — the sound becomes thin and diffuse rather than spatially confused. On a mono speaker (or if your setup sums to mono), out-of-phase nearly disappears entirely.