Stereo Test

What is this tool?

The Stereo Test checks that two channels exist, that they are the right way round, and that they combine correctly. Those are three separate questions, and this tool separates them with dedicated controls rather than leaving you to infer anything from music.

The engine isolates LEFT, RIGHT, BOTH, CENTER and MONO. Centre and mono are the interesting ones: a centre signal should appear to come from a point between the speakers even though no speaker sits there, and a summed mono signal reveals phase problems that stereo content hides.

Balance can be set to 100% LEFT, 50% LEFT, CENTER, 50% RIGHT and 100% RIGHT, and the panning sweeps move the sound continuously between the two. Ten frequency presets run from 100 Hz to 20 kHz.

How to use it

1. Start with left and right in isolationConfirm each channel produces sound and that the side matches the label. Do this before anything else — every later test assumes both channels work and are correctly assigned.
2. Play the centre channelPLAY CENTER CHANNEL sends an equal signal to both sides. It should sound like it is coming from a single point midway between the speakers, or from the middle of your head on headphones. That illusion is the phantom centre, and it is the best single indicator that a stereo setup is intact.
3. Play mono (summed) and comparePLAY MONO (SUMMED) combines both channels. If the sound becomes noticeably thinner or the bass drops away compared with the centre signal, the channels are partly out of phase — the two waveforms are cancelling each other.
4. Sweep left to right and backLEFT → RIGHT SWEEP and RIGHT → LEFT SWEEP move the sound smoothly across. Listen for a continuous journey. A sound that jumps from one side to the other, or that vanishes in the middle, indicates a problem with the centre image.
5. Run the alternating testALTERNATING TEST (L ↔ R) switches sides repeatedly, which makes a level difference between channels far easier to hear than playing each side once. Small imbalances become obvious when the comparison is immediate.
6. Check separation at several frequenciesStep through the presets from 100 Hz to 20 kHz while playing one channel only. Low frequencies are much harder to localise than high ones, so bass appearing to come from both sides is normal physics rather than crosstalk.

What the results mean

Both channels work and directions match

The baseline result. Left sounds left, right sounds right, both together produce a solid image. Everything else on the page builds on this being true.

The centre image is centred

The phantom centre appears midway between the speakers. This requires both channels at matched level and in phase, so a correct centre confirms both at once — it is the most informative single check on the page.

A pulled or vague centre

The centre appears off to one side, or refuses to settle in one place. A pulled centre means a level difference between channels; a vague one usually means the two signals are not fully in phase, or you are sitting off-axis.

Mono sounds thinner than centre

The classic phase or polarity problem. When one channel is inverted, summing the two cancels much of the signal — bass suffers most. On a speaker system this often means one speaker's wires are connected the wrong way round.

Stereo separation is clear

A signal sent to one channel is clearly heard from that side only. Perfect separation is not expected on speakers, because both ears hear both speakers — that acoustic crosstalk is normal and is why headphones always separate better.

Left → Right movement works

The sweep travels smoothly. A sweep that steps or drops out mid-journey points at a channel level problem, or at processing between the browser and the speakers.

What it can detect

• A dead channel, or channels wired the wrong way round
• A level imbalance between channels, most easily heard in the alternating test
• Phase and polarity problems, revealed by comparing centre against summed mono
• A phantom centre that will not sit in the middle
• Separation that changes with frequency

Limits worth knowing

A browser can only report what the platform gives it. These are the honest boundaries of this page, so a result is never read as more than it is.

• The page has a “Browser Stereo Test Limitations” section and its FAQ asks “Can a browser measure stereo separation?”, “Can this test measure speaker crosstalk?” and “Can this test measure SPL?” No to all three, and this documentation does not claim otherwise.
• There is no microphone involved, so nothing acoustic is measured. Separation and balance are reported as what you heard, from signals whose digital values are known exactly.
• Crosstalk measurement requires a calibrated microphone and a controlled space. What you can do here is compare, which is often enough to find the fault.
• Room acoustics and your seating position affect every stereo judgement. A reflection from a nearby wall can shift the apparent centre without anything being wrong with the equipment.

Tips

• Use headphones to establish the truth about the signal, then speakers to judge the room. If the centre is solid on headphones but wanders on speakers, the room and placement are the cause.
• For a phase check, compare centre and summed mono back to back at 100 Hz or 250 Hz. Low frequencies show cancellation most clearly.
• Close your eyes during the sweeps. Visual cues about where the speakers are will bias what you think you hear.

Troubleshooting

Related documentation

• AudioLeft Right Audio TestThe quickest way to prove which side is which, with a blind identification game.
• AudioSpeaker Balance TestA nine-position balance stepper from full left to full right, with separation checks.
• AudioSpeaker TestLeft, right, stereo, bass, treble and sweep tones for laptop or desktop speakers.
• AudioHeadphone TestEars, balance, separation, bass and treble checks built for headphones and earbuds.
• AudioAudio Frequency TestA 20 Hz to 20 kHz tone generator with sweeps, steppers and A/B comparison.