**1. Introduction**

Acoustic emission (AE) is a relatively new non-destructive Testing (NDT) technique. Structural testing and assessment, material characterization, and process monitoring are three important application areas of AE. A comprehensive introduction to the AE technique can be found in [1].

The AE technique is one of the most reliable NDT techniques for detecting and monitoring damages and defects in different structures. AE has been effectively employed for fracture behavior monitoring and fatigue detection in various materials including composites, metals, concrete, fiberglass, ceramics, plastics, and wood. It has also been used for fault and pressure leak detection in pipes, tanks, and vessels.

There are several electronic instruments that can be used to digitize and store large numbers of high-speed digital waveform signals of AE. Common instruments used in AE include preamplifiers, amplifiers, filters, sensors and other data collection, analysis, and storage equipment such as computers, oscilloscopes, and voltmeters. Preamplifiers are used to amplify a weak signal and to reduce the interference from noise, while the piezoelectric sensors are used for the conversion of mechanical AE waves into electrical voltages. The overall objective of the measurement is to determine the various AE parameters such as the frequency range (controlled by filters) that exist in the system by observing and measuring the performance of AE amplifiers and sensors. These are very useful mechanisms for measuring the essential AE parameters such as event, count, energy moment, maximum amplitude, hit, energy, arrival-time difference, RMS (root mean square) voltage, rise time, spectrum, frequency, and duration [2].
