Summing a stereo mix to mono lifts centred sounds ~3 dB relative to edge-panned ones

When a stereo mix is summed to mono, sounds do not keep their relative levels: centred sounds rise by roughly 3 dB relative to those panned to the edges, because equal-in-stereo sounds at different distances from centre end up at different mono levels. This is one reason a balance can shift audibly in mono, over and above outright cancellation of polarity-inverted content.

Examples

A backing part balanced against a centred vocal in stereo can sit ~3 dB lower once summed to mono, since the centred vocal gains level relative to the edge-panned part.

Assessment

State how central and edge-panned levels change when a stereo mix is summed to mono, and why.