Stereo Widener and Narrower

Mid-Side Processing: How Stereo Width Works and Why It Matters for Your Master

Every stereo audio file contains two channels: left and right. What most producers do not realize is that those two channels can be mathematically decomposed into two completely different signals with entirely different properties. The Mid signal is the sum of the left and right channels. The Side signal is the difference between them. This decomposition is called Mid-Side encoding, and it is the foundation of every professional stereo widener, stereo narrower, and MS EQ plugin on the market.

The math is straightforward. Mid = (L + R) / 2. Side = (L - R) / 2. To reconstruct the stereo signal, you reverse the process: Left = Mid + Side, Right = Mid - Side. This tool uses exactly that math. The Width slider multiplies the Side signal by a factor before re-encoding back to stereo. At 1.0x, the Side signal is unchanged and the output is identical to the input. Below 1.0, the Side signal is attenuated and the image narrows toward mono. At 0, the Side signal is completely removed and the output is pure mono. Above 1.0, the Side signal is boosted and the perceived stereo width increases beyond the original.

Why the Bass Mono Crossover Is Not Optional

Bass frequencies below roughly 100 to 150 Hz should always be mono in a commercially released master. Your ears cannot accurately localize frequencies at those wavelengths, so wide stereo bass adds no perceptual benefit. What it does add is a set of phase relationships that cancel directly when a system sums left and right to mono. Club PA systems sum the sub channel to a single driver in the center. DJ booth monitors are typically mono below 100 Hz. Bluetooth speakers and phone speakers produce a mono output. The KERN Audio stereo guide documents this precisely: bass in stereo field adds phase differences that cancel on mono fold-down, costing 3 to 6 dB of low-end at exactly the frequencies where the room is loudest.

Vinyl adds a second reason. The cutting stylus on a lathe moves horizontally for stereo information and vertically for the sum. Stereo content below 80 to 100 Hz causes the stylus to move vertically, which can force the needle out of the groove during playback. Every vinyl mastering engineer applies an elliptical filter or a Side channel high-pass at the cutting stage for exactly this reason. If you plan to release on vinyl, apply the bass mono crossover at 100 Hz minimum before your master leaves your DAW. Our default of 120 Hz matches the guidance from most professional mastering engineers for pop, electronic, and hip-hop releases.

The Bass Mono control in this tool applies a second-order Butterworth lowpass filter to the original left and right channels. The filtered (bass) portion is summed to mono before the widened output is reconstructed. The result is a signal where the sub and low bass are perfectly mono, and the mids and highs carry the adjusted stereo width. You can set the crossover anywhere from 40 Hz to 300 Hz depending on your material. Electronic and club-focused music typically uses 100 to 150 Hz. Sparse acoustic and folk recordings can often get away with 80 to 100 Hz.

How to Read the Phase Correlation Number

The Phase Correlation meter in this tool shows the Pearson correlation coefficient between the processed left and right channels, computed across the first second of the file. The range is -1 to +1 and the interpretation is direct:

• +1.0: The left and right channels are identical. The signal is fully mono regardless of what the stereo field looks like on a meter.
• +0.4 to +1.0: The standard target range for a commercially released master. The Audio Engineering Society recommends maintaining average correlation above +0.4 throughout the full track.
• 0 to +0.4: The channels are significantly different. The mix will likely sound thinner and quieter on mono devices, but will not collapse completely. Audition in mono before exporting.
• -1.0 to 0: The channels contain out-of-phase content. On any mono device, portions of the signal will cancel. A reading of -1.0 means complete cancellation: the mono sum is silent.

When you move the Width slider above 1.5x without the Bass Mono crossover active, you will typically see the correlation reading drop below 0.4 and in some cases below 0. The tool shows a warning when this happens. Reducing the width or enabling the Bass Mono crossover usually brings correlation back into the safe range, because the largest source of out-of-phase energy in most mixes is sub and low bass content that varies between channels due to reverb, room acoustics, or stereo miking.

When to Use Width Below 1.0: Narrowing and Mono Compatibility

Not every use case calls for more width. Narrowing is a legitimate and common mastering move. If a mix arrives with an excessively wide stereo image where the center feels hollow or elements are spread too far left and right, pulling the Side signal down with a width of 0.7 to 0.9 brings the image inward and improves playback on mono devices and narrow speaker setups.

Setting width to exactly 0 collapses the signal to mono. This is useful for checking mono compatibility without leaving this page. Play the audio at width 0.0 and listen for elements that disappear, drop in level by more than a few dB, or change tonal character. If the kick drum becomes hollow at 0.0, there is likely a phase issue in the low end of the original mix. If a synth pad vanishes, the widening used on that element during mixing was creating destructive cancellation.

Use our Stereo Width Meter to analyze the original file before processing, then come back here to apply the width adjustment and download the result. The Stereo Width Meter shows a live goniometer and correlation reading in real time during playback, which gives you a visual reference for the pre-processing state.

Practical Width Settings by Genre and Use Case

These are starting-point ranges based on common mastering practice for each genre. Your specific mix may need different treatment depending on the source material.

• Pop and commercial music: 0.9 to 1.2x. Most pop mixes arrive with a well-calibrated stereo image. A subtle narrowing to 0.9 often improves translation across mono devices without audible loss on stereo systems. Going above 1.2 on already-wide pop mixes tends to push correlation below 0.4.
• Electronic and club music: 1.0 to 1.4x with Bass Mono at 120 to 150 Hz. The wide pads and synth layers in electronic music can absorb additional Side signal well because they already occupy the sides. The Bass Mono crossover is mandatory here: club systems will expose any stereo content below 100 Hz immediately.
• Hip-hop and trap: 0.9 to 1.1x with Bass Mono at 80 to 100 Hz. The low-end punch in hip-hop depends on a tight, mono-compatible sub. Narrowing slightly often improves the perceived impact of the kick and 808 on small speakers and earbuds, which is where the majority of the audience listens.
• Rock and live recordings: 0.8 to 1.1x. Drum overheads and room mics produce naturally wide stereo content with complex phase relationships. Narrowing by 10 to 20 percent often improves mono compatibility without affecting the drum room sound on stereo systems.
• Classical and acoustic: 1.0 to 1.2x. Orchestral recordings typically have excellent mono compatibility already due to the careful microphone placement used in classical recording sessions. A small amount of additional width can improve the sense of hall space in headphone listening.

How the AudioWorklet Processor Works

Real-time preview in this tool uses the Web Audio API's AudioWorklet interface. An AudioWorkletProcessor runs in a dedicated audio rendering thread, separate from the main JavaScript thread. This guarantees glitch-free real-time processing at the audio block rate (typically 128 samples per block) without JavaScript garbage collection or UI rendering affecting the audio thread.

The processor receives the Width, Bass Mono frequency, and Bass Mono enable state as AudioParam values, which means they can be updated without restarting playback. When you move the Width slider during preview playback, the new value is scheduled on the audio thread in the next render block. The transition is sample-accurate.

The export function bypasses the real-time graph entirely. It runs the same M/S math directly over the raw Float32Array channel data in the main thread, in a single synchronous loop. This is simpler and faster than using an OfflineAudioContext for the same processing, and it produces a bit-for-bit identical result to what you hear in the preview. The output is written to a WAV file using a PCM 16-bit encoder with a standard 44-byte RIFF header. No external libraries are required.

For a deeper look at how stereo phase issues affect playback translation, see the Stereo Width Meter which shows a live goniometer and correlation reading, and the LUFS Loudness Meter to check integrated loudness and true peak after widening. Stereo widening changes the level relationship between the Mid and Side signals, which means your true peak ceiling may shift after processing. Re-check loudness after any width adjustment before submitting to distribution. Spotify targets -14 LUFS integrated with a -1 dBTP true peak ceiling. Apple Music targets -16 LUFS.

M/S Processing Versus Stereo Widener Plugins

Most consumer stereo widener plugins use one of three mechanisms: Haas delay (offsetting one channel by 5 to 40ms), allpass decorrelation (shifting phase by different amounts across the frequency spectrum), or direct Side signal boost, which is M/S processing. Each mechanism has a different mono fold-down characteristic.

Haas delay creates comb filtering when the signal is summed to mono. A 10ms delay between channels produces notches every 100 Hz across the frequency spectrum. This sounds like thinning and tonal change on mono devices. Allpass decorrelation produces gentler, more random phase ripple on mono fold-down, typically less than 1 to 2 dB of variation. Direct M/S Side boost produces the cleanest mono fold-down of the three: the Side signal cancels by definition when left and right are summed, so only the Mid survives. The energy loss on mono is proportional to the amount of Side content, which is predictable and measurable with the correlation meter.

This tool uses direct M/S processing, which makes it the safest widening approach for mono compatibility. The trade-off is that it does not generate new stereo information. It only scales what is already in the Side channel. If the original mix is nearly mono, there is very little Side signal to boost and the widening effect will be subtle. Haas delay and allpass decorrelation can create apparent width on a mono source, but at the cost of the mono compatibility issues described above.

For a full workflow, use this tool alongside the Online Mastering Tool which applies EQ, compression, and limiting after you have set the stereo width. Once the master is finalized, run it through the LUFS Loudness Meter to confirm your integrated loudness target before distributing.