Acoustic beams with angular momentum

A family of exact solutions of the Helmholtz equation is used to represent transversely bounded helicoidal sound beams. Simple results are obtained for the energy content per unit length, the momentum content per unit length, and the angular momentum content per unit length. The analysis is restricted to lossless media; scattering and viscous damping are neglected. The energy, momentum, and angular momentum are calculated to second order in the velocity potential. The angular momentum content is always equal to m/omega times the energy content, where m (an integer) is the topological charge and omega is the angular frequency.