Stereo Width Meter and Phase Correlation Meter

Phase Correlation and Stereo Width: What Every Producer Needs to Know

A significant portion of music is played back in mono. Smartphones play audio from a single speaker. Club PA systems often run L+R summed to a center fill. Smart speakers like the Amazon Echo and Apple HomePod Mini are mono devices used by millions of listeners every day. If your stereo mix collapses to silence or sounds thin and hollow when summed to mono, you have a phase problem. This tool gives you the measurement data to find and fix it before your track is released.

What Phase Correlation Actually Measures

Phase correlation is the Pearson correlation coefficient between the left and right audio channels. It ranges from -1 to +1, and the interpretation is straightforward:

• +1.0: The left and right channels are identical. The signal is perfectly mono. Any stereo width you see on a metered signal at +1.0 is coming from the monitor panning alone, not from actual difference between channels.
• 0.0: The two channels are completely uncorrelated. There is no relationship between what is in the left and the right. This is common with heavily processed reverb tails or synthesized stereo content.
• -1.0: The left channel is the exact inverse of the right. When you sum these two channels to mono, they cancel each other completely. You hear nothing. This is full phase cancellation.

The standard recommendation for a commercially released mix is to keep average correlation above +0.4. Readings between 0 and +0.4 are not immediately dangerous but should be listened to in mono before export. Readings below 0, even briefly, indicate portions of the signal that will audibly thin out or disappear in mono. The Audio Engineering Society recommends regular mono compatibility checks throughout the mix process for exactly this reason.

How to Read the Goniometer

The goniometer, sometimes called a Lissajous display or vectorscope, plots the left and right channel samples on a rotated XY graph. The X axis shows the difference signal (L minus R) and the Y axis shows the sum signal (L plus R). This 45-degree rotation is what gives the goniometer its distinctive diamond orientation.

The shape of the plot tells you about the stereo content at a glance:

• Vertical line: The signal is mono. Left and right are identical, so the difference (horizontal axis) is zero.
• Wide oval or round cloud: Healthy stereo spread. There is significant content across the full stereo field.
• Horizontal line: The signal is fully out of phase. Left is the inverse of right. This will null completely in mono.
• Narrow diagonal shape: A mix that leans toward one side or has a lot of correlated low-end with some stereo content in the mids and highs.

The goniometer in this tool uses a fade trail effect during playback. Each animation frame draws a semi-transparent black overlay before drawing the new sample points. This creates the persistence effect you see on hardware goniometers, where recent samples are bright and older samples fade. It makes transient behavior much easier to spot.

Stereo Width Percentage and the Mid/Side Breakdown

The stereo width percentage displayed here is derived from the correlation reading using the formula: width = (1 - correlation) / 2 * 100%. At a correlation of +1.0, width is 0% (pure mono). At a correlation of -1.0, width is 100% (fully anti-phase). A healthy commercial pop or rock master tends to sit around 30 to 60% on this scale.

The Mid/Side (MS) breakdown separates your stereo signal into two components. The Mid signal is L+R, which is what you hear on a mono speaker. The Side signal is L-R, which is the stereo difference content. These are shown as separate level meters in the tool. A high Side level relative to Mid means your mix is heavily stereo, which can sound impressive on headphones and wide stereo systems but risks sounding empty or phase-cancelled in mono. Use our LUFS Loudness Meter to cross-reference the integrated loudness of your mono sum against the original stereo version.

Why Clubs, Smartphones, and Smart Speakers All Matter

A 2023 survey of streaming listening environments found that over 40% of mobile listeners use their phone speaker, not headphones, for casual listening. Phone speakers are a single transducer producing a mono signal. In a club, the DJ booth monitors are typically mono, and the engineer checks the mix in mono before deciding whether to play it. Smart speakers like the Amazon Echo Dot have a correlation-summing mono output by design.

This is not a niche concern reserved for club music. Lo-fi hip-hop, acoustic singer-songwriter releases, and podcast beds are all commonly heard in mono. A mix that sounds lush on headphones but collapses on a phone speaker will lose listeners before the chorus hits. The Sum to Mono button in this tool lets you audition that scenario in real time.

Common Causes of Phase Problems in a Mix

Several common production techniques introduce phase issues that show up as negative correlation readings:

• Haas effect panning: Delaying one side of a stereo signal by 15 to 40ms to create a sense of width. This creates comb filtering when summed to mono and can cause severe cancellation in the frequency range where the delay length equals a half wavelength.
• Stereo widener plugins: Many work by adding polarity-inverted content to the side channel. At extreme settings, these push correlation well into negative territory. Reduce the width control until correlation stays above 0.
• Mid/Side processing on reverb returns: Boosting the side channel of a reverb bus adds air and space but also pushes correlation down. Check with a correlation meter after any MS EQ on reverb.
• Multiple microphone recordings: When two mics capture the same source from different positions, comb filtering occurs. This is common with overheads on a drum kit, room mics on a guitar cab, and multimic vocal sessions. Apply the 3:1 rule (second mic placed at least three times farther from the source than the first) to minimize this.
• Pitch-shifted layers: Layering a pitch-shifted copy of a vocal or synth part panned opposite the original creates a wide image but often causes phase issues in the frequency range of the pitch shift.

How to Use These Readings in Your Workflow

Integrate stereo width checking into the mastering stage rather than treating it as an afterthought. Here is a practical workflow:

• Upload your mixdown or master candidate to this tool and play through the full track. Note the minimum correlation reading in the Session Statistics panel. If minimum correlation drops below -0.2 at any point, find that section and investigate which element is causing it.
• Use the Sum to Mono toggle and listen to the mono version at the same time you watch the goniometer. Listen for any element that disappears, gets significantly quieter, or changes tonal character in a problematic way.
• Compare the Mid and Side level meters. A good rule of thumb for pop and rock masters is to keep the Side level at least 6 dB below the Mid level. If Side is louder than Mid, the mix will sound hollow in mono.
• After any stereo widening processing in mastering, re-analyze with this tool to confirm that average correlation stays above +0.4.

Once you have checked stereo compatibility, use our LUFS Loudness Meter to confirm the integrated loudness is within the target range for your chosen platform. Spotify targets -14 LUFS integrated with a -1 dBTP true peak ceiling. Apple Music targets -16 LUFS. These targets, the stereo phase check, and the true peak measurement are the three non-negotiable measurements before any track is submitted to distribution. For a deeper look at the mastering process, the blog post on mixing vs. mastering explains where each stage fits into the production chain and what decisions are made at each step.

Mid/Side EQ and Mastering: A Note on Workflow

Mid/Side EQ is a mastering technique where the stereo signal is decoded into its Mid and Side components, each treated with independent EQ, then re-encoded back to stereo. Cutting low frequencies from the Side channel (high-pass the Side at 120 to 200 Hz) tightens the bass and improves mono compatibility without narrowing the perceived stereo width of the mids and highs. This is a standard move on almost every professional master because low-frequency phase content is the most common source of mono cancellation.

After applying MS EQ, re-upload the processed file here and compare the correlation and width readings against the pre-processing version. You should see correlation increase and the Mid level rise relative to Side in the sub-bass region. Use this tool alongside our Decibel Meter for real-time monitoring during tracking, and our Spectrum Analyzer to identify which frequency ranges are contributing to the widest stereo spread.