Sound waves used in most engineering applications are longitudinal waves. These mechanical waves can propagate in solids, liquids, and gases. The waves originate at a vibration source and move through a medium by material particles oscillating in the direction of the propagation of the wave. Figure 12.1 illustrates the propagation of a longitudinal sound wave in terms of compressions and rarifications (or pressure pulses) in the direction of travel. This chapter focuses on this type of sound wave, specifically in the ultrasonic frequency range. The ultrasonic range is considered to be at frequencies over 20,000 Hz. In contrast, sound waves with frequency in the range of 20 to 20,000 Hz are in the audible range; sound waves with frequency below 20 Hz are in the infrasonic range. For engineering applications, ultrasound frequency in the range of 2 to 20 MHz provides a balance between attenuation, back scatter, and beam width.