Acoustic waves are produced by successive phases of volume expansion and contraction. In contrast to electromagnetic waves, their polarization is longitudinal. Depending on the number of sources and their geometry, different patterns can be created. The simplest case, with spherical symmetry, is the monopole (e.g. from a boxed loudspeaker). Other common cases are the dipole (from an unboxed loudspeaker) and the lateral quadrupole with four monopoles, with the two pairs out of phase with one other.