# Machine-Learning-Based Methods for Acoustic Emission Testing: A Review
## 2 Acoustic Emission Testing Methods
### Acoustic Emission Detection
Along with various amplification stages, filters are incorporated into the instrument. These define the frequency range to be used and attenuate low-frequency background noises. These amplification and filtering processes are called signal conditioning. They clean the signal and prepare it for the detection and measurement process. After conditioning, the signal is sent to the detection circuit: it is an electronic comparator that compares the amplified signal with a threshold voltage defined by the operator. Whenever the voltage exceeds the threshold, the comparator generates a digital pulse. The first impulse produced marks the beginning of the phenomenon (hit). This pulse is used to activate the signal measurement process. As the signal continues to oscillate above and below the threshold level, the comparator generates additional pulses, and the electronic circuits actively measure the key characteristics of the signal. Over time, the amplitude of the signal is reduced to a level where the threshold is no longer crossed. After a predetermined time, called the hit definition time (HDT), any further impulse from the comparator determines the end of the event. The control circuit ends the measurement process and passes the results to a microprocessor. Finally, the measurement circuit is reset and re-prepared for the next event [13]. In many cases, the discrimination of the signal is very difficult: this is due to the temporal overlap of different processes, including secondary products such as reflections. In the case, for example, of the growth of fractures in heterogeneous materials, the effects due to the friction between the faces formed by the fracture itself or by preexisting fractures are superimposed on the primary source represented by the motion of the fracture tip. Preexisting defects can act as resonators and diffusion centers, modifying the signal. On the other hand, regions with lower fracture strength or different composition may have a more fragile behavior with a consequent increase in the amplitude of the signals. Therefore, the  
Figure 2: Piezoelectric sensor scheme: they consist of thin layers of single crystal, which produces an electrical charge when subjected to compressive force. nature of the phenomena is so articulated that a univocal definition of acoustic emission appears difficult [12]. In general, the signals appear as isolated pulses over time (burst), formed by a very rapid rising edge followed by an exponentially decreasing trend, also due to the response of the transducer (Figure 3). Furthermore, it is possible to identify stationary or quasi-stationary components, free from decays, in which the single AE events are so close that they are not distinguishable. These contributions are of smaller amplitude than the burst event due both to the dispersion of the medium and to other causes such as viscoelasticity, internal friction, etc. The impulsive components (bursting signals) are caused by the initiation of fractures and their growth, whereas the continuous part of the signal is due to plastic deformations as well as external noise: in plastic deformations, the energy involved translates into work of plastic deformation used to overcome the resistance to motion of dislocations. This movement produces within the material the propagation of a stress wave that reaches the surface and manifests itself as an AE wave [14]. Acoustic emission has the appearance of a damped sinusoid and is characterized by some characteristic quantities. Conventionally, referring to the sine wave (waveform), we speak of Hit of acoustic emission when the AE transient is identified and processed by a certain channel. We speak instead of Event when the localization of the source of this same acoustic emission is carried out. With reference to the ASTM E1316 [31] standard, the following definitions are given:  
* Hit: term to indicate that a given channel has identified and processed an acoustic emission transient;
* Event: an acoustic emission wave can be identified in the form of a hit on one or more channels.