How to mix bass: sub-bass correlation and mono compatibility

Sub-bass phase is the #1 issue PRISM detects across all genres. It affects electronic music, hip-hop, pop, and anything else with bass content below 80 Hz — which is nearly every track. And unlike most mixing problems, it's invisible in a stereo mix. You only hear it when the track goes mono.

What mono compatibility actually means

Mono compatibility isn't about making your mix sound good in mono — it's about making sure your track doesn't disappear in mono. When a stereo signal is summed to mono, the left and right channels are added together. Any frequencies where the left and right channels are out of phase cancel each other out. At sub-bass frequencies, even a small phase offset can cause 3–6 dB of cancellation. Your 808 vanishes. The kick loses its body.

This matters more than ever. Bluetooth speakers, phone speakers, smart home devices, and club PA systems that route both channels to a single sub driver all sum low frequencies. If your bass doesn't survive that sum, it doesn't survive those playback scenarios.

What PRISM measures

PRISM calculates sub-bass correlation using a lowpass filter at 80 Hz applied to both channels, then computes the Pearson correlation coefficient between the filtered left and right signals. A correlation of 1.0 means the sub-bass is perfectly mono — identical in both channels. A correlation of 0 means phase-random. A correlation below 0 means inverted — active cancellation in mono.

Why this happens

Sub-bass phase problems usually come from one of three sources. First, a stereo bass instrument or sample where the designer added stereo width via chorus, flanger, or sample offset — these processes inevitably create phase difference at sub frequencies. Second, a mid/side EQ or stereo widener applied to the full mix that widens bass content. Third, two layered bass elements (sub + bass guitar, or 808 + sine sub) that overlap in frequency and are slightly out of time.

How to fix it

• Make the sub-bass mono. The simplest fix: send all bass elements to a group bus and add a Mid/Side or Dual Mono EQ. Apply a lowpass at 80–120 Hz to the sides channel only, setting it to 0 gain (or simply attenuating the sides below that frequency). This removes stereo information from the sub-bass without affecting the rest of the mix.
• Use a Bass Mono plugin. Ableton's Utility at the end of a bass chain has a "Bass Mono" switch that automatically sums frequencies below 250 Hz to mono. FL Studio has Stereo Enhancer with a similar mode. Logic users can achieve the same with the Direction Mixer, setting frequencies below 120 Hz to centered.
• Check layer alignment. If you're layering two sub elements, nudge them to be sample-accurate in time. Phase cancellation between two sines of similar frequency but different timing is extremely destructive. Use the spectrum analyzer to confirm they're adding constructively, not cancelling.
• A/B in mono. Solo your bass elements, switch your master to mono (Utility plugin, or the mono button in your interface monitor section), and confirm the bass is still present and punchy. If it drops 3+ dB, there's phase cancellation to fix.

The distribution angle

If you're releasing through DistroKid or TuneCore, your track will be submitted to Spotify, Apple Music, and YouTube — all of which serve content on mobile devices and Bluetooth speakers. Those playback systems are mono or near-mono at bass frequencies. A track that's clean at sub-bass correlation ≥ 0.72 will sound consistent across all of them. One that isn't will lose energy on exactly the playback systems your audience uses most.