Abstract
This chapter presents the background of ultrasonics along with some common definitions. The chapter defines wave as a moving transition between two states of a medium. The state in an elastic wave is the stress acting on the medium and its velocity. An elastic wave carries changes in stress and velocity. The state can take a jump, oscillate periodically in time and position, or vary in some arbitrary fashion. The chapter explains the concept of waveform as the sequence in time of the motions in a wave. A typical signal includes several waveforms such as the initial excitation (main bang), a front-face echo (in a system with a stand-off transducer, such as in a tank or a squirter), and one or more back-face echoes. A waveform can be displayed in three ways: the RF, or radio-frequency; the video waveform after rectification; and the analytic envelope, constructed through a data processing algorithm. The chapter also elaborates the basic concepts related to wavefront, wave surface, wave pattern, propagation and polarization vectors, phase, and frequency. Quantitative measures of a wave are also discussed in detail in the chapter.
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