First-order Ambisonic is an expansion of the MS format. It includes the horizontal plane. For 2D recordings, we saw that a Double-MS microphone arrangement can encode the signal in the frontal and median planes.
To record in 2D First-Order Ambisonic format we can use a Double-MS arrangement as shown in the previous section. It is possible to add another figure-of-8 microphone and create a Triple-MS array to record channels W,X,Y,Z/. Using the same principle, a coincident tetrahedron microphone array of cardioids with aiming directions FLU-FRD-BLD-BRU works given:
FLU = Front Left up
FRD = Front Right Down
BLD = Back Left Down
BRU = Back Right up