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Phase Test Tool

Explore the phase relationship between your left and right channels. Sweep the phase offset and watch the vectorscope and correlation meter respond, flip channel polarity, and switch to mono to hear how out-of-phase audio cancels. A deeper companion to the quick Stereo Channel Tester. No microphone needed.

🎧 Use headphones or speakers. The vectorscope and correlation update even before you press Play, so you can see the phase relationship; press Play to also hear it.

Signal

Polarity & routing

Idle — adjust phase to preview, press Play to hear it.

Vectorscope (L × R)

Diagonal "/" = in phase · circle = 90° · "\" = out of phase

Phase correlation

-10+1
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Understanding Phase & Polarity

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 phase does.

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.

Frequently Asked Questions

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.

Related Tools

Stereo Channel Tester
Quick left/right, swap and mono check.
Tone Generator
Generate tones with panning and waveforms.
Audio Loopback Tester
Confirm output→input routing.
Microphone Test
Check that your mic/input works.
Left/Right Balance Test
Fine-tune your stereo balance.
Goniometer / Vectorscope
Full stereo-field vectorscope for any signal.
← All Microphone Diagnostics Tools