Acoustic Emission (AE) testing is an advanced non-destructive testing (NDT) technique that measures stress wave, or ultrasonic waves, that are generated by active flaws. AE testing is a passive technique that analyzes the stress waves emitted by structural defects activated during loading. Some common examples of AE in materials include fatigue crack growth in steel pressure vessels and fiber breaks in composite materials. The sudden release of energy caused by these phenomena generates elastic waves that propagate through the volume of the material and guided by the boundaries of the material. AE may travel through the bulk of a material as longitudinal and shear waves. Along the boundaries of the material, AE can be detected via measurement of guided and surface waves. Typically, stimuli such as mechanical loads, pressurization, thermal stresses and magnetic forces, can produce AE.
Figure 1 Acoustic emission hit showing first threshold crossing, peak amplitude, duration, and rise time features .
AE testing is widely used to detect and assess the severity of fatigue crack and fatigue crack growth rates in steel structures including fracture critical bridge structures and metallic pressure vessels. Compared to other NDT techniques, AE testing has several advantages over traditional NDT and even hydrotesting of pressure vessels: (i) in-service testing; (ii) no contamination due to humidity; (iii) remote inspection and (iv) sensitive to active flaws as they propagate. However, there are some important limitations associated with the technology as well: (i) controlled loading of the asset is required; (ii) flaw sizing is generally not possible; (iii) sophisticated instrumentation with operated by highly trained personnel is required; and (iv) only active defects are detected. In this article, the concepts of AE source location are introduced.
Figure 2 Schematic of the 1D linear location 
Source Location in AE testing
Source location is one of three basic objectives of an AE test, which are: (i) detecting AE activity; (ii) locating the source of the activity and (iii) evaluating the material defects causing the activity. Source location is also referred to location calculation in technical literature. Source location is perhaps the most important feature of AE testing since knowing the location of an event abets reduction in the number of source mechanisms that may be possible. This is because source mechanisms are dependent on particular geometric features and thus, accurate AE source location results can aid in inference of crack initiation and propagation.
In this article, the focal point is a class of source location techniques used in conjunction with AE testing for pressure vessels/cylinders. These techniques are (i) 1D linear location; (ii) 2D planar location and (iii) 3D location. They are based on the time difference of arrival (TDOA), wave speed and distance between the sensors. The two underlying assumptions for this class of source location methods are that (a) wave speed remains constant from the source to the sensor and (b) there is a direct wave path between the source and the sensor.
Linear Location (1D): Linear Location is most suited for vessels where the length is much larger than the diameter i.e., a rod-like shape. Figure 2 shows the schematic of the linear location setup.