1 speaker vs 2 speakers

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'Two different guitarists, each playing through his own Champ, are just 3dB louder than either guy on his own, not 6'

Would that hold true if the guitarists were replaced with white or pink noise sources - could the constructive interference take place?
I'd guess not, as even though all frequencies might be present in the same amount, their phase relationship wouldn't be locked together.
But it seems counter-intuitive that with 2 champs and 2 noise sources, the SPL would increase 3dB if 1 of the noise source was switched out such that both amps were fed the same input.
Perhaps not though - it could be the same effect that makes stereo centre mix tracks become louder when mixed to mono.
Apologies for documenting the mental cog's slow rotation.

[Steve Conner]

Would that hold true if the guitarists were replaced with white or pink noise sources - could the constructive interference take place?
I'd guess not, as even though all frequencies might be present in the same amount, their phase relationship wouldn't be locked together.
Perhaps not though - it could be the same effect that makes stereo centre mix tracks become louder when mixed to mono.

You're exactly right (well as far as I know! )

If you replaced the two guitarists with two different noise sources, those are uncorrelated because of the phases, as you said, so you only get 3dB.

If you routed the SAME noise source into both amps, boom, another 3dB as their outputs are now correlated.

And yes, instruments panned to the centre of a stereo mix are correlated in the left and right channels, by definition. If they weren't, your ears wouldn't hear them in the centre. So I guess they get 3dB louder than uncorrelated information in the mix, when mixed to mono.

What's the difference between a guitarist and a white noise source?
The guitarist drinks more.

[Glass Snuff]

And yes, instruments panned to the centre of a stereo mix are correlated in the left and right channels, by definition. If they weren't, your ears wouldn't hear them in the centre. So I guess they get 3dB louder than uncorrelated information in the mix, when mixed to mono.

Which is why stereo balance controls and pan pots are -3dB in the center.

BTW, there's a lot of good stuff here:

FAQ in Music Acoustics