Acoustic Pressure Equaliser (APE) — A smooth spherical construction with a small-diaphragm omnidirectional microphone capsule mounted flush to the surface. APEs are typically 30-50mm in diameter and serve to modify the frequency response and polar pattern through surface diffractions. These create an HF boost for on-axis sounds, and greater directivity at high frequencies giving more forward focus and, when used in a spaced array, enhanced stereo imaging. The Decca Tree is famous for using Neumann M50 microphones in which the omni capsule is embedded within a 40mm APE.