Ultrasound and elastic waves : frequently asked questions Definitions and background Wave propagation concepts Wave interactions Hardware : equipment concepts Software : data processing Stress strain and elasticity (also vectors and tensors) The generalized Hookes Law States of stress or strain in waves Balance of forces and Newtons law of inertia Theory of wave propagation Solutions to the wave equations Dispersion group velocity Transducer beam forming Solutions for anisotropy Oblique interactions between waves and boundaries Lateral stress and strain in rods under axial loads Bending waves in beams and plates Time domain analysis Ultrasound has found an increasing number of applications in recent years due to greatly increased computing power. Ultrasound devices are often preferred over other devices because of their lower cost portability and non invasive nature. Patients using ultrasound can avoid the dangers of radiological imaging devices such as x rays CT scans and radioactive media injections. Ultrasound is also a preferred and practical method of detecting material fatique and defects in metals composites semiconductors wood etc. Detailed appendices contain useful formulas and their derivations technical details of relevant theories The FAQ format is used where a concept in one answer leads to a new Q & A