When the same sound reaches both channels, the phase relationship between them shapes the stereo image and decides whether the audio survives being summed to mono. This tool plays one tone to both channels and lets you slide the right channel’s phase offset from 0° to 360°, so you can hear and see exactly what stereo phase does to your signal.
The vectorscope plots the left channel against the right (an X-Y / Lissajous figure). In phase (0°) the two are identical, so the dot traces a diagonal line "/". At 90° they trace a circle. Fully out of phase (180°) they trace the opposite diagonal "\". The correlation meter turns this into one number — the cosine of the phase difference — from +1 (perfectly in phase, mono-safe) through 0 (decorrelated, very wide) to −1 (out of phase, cancels in mono).
Phase vs polarity
They’re related but distinct. Phase is a time/angle shift that depends on frequency; polarity is a full ± flip of the whole signal (equivalent to 180° at every frequency). The Invert L/R buttons flip polarity; the slider shifts phase at the chosen frequency. Inverting one channel sends correlation to −1 and makes the audio cancel in mono — the classic "out of polarity" problem from a miswired cable.
Why mono compatibility matters
Lots of playback is mono or near-mono — phone speakers, club PAs, smart speakers, Bluetooth pucks, and many TV setups. If your left and right are out of phase, summing to mono makes them cancel and the sound drops out or goes thin. Press Mono sum here with the phase at 180° (or one channel inverted) and you’ll hear it nearly disappear — that’s what a mono listener would experience.
Comb filtering and partial phase shifts
While the most obvious case is a full 180° inversion, partial phase offsets (30°–150°, 210°–330°) cause a subtler problem: comb filtering. When a delayed copy of a signal is mixed with the original — which is exactly what a phase-shifted version is — frequencies whose half-wavelength equals the delay are cancelled while adjacent frequencies reinforce. In a multitrack recording the same issue appears whenever two microphones capture the same acoustic source at different distances. The classic test is the 3:1 rule — keeping the second microphone at least three times the distance from the source as the first. When checking microphone time-alignment in a session, measuring the actual phase difference at several frequencies using the Phase & Frequency Analyzer is more revealing than a single-tone check.
Headphones vs speakers: why phase sounds different
On headphones each ear receives exactly one channel with no acoustic crosstalk, so a 180° phase flip produces an uncanny "inside-the-skull" or outside-the-head sensation rather than the cancellation you get from speakers. This is why headphone mixes are checked for mono compatibility separately from speaker mixes: the correlation meter above gives you the objective number, but the subjective perception shifts depending on the transducer. For broadband signals where you want to verify frequency-domain symmetry between channels, the FFT frequency analyser can reveal level and spectral differences that a single-tone phase test won't catch.
What does the correlation number mean?
It’s the cosine of the phase difference between the channels: +1 means perfectly in phase (and mono-safe), 0 means decorrelated (a very wide, diffuse image), and −1 means fully out of phase (the channels cancel when summed to mono). Pro audio meters use the same +1…−1 scale; keeping it positive avoids mono problems.
What’s the difference between phase and polarity?
Polarity is a complete ± flip of a signal — the same inversion at every frequency (a 180° shift everywhere). Phase is a frequency-dependent time shift. The Invert buttons change polarity; the slider changes phase at the chosen frequency. They look the same on this single-tone test, but on real music polarity inverts everything while a phase shift affects frequencies differently.
How do I read the vectorscope?
It plots left (horizontal) against right (vertical). A tight diagonal line leaning "/" means the channels are in phase; a line leaning "\" means out of phase; a circle or ellipse means they’re partly decorrelated (around 90°). The narrower and more diagonal, the more mono-compatible.
Why does the sound vanish in mono at 180°?
Because left and right become exact opposites — for every peak on one side there’s an equal trough on the other. Adding them (which is what mono does) gives zero. On stereo speakers/headphones you still hear it (each ear gets one channel), but any mono playback cancels it. That’s why out-of-phase audio is a real-world problem.
My one channel is inverted — is that bad?
Usually yes. A single inverted channel (often a miswired speaker cable with + and − reversed) puts your whole mix out of polarity: a vague, "outside-the-head" image on headphones and serious cancellation in mono. Fix the wiring or use the invert here to confirm it’s the culprit (inverting the bad channel back should restore a solid center).
Does the frequency choice matter?
For polarity (a full flip) no — it affects all frequencies equally. For a fixed time delay, a given delay equals different phase angles at different frequencies, so the slider’s degrees are defined relative to the chosen tone. Try 200 Hz vs 1 kHz to feel how the same phase angle sounds at different pitches.
Do I need a microphone?
No. This is a synthesis/output tool — it generates the tones and computes the vectorscope and correlation directly from the signal it’s producing. There’s no microphone access and nothing is recorded.
How is this different from the Stereo Channel Tester?
The
Stereo Channel Tester is a quick check for swapped/dead channels with a simple in/out-phase button. This tool is the deeper phase instrument: a continuous phase sweep, a vectorscope, a correlation meter, independent polarity inversion, and a mono-sum monitor.
What correlation value is acceptable for a stereo mix?
Broadcast and mastering standards typically require a correlation of +0.5 or higher for mono-safe content, and many loudness standards (EBU R128, ATSC A/85) treat anything below 0 as a warning. A healthy stereo mix ranges from about +0.7 at dense, centered passages to near 0 on reverb tails or very wide panned content. Sustained readings below −0.2 on a full mix are a sign of a phase or polarity problem rather than creative width.
Can this tool detect phase problems in a recording through my microphone?
No — this is a synthesis and output tool only. It generates a test tone and measures the phase it applied itself; it does not analyse incoming audio. To check the phase relationship between two microphones or two lines in a real recording, use a DAW with a phase-correlation plug-in, or measure the frequency-dependent phase offset with an input-capable analyser. This tool is the reference for understanding what different phase angles look and sound like so you recognise the problem when you encounter it.
How does a 90° phase shift affect audio differently from 180°?
At 90° the correlation is 0, meaning the channels are orthogonal — they carry no correlated energy at all. In mono they partially cancel but not completely, because the overlap integral of a sine and a cosine is zero; you lose 3 dB of power rather than complete silence. The vectorscope shows a circle. You'll hear it as a change in stereo width and timbre (the fundamental and its harmonics each experience different absolute delays on a broadband signal), but the signal survives mono summing. At 180° the cancellation is total for a single tone, and on the vectorscope the line swings to "\".