Horizontal and vertical reflections dominate because they are the strongest first reflections.

Evidence

The floor, ceiling and walls in most listening rooms are horizontal and vertical, and these provide the strongest first-reflected sounds. All other reflections travel farther (more inverse-square loss and air attenuation) and encounter multiple reflecting surfaces, losing energy at each contact. Consequently, measurements were made on horizontal and vertical orbits at 15° intervals in the front hemisphere and 30° intervals in the rear hemisphere; 34 measurements in all. Comparisons, sums and differences of curves, spatial averages of multiple curves and total sound power output estimates were then possible. — Toole, Floyd E, Sound reproduction- the acoustics and…, 165

Categorization

• DEFINE: Explains the design logic behind full-orbit loudspeaker measurement: the geometry of room reflections dictates which spatial directions matter most.