Standard Ambisonics microphones use four capsules to capture omnidirectional and three-dipole signals.

Evidence

The most common microphone device for Ambisonics is the first-order, four-capsule Bformat microphone, producing signals with directional patterns having spherical harmonics up to the first order. The omnidirectional signal is denoted w(t), and the three dipole signals are denoted x(t), y(t), and z(t). — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 1856