Amplitude panning fails front-to-back because ear placement suits horizontal localization.

Evidence

There were even notions of “panning” images front to back using conventional amplitude-panning techniques, something that Ratliff (1974) and others found problems with—the ears are in the wrong locations for that to work. The quadraphonic square array of left and right, front and rear, was still an antisocial system, with the sweet spot now constrained in the front-back as well as the left-right direction. — Toole, Floyd E, Sound reproduction- the acoustics and…, 758

Categorization

• DEFINE: Explains why the quadraphonic array’s sweet spot is limited in both axes: physical ear geometry sets the constraint.