Stereo Test
Check that your speakers or headphones place sound where they should. Play a knock hard left, hard right, or in the phantom center, isolate the three cues your brain locates it by, then quiz yourself on ten knocks from five positions.
Left should sound from the left, Right from the right, and Center should float exactly between the speakers. If the sides are reversed, your channels are swapped.
Use headphones, or sit exactly between your speakers — off-center listening skews every result.
Timing and head-shadow cues survive intact on headphones. On speakers, a foot off-axis drags the whole image sideways.
Center is an illusion: equal level from both speakers, and your brain invents a source between them. Off to one side? Check balance and your seat.
The quiz combines realistic level, timing, and head-shadow cues. 7+ out of 10 means your stereo image is doing its job.
Stereo works by trickery. There are only two speakers, yet a good recording puts a voice dead center, a shaker half left, and a door slam far right — positions where no speaker exists. Your brain builds those positions out of tiny differences between what each ear receives, and a stereo chain images correctly only when it preserves them: channels on the correct sides, both at equal level, nothing collapsing the mix to mono along the way. A swapped cable, a balance control nudged off center, or a Bluetooth codec falling back to mono breaks the illusion while each speaker still passes a one-channel test — which is why this page tests imaging, not just whether each side makes sound.
How does the test work?
The sound is a short knock rather than a steady tone, because your brain localizes sharp transients far better. Channels mode plays it hard left, hard right, or equally on both — the last should float as a phantom center exactly between the speakers. Cues mode isolates the three tricks one at a time: a level difference between the channels (panning, how nearly every mix places sound), a timing difference (exaggerated to 10 ms here so the pull is obvious; real ears work with under a millisecond), and timing plus a head shadow (the far channel also muffled above 800 Hz, the way your head blocks treble from the far ear). The quiz then combines realistic versions of all three and plays ten knocks from five positions, far left to far right, for you to place. Everything is synthesized live with the Web Audio API — nothing is downloaded or uploaded.
What if sounds end up in the wrong place?
Score low on the quiz, or hear the center image pull to one side, and the fault is almost always the setup rather than your ears. Check for swapped channels first: if the Left button sounds from the right, reverse the speaker cables or flip the earbuds. Then check the balance sliders in your OS and app, mono-audio accessibility switches, and any "audio enhancements" — each of them can narrow or shift the image. On speakers, sit centered: the phantom center collapses toward whichever speaker is closer. If one side is quiet or silent altogether, the fault is the driver or the wiring, not the imaging.
The three cues behind stereo localization
| Cue | What it is | How stereo uses it |
|---|---|---|
| Level difference (ILD) | A sound on your left is simply louder in the left ear | Panning — how mixes place sound; robust on any speakers or headphones |
| Timing difference (ITD) | The same sound reaches the near ear up to ~0.6 ms before the far one | Rarely mixed in deliberately, but your brain still reads it — the leading channel pulls the image toward it |
| Head shadow | Your head blocks treble, so the far ear hears a muffled copy | A low-pass filter on the far channel; adds realism on headphones |
| Pinna filtering | The outer ear colors sound by direction — how you tell front from back and up from down | Unused in normal stereo: it is unique to each person, so recordings cannot bake it in |