Equal-level stereo speakers create a phantom image midway between them

Evidence

However, both amplitude and phase response matter in the matching of stereo left and right loudspeakers that perform in double-mono to generate the phantom images in the soundstage. It is the similarity of the loudspeakers that is important, though, not the actual responses. Credible phantom images can be created by two flawed, but identical, loudspeakers. — Toole, Floyd E, Sound reproduction- the acoustics and…, 117

In a traditional stereo arrangement, the subject faces two loudspeakers positioned in front, each at an angle of about 30° from straight ahead. If both loudspeakers radiate the same sound pressure level at the same time, a summing localization occurs and the sound is perceived as coming from a location midway between the two loudspeakers. If the sound pressure level from the right loudspeaker is 30 dB higher than that from the left speaker, the sound is perceived as originating in the direction of the right loudspeaker. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1721

Categorization

• XREF: This is the soundstage analogue of the reader’s grafting piece: compatibility between matched units matters more than the absolute quality of each — parallel to how grafted trees depend on the union, not each rootstock.
• ADDENDUM: Offers a sensory-system parallel to ‘Grafting & Growth’ — the union or match matters more than each unit in isolation, worth a footnote-style follow-up.
• corroborated: 2 independent passages support this claim — Toole, Floyd E, Sound reproduction- the acoustics and…, 117; Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1721