ABSTRACT
US-NDT language often uses the term “ultrasonic beam” instead of “ultrasonic wave”; it is advantageous that we may distinguish between “beams” and “rays” for a correct mathematical definition of respective approximate solutions of homogeneous wave equations: Basically, rays come as generalization of time harmonic plane waves to time harmonic locally plane waves in inhomogeneous materials-they are geometric trajectories of a wave normal-whereas time harmonic beams are paraxial solutions of homogeneous reduced wave equations propagating along rays and keeping their functional, for example, Gaussian, structure; as wave packets or pulsed beams, they represent respective impulse solutions physically visible in the time domain. Beams have a lot of conceptual, mathematical, and practical benefits as compared to rays.
