Polarity Check

Play the same low tone in phase and then inverted. Out of phase sounds hollow, with no centre.

Channel swap detection

Stereo imaging

What out-of-phase actually sounds like

The polarity check plays the same tone twice: once normally, once with one channel's waveform flipped upside down. In phase, the two copies reinforce each other and the sound gathers into a solid point. Out of phase, they partially cancel instead, and the result is not simply quieter — it is hollow and diffuse, as if the sound has no single place it is coming from. The centre image is the first thing to go: rather than sitting between the two sides, it seems to smear across the whole space, or vanish from the middle while the sides stay present. Bass thins out noticeably, because cancellation is most complete at low frequencies where the wavelength is long compared to the distance between the two sources. On headphones the effect is stranger still: instead of feeling like it is happening in front of you, out-of-phase sound tends to feel like it is happening inside your head, with no external location at all. That inside-the-head quality, more than any loss of loudness, is the giveaway. For why that happens, how it differs from a genuine phase shift, and why a single speaker alone can never demonstrate it, see this site's guide to out of phase and reversed polarity.

Why polarity is one of the few certain fixes in audio

On a wired speaker, reversed polarity almost always means one thing: the two conductors of a cable are connected the right way round at one end and swapped at the other. A speaker cable carries no "left" or "right" marking that matters electrically, only positive and negative, and it is easy to land red-to-black on one end and red-to-red on the other without noticing, especially on bare-wire terminals. This makes it one of the few faults in audio with a genuinely certain repair: find the reversed pair and swap it back, and the fault is gone completely, not just reduced. Contrast that with a resonant cabinet buzz or a driver nearing the end of its life, where the fix is partial at best. Headphones and most consumer electronics wire polarity correctly at the factory and give no way to reverse it yourself, so a problem found here on headphones more likely sits in the source device or a third-party cable, not in the headphones themselves.

The phantom centre, and why it collapses

Play an identical signal from two speakers positioned apart from each other, in phase, at equal level, and the ear does not report two sources — it fuses them into one, imaginary source sitting in the middle, where no physical speaker exists. This is the phantom centre, and it is the basis for how stereo recordings put a lead vocal or a kick drum in the middle of the stage without a centre speaker at all. The fusion depends on both signals arriving in step. Reverse the polarity of one side and that timing relationship inverts: instead of reinforcing in the middle, the two signals fight there, and the centre image collapses into the diffuse, locationless smear described above. A phantom centre that refuses to form, or that only forms when you swap which side is inverted, is a direct, audible sign of a wiring fault — no measurement equipment required.

A swapped channel is easy to miss for years

Unlike a dead speaker or reversed polarity, a straight left/right swap produces sound that is completely normal on its own. Both channels work, both are in phase, levels match, and nothing sounds broken in isolation — the only thing wrong is which side each channel comes out of. Most music is close enough to symmetrical, and most listening positions forgiving enough, that a swap can sit unnoticed for years of ordinary use. What it costs is precision: a mix engineer chose to put that guitar on the right and that harmony vocal on the left for a reason, and a swap quietly undoes every one of those decisions. It also breaks anything relying on a real stereo relationship — game audio placing a sound behind and to your left, or a film mix panning left to right — without any obvious fault to chase. The channel-swap test above exists because a swap this quiet is exactly what nobody goes looking for.

When "both sides identical" is the device, not the wiring

Some Bluetooth speakers, soundbars, and USB audio devices report themselves to a computer or phone as full stereo, while internally summing both channels to mono before they reach a driver. Run any tool on this page against one of those and every round reports "both sides the same," including a phantom centre that never moves or separates — not because anything is wired backwards, but because there was only ever one signal path after the device's own mixing stage. That is a real limitation of the hardware, not a fault this page can detect or fix, and it is worth ruling out before blaming a cable: try a different device, or a wired pair of headphones with a separate driver in each ear, and see whether the picture changes.

What these checks cannot tell you

Everything above tests the signal from this browser tab, through your operating system, out through whatever cable or wireless link carries it, to the drivers that turn it into sound. That is a real and useful path to check — it is also the whole of what a web page can reach. It cannot distinguish a fault sitting inside your headphones from one sitting in an amplifier, a DAC, or a mixer ahead of them: a reversed pair of wires inside a headphone cable and a reversed output stage in an amplifier produce the identical result here. Nor can any of this measure absolute level, distortion, or frequency response — there is no calibrated reference on the far end of a consumer headphone jack for a browser to compare against. What you get is a clear, repeatable answer to "is the phase right, is the wiring the right way round, does the centre hold together" — genuinely useful questions, and the honest limit of what they answer.

Not sure where the problem is? Run the full sound check: six quick questions, and one summary that points you at the right tool.