Stereo Test
Test your speakers' stereo image: check the phantom centre, step a sound across nine pan positions, take a localisation quiz, set up the stereo triangle and hear what your system loses in mono.
This page is for speakers, not headphones. Sit in your normal listening spot, start quiet and raise the volume until the sound is clear but comfortable.
1. Phantom centre test
Good centre
Centre is the same signal from both speakers at the same level, not a third speaker. It should sound like one small source floating exactly between them. Close one eye and it still stays put. Moving your head 10 cm sideways shifts it a little, not all the way to one side.
Bad centre
It sits off to one side, sounds wide and smeared, splits into two sources, or seems to come from each speaker separately. Out of phase should sound diffuse, hard to place and thin in the bass. If Centre and Out of phase sound alike, one speaker is probably wired with + and - swapped.
Check Left and Right first: if they come from the wrong speakers, swap the cables before anything else.
2. Imaging sweep: 9 pan positions
Each burst is panned with a constant-power law, so it should keep the same loudness as it moves. Listen for nine evenly spaced steps along a straight line. Steps that bunch up near the speakers with a gap in the middle mean a hole in the middle (speakers too far apart). Steps that pile up in the centre mean the stage is narrow. Tap any position to repeat it.
3. Localisation quiz
Ten short bursts play from random spots among seven pan positions between your speakers. Click where each one seemed to come from. Keyboard: arrows to aim, Enter to answer, R to replay.
Target angles assume the standard 60° stereo triangle and use the classic tangent law, which is an approximation. With speakers set wider or narrower, everything scales with them, so judge the pattern of your errors more than the exact number.
4. Placement helper and balance check
Spacing is centre to centre (tweeter to tweeter). Listener distance is from your head straight forward to the line between the speakers.
Close to the ideal 60° stereo triangle. This is the setup most studio and hi-fi advice aims for.
Toe-in to aim straight at you: about 30° per speaker.
Solid: your setup. Dashed: the 60° equilateral triangle for the same spacing.
- Start with the tweeters aimed at your ears, then reduce toe-in until the centre is still solid but the stage feels wider.
- Keep both speakers the same distance from side walls and the back wall.
- Put tweeters at ear height and keep the space between the speakers clear.
Both channels of this signal are equal. If the sound already sits in the middle, you do not need an offset.
5. Width and mono compatibility
The loop has a voice in the centre, ticks hard left, plucks hard right and a low pad widened with a polarity trick. In stereo the pad should feel wide and vague. Switch to mono while it plays: the voice stays, the ticks and plucks move to the middle, and the pad almost vanishes. Side only removes the voice, leaving only what differs between the speakers. This is why one-box Bluetooth speakers and phones can lose parts of a mix, and why audio "widening" modes can sound great in the sweet spot and hollow everywhere else.
The stereo triangle
Stereo works by fooling your ears. When both speakers play the same sound at the same level and time, you hear a single source between them, the phantom centre. That illusion is strongest when you and the two speakers form an equilateral triangle: the speakers about 60° apart as seen from your seat and the same distance from each ear. Narrower than about 45° and the stage shrinks. Wider than about 75° and you get a hole in the middle, with vocals pulling to whichever speaker is nearer.
Why the phantom centre collapses
Your brain places the centre using tiny differences in level and arrival time. Sitting 30 cm closer to one speaker makes its sound arrive about 1 ms earlier, which is enough to drag the image to that speaker. A 1 to 2 dB level mismatch does the same, whether it comes from a balance control, a dusty pot, mismatched speakers or a TV's "sound mode". Strong early reflections from a nearby wall, desk or window add a second, smeared copy on one side. The out-of-phase test shows the opposite extreme: when one speaker is inverted, there is no single location to hear, so the sound spreads out and the bass thins.
Toe-in and nearby walls
Toe-in aims the speakers at you. More toe-in gives a firmer centre and less wall reflection; less gives a wider, more spacious stage. Most speakers land somewhere between aimed at your ears and aimed just behind your head. A speaker close to a side or back wall gets louder bass and stronger reflections, so put both the same distance from their walls. If one has to sit in a corner, expect the image to lean that way.
Rooms beat specs
Be careful with small differences. Moving your head a few centimetres, a sofa cushion or an open door can shift the image more than swapping cables or amplifiers ever will. If a box promises a "3D soundstage" from two drivers a few centimetres apart, listen with Centre and the sweep. The quiz only compares your clicks with pan angles. Repeat a test before changing anything, and change one thing at a time. For room problems, the room acoustics test is the next step.
Related tools
- Headphone Test for left/right, balance and polarity checks on headphones.
- Surround Sound Test to check every channel of a 5.1 or 7.1 system.
- Room Acoustics Test to find echoes and room modes that blur the image.
- Subwoofer Test to set crossover, phase and placement for the low end.
Frequently asked questions
What is the phantom centre?
Centre does not add a third speaker. It plays one signal on the left and the right at the same time and the same level, with equal constant-power gains. On a matched pair of speakers, it can sound like a single source in the middle. Left plays the left output only. Right plays the right output only. Out of phase plays both outputs and inverts only the right channel.
What should Left and Right sound like?
Left is a hard left pan: full level on the left and silence on the right. Right is the opposite pan. If those two come from the wrong speakers, swap the cables before you judge the centre. The sweep uses the same pan law across nine steps, from hard left to hard right. The middle step is the same equal left and right signal as Centre.
Can I use headphones instead of speakers?
This page is for speakers. Headphones send left and right to separate ears, so an equal left and right signal sits inside your head rather than between two speakers. Left, Right, and the inverted right channel still show a swapped cable or a polarity flip. If the browser reports one output channel, both sides play the same thing and these stereo checks cannot work.
How do I stop the sound?
Press Stop, or press the control that is already playing. That includes Centre, Left, Right, Out of phase, the sweep, a single pan step, Play centre signal, and the width demo. Quit ends the quiz and stops its bursts. Changing pink noise to the voice-like signal, or the other way, also stops playback, except during the quiz and the width demo. The volume slider at 0 is silent. Leaving the page stops the sound.
What does the balance slider do?
Play centre signal starts with the same signal on both channels. The slider runs from -6 dB to +6 dB in steps of 0.5 dB. Moving it right turns the left channel down. Moving it left turns the right channel down. The decibel number is the offset you set. It is not a level reading from the room. If the image sits in the middle only after you add an offset, that offset is how much you had to turn the stronger side down.
What happens in mono and side?
The demo is a short loop, not your own files: a voice in the centre, ticks hard left, plucks hard right, and a low pad that is positive on the left and inverted on the right. Stereo plays that loop unchanged. Mono sum plays L+R on both channels, so the voice stays, the ticks and plucks move to the middle, and the pad mostly cancels. Side only plays the same L minus R signal on both channels, so the centre voice drops out.
What does the quiz score mean?
You get ten rounds. Each round plays a short burst from one of seven pan positions, and you click the arc where it seemed to come from. The degree figure is the gap between your click and the angle that pan would have on a 60 degree speaker triangle, using the tangent law as a rough guide. It is a listening quiz, not an imaging measurement of your speakers, your room, or your hearing. The best average is saved in this browser only.