# Machine-Learning-Based Methods for Acoustic Emission Testing: A Review
## 2 Acoustic Emission Testing Methods
### Acoustic Emission Detection  
The data recorded in the form of electrical impulses return different information depending on the type and number of sensors and the acquisition system. With normal transducers available on the market, the parameters that can be evaluated (counts, amplitude, duration, and energy of the events) only allow a qualitative analysis of the phenomena to be monitored. To obtain quantitative data, the use of high-tech transducers and an extremely fast data acquisition system is required, so that information relating to the shape of the acoustic waves is available [28]. A system for measuring AE consists of the following elements (Figure 1):  
* AE sensor;
* Preamplifier or an impedance matching transformer;
* Band-pass filter;
* Main amplifier;
* Cables;
* Detector or processor. The AE sensors generate an electrical signal when they are hit by an acoustic wave. The principle of operation of these sensors can be different: signals can be generated by electromagnetic, magnetostrictive, and piezoelectric devices and using laser interferometers. The most widely used sensors are piezoelectric ones, which generate an electrical voltage and a corresponding charge separation when deformed. The deformation is produced by the motion of the wave, and it is the elastic response of the piezoelectric crystal when it is hit by the incoming stress wave. The electrical signal is generated by the material, which does not need to be powered from the outside: when hit by an impulse, it vibrates at its resonant frequency (Figure 2). There can be many resonant frequencies simultaneously excited. If shaken by a vibratory motion, the piezoelectric element will produce a corresponding oscillating voltage at the same frequency of the motion. The element also has a linear response: if the input motion is doubled, the output voltage will also double [29]. The relationship between the amplitude of the output-input voltage is a measure of the sensor sensitivity: this strongly depends on the motion frequency (the number of oscillations per second) and is better at the element resonant frequency. The sensitivity of the sensor depends not only on the frequency but also on the motion direction. Unlike accelerometers, which are carefully designed to measure only the motion component parallel to their axis, acoustic emission sensors respond to motion in any direction. In the choice of sensors, the priorities are high sensitivity, well-defined and consistent frequency response, high performance in the working environment, and immunity to unwanted noise. Noise sensitivity has been greatly improved with the development of sensors with integrated preamplifiers. These types of sensors have a preamplifier built into the housing together with the piezoelectric element, and for field tests, these have great advantages compared with previous types of sensors that required separate preamplifiers to be mounted a few meters from them [30]. Figure 1: Acoustic emission detection scheme. AE sensor captures the signal and passes it to a preamp, which then sends it to a band-pass filter. Subsequently, the selected spectrum is amplified and passed to the signal processor. The sensor must be properly placed in contact with the surface of the material to be monitored to identify the motion of the AE wave and provide a strong signal. The coupling and assembly techniques are, therefore, very important. For example, an acoustic couplant in the form of an adhesive, viscous liquid, or grease can be used and applied to the surface of the sensor, which is then pressed against the structure, the surface of which must be smooth and clean. The sensor must then be firmly held in position using adhesives, magnetic bases, or other means. Finally, after assembly, the performance of the system is verified by simulating an AE signal and checking the response of the system. The signal produced by the sensor is amplified and filtered, identified, and measured. The amplifiers increase the signal voltage to bring it to an optimal level for the measurement circuit.