How much louder does a bus get when I add channels?

For incoherent (uncorrelated) signals like different instruments, the level increases by 10 × log10(N) dB, where N is the number of channels. So 2 channels = +3 dB, 4 = +6 dB, 8 = +9 dB, 16 = +12 dB. For coherent (identical) signals, the increase is double: +6 dB per doubling.

What is the difference between coherent and incoherent summing?

Coherent summing occurs when signals are identical (e.g. the same signal on two channels) — voltages add directly (+6 dB per doubling). Incoherent summing occurs with uncorrelated signals (different instruments) — only power adds (+3 dB per doubling). In a real mix, signals are mostly incoherent.

Why does my bus clip when individual channels are not clipping?

When multiple channels sum to a bus, their levels combine. Eight channels at -12 dBFS can sum to approximately -3 dBFS (incoherent) or even higher during momentary correlations. This is why gain staging throughout the mix is important — leave headroom on each channel.

How should I set channel levels to avoid bus clipping?

A good rule of thumb: if you have N channels, set each channel to -10×log10(N) dB below your target bus level. For 16 channels targeting -6 dBFS on the bus: each channel at -6 - 12 = -18 dBFS average. Leave extra headroom for peaks.

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How We Calculate This

Incoherent Sum (uncorrelated signals)

Bus Level = Channel Level + 10 × log10(N)

This applies to independent signals like different instruments. Power adds, not voltage.

Coherent Sum (correlated signals)

Peak Level = Channel Level + 20 × log10(N)

Worst case where all signals align in phase. This represents the theoretical maximum bus level. For independent musical sources this in-phase alignment is essentially never reached, so it is shown only as an informational worst case.

Phase-alignment margin

The gap between the coherent and incoherent sums equals 10 × log10(N) dB — the extra level you would see if all channels happened to align perfectly in phase versus the typical uncorrelated sum. (Note: this is not crest factor, which is the peak-to-RMS ratio of a single waveform.)

Headroom

The headroom target defines the safety margin below 0 dBFS. The remaining headroom figure shows how far the typical (incoherent) bus sum sits below that target — a negative value means the typical sum is already hotter than your chosen margin allows.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates based on standard formulas. Always verify results for critical applications.

Bus Routing Calculator — Channel Level Summation