Higher-Order Ambisonics (HOA)
As the spatial resolution of 1st order Ambisonics is relatively low and the listening area's sweet spot is small, enhancements of both parameters are required for some purposes.

The idea behind the HOA is to increase the resolution and enlarge the sweet spot by adding more selective directional components to the B-format.

No single-element microphone can provide for that. In practice, higher-order microphones are designed from several pressure microphones (i.e., 8, 16, 32 pcs.) equally spaced on a solid sphere, with a typical 5-15 cm diameter.

The signals from the individual microphones are mixed/processed to obtain improved directionality and, to some degree, an enlarged sweet spot.

HOA finds many applications in scientific research. Further, HOA sound is valid for virtual reality (VR) and the like, where a full sphere resolution is desired.

Figure 1  This is a mapping of directional patterns possible to obtain from the Ambisonics principle from order 0 (top row) to order 4 (bottom row). The 1st order yields three orthogonal- oriented bidirectional characteristics while the higher-order systems exhibit multiple lobes.