Acoustic beam invariants.
Transversely finite sound beams traveling in fluid media with negligible viscous damping and scattering have seven quantities constant along the length of the beam. The simplest of these is the cycle-averaged momentum per unit length of the beam. The seven constant quantities are proved to be invariants from conservation laws. An angular momentum flux density tensor is introduced, in the formulation of conservation of angular momentum. Examples of the invariants are given, for approximate solutions of the wave equation (Gaussian beams), and for a set of exact solutions (generalized Bessel beams).
Lekner
John
J
School of Chemical and Physical Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand.