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I got 16 channels of a jazz ensemble, sending them to groups by instrument.

My problem is that for a range here in Logic Pro 10.6.3, the base and piano groups buses (we got several microphones for the instruments) causes Stereo Out to clip by 0.3 when solo’d together, but don’t when I solo them individually.

It’s as if there’s some kind of interaction/“additive” effect going on between these groups/tracks.

Anyone has reading/watching material or can explain what is going on?

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    You said it yourself. The tracks are added together or summed. That’s part of what a DAW does: sums the outputs of several tracks together to make one mixed track Jul 18 at 20:48
  • Sure, this is beyond my knowledge in signal theory, or whatever it is. So mixing two audio signals is adding the wave amplitudes of them? I don't know if I can intuitively confirm that. Say, three people talking simultaneously doesn't to me appear as one person talking three times as loud/yelling. Wikipedia says "Audio mixing is the process by which multiple sounds are combined into one or more channels. In the process, a source's volume level, frequency content, dynamics, and panoramic position are manipulated or enhanced." which isn't specific.
    – Frans
    Jul 18 at 20:56
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    @Frans summing two signals doesn't double the "volume" (or "loudness") as this is not how we perceive loudness (which uses a logaritmic scale). Also, three people talking simultaneously means having three different sources of sound, which is not the same due to the way we can hear things. Jul 18 at 21:11
  • Just because the sum of two signals isn’t double the loudness doesn’t mean it’s not louder. But think about this: a whole crowd cheering in a stadium is much louder than one person cheering. Jul 19 at 2:22
  • Is soloing those two busses at the same apparent level as muting all the others? [It should be on a modern DAW, but just to check you've not got something else going on]. If so, then you're going to have to pull all the group busses back, as with everything in the mix you'll be well in the red.
    – Tetsujin
    Jul 19 at 6:41
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It’s as if there’s some kind of interaction/“additive” effect going on between these groups/tracks.

When you sum two channels, the resulting waveform is a sum of the two added waveforms.

three people talking simultaneously doesn't to me appear as one person talking three times as loud/yelling

Yes, this is a bit nonintuitive. If you add two exactly identical signals, the resulting signal would be louder by 6dB. However, adding two uncorrelated sources of the same volume like two different people talking, or even two instruments, even if they play in unison, results in loudness increase by 3dB. Three sources of the same volume mixed together are 5dB louder than a single source. Note here I'm speaking about averaged loudness, the peak volume may occasionally go higher, causing risk of clipping.

You may want to read about decibel scale. It's logarithmic and might be not intuitive at first.

On the contrary, what people intuitively call "twice louder" corresponds to around 10dB of difference.

Nevertheless, coming to your main question: a single source was below the clipping level, but summed with another source it went above. Possible solutions:

  • roll down the volume. If you are not mastering the recording, this might be the preferred way. Make sure your track doesn't clip and let the mastering engineer set the final volume of the track
  • apply dynamic compression or limiting either to individual tracks or to the sum. Compression automatically reduces the signal volume when it crosses a threshold you define. Compression may actually improve the sound quality if used correctly, it may also cause several damage if set wrongly, or applied excessively. There are plenty learning materials in the internet.

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