Mono compatibility

Definition

Mono compatibility is the ability of a stereo mix to preserve balance, timbre and intelligibility when the channels are summed into a single channel, as happens on smartphones, Bluetooth speakers, many club and public address systems, AM radio and some low-bitrate streaming systems.

The most common mono sum is M = (L + R) / 2. The level change relative to stereo is:

ΔL = 10 · log10( P_M / ((P_L + P_R) / 2) )

For channels of equal power with correlation ρ: ΔL = 10 · log10((1 + ρ) / 2): ρ = 1 → 0 dB; ρ = 0.5 → −1.25 dB; ρ = 0 → −3.01 dB; ρ = −0.5 → −6.02 dB; ρ = −1 → silence. All the Side (L − R) content disappears in the sum.

When the two channels contain the same source with a delay τ, the sum produces a comb filter: cancellations at the frequencies f_k = (2k + 1) / (2τ), with k = 0, 1, 2… and reinforcements at k / τ. With τ = 1 ms the cancellations fall at 500, 1500, 2500 Hz.

Parameters and units

• Mono drop: dB, on the whole mix and per element.
• Correlation: −1/+1.
• Side/Mid: dB.
• Cancellation frequencies: Hz, as a function of the delay between channels.
• Reference values: a well mono-compatible pop mix: mono drop ≤ 1–1.5 dB; vocal, kick, bass and snare: drop ≈ 0 dB (centered); wide pads and backing vocals: a drop of 2–4 dB is acceptable.

Musical function

A mono-compatible mix keeps vocals, bass and drums at the same relative level on any system. Wide elements lose some level in mono, and that is acceptable; it becomes a problem when they change timbre (comb filtering), when the vocal sticks out too much because everything else drops, or when the bass cancels. For this reason the load-bearing elements should be kept centered and width should be built with distinct takes, panning and reverbs rather than with delays between channels.

Practical example

• Haas-effect widener on a guitar: 15 ms delay on the right channel. In mono, cancellations appear at 33.3 Hz, 100 Hz, 166.7 Hz and then every 66.7 Hz: the guitar sounds thin and metallic. Replacing the delay with a second take played and panned to the right makes the cancellation disappear.
• Very wide pad: mono drop −4.5 dB. Reducing the Side by 3 dB brings the drop down to −2.8 dB and the pad stays wide in stereo.
• Full mix: mono drop −0.9 dB, correlation 0.62: good compatibility.

Common mistakes

• Using delays between channels (Haas, heavy stereo chorus) on vocals, bass or load-bearing elements.
• Leaving side content below 120 Hz.
• Checking mono only at high volume: at low volume level drops are easier to perceive.
• Making the entire mix mono to solve the problem of a single track.
• Confusing the drop caused by the panning law with a phase cancellation.

How to check it

• App analysis: Mono sum drop = 10 · log10 of the energy of (L + R)/2 relative to the average of the energies of the two channels; Stereo correlation; Side / Mid (total energy) and per-band width to locate the responsible zone.
• Listening in mono: in FL Studio set the master's stereo separation control to mono or use a utility plugin (Fruity Stereo Shaper) on the master bus; listen at low volume and compare the balance of vocals, bass and drums.
• Per track: analyze the individual wide track in the app; a drop beyond 4–5 dB or a negative correlation indicates that intervention is needed.