The B-format signals comprise a truncated spherical harmonic decomposition of the sound field. They correspond to the sound pressure formula_1, and the three components of the pressure gradient formula_6 (not to be confused with the related particle velocity) at a point in space. Together, these approximate the sound field on a sphere around the microphone; formally the first-order truncation of the multipole expansion. formula_1 (the mono signal) is the zero-order information, corresponding to a constant function on the sphere, while formula_6 are the first-order terms (the dipoles or figures-of-eight). This first-order truncation is only an approximation of the overall sound field.