Ambisonics is a method of codifying a sound field taking into account its directional properties. Instead of every channel having the signal what every loudspeaker should be emitting, as in stereo or 5.1 surround, every Ambisonics channel has information about certain physical properties of the acoustic field, such as the pressure or the acoustic velocity.
Since it is relatively difficult to find good introductory material about Ambisonics, you can find some preliminary lecture notes at:
This material has been elaborated by Daniel Arteaga and published under Creative Commons Attributtion-ShareAlike 4.0 International License.
Many things are missing or can bee improved, like a better list of references, historical remarks, a better exposition of concepts, et a revision of the English, typographical revision, etc. Additionally, it will probably contain errors. Use it under your own risk!