# Acoustic Emission for Structural Monitoring of Historical Masonry: An In-Field Application
## 3 Theoretical Framework  
### AE for Structural Applications  
An AE signal consists of a sequence of waves whose velocity depends on the nature of the material through which they propagate. The structure of an impulsive signal (Burst Event) is characterised by a rapid increase in the energy of the generated elastic waves, followed by a decrease after reaching the peak amplitude. A structural monitoring campaign is generally focused on the analysis of Burst Events. This type of event differs from those classified as continuous signals, which are associated with plasticisation phenomena. These continuous signals have low energy content and are characterised by stationary waveforms that are difficult to distinguish from background noise. The analysis of AE phenomena begins with the definition of the following two key concepts:  
* Acoustic Emission Hit (hereinafter referred to as hit): An impulse detected and processed by a specific channel within a transient interval. During this interval, the signal transitions from a stationary condition, where no significant variations occur, to a state in which the signal exhibits substantial amplitude variations, indicating an event that has generated an elastic wave. * Acoustic Emission Event (hereinafter referred to as event): A group of hits received by two or more channels from a single localised source. In the case of a single channel, the number of hits is equal to the number of events. An AE signal generated by a source is detected and processed only if it exceeds a predefined minimum voltage value, known as the threshold level. This level is initially set by the operator and depends on the environment in which the test is conducted. Determining the appropriate threshold level allows for the maximum possible reduction in background noise. When the signal first exceeds the threshold level, it is referred to as a trigger, initiating the recording and processing of the signal by the dedicated software. The standard terminology used to define the parameters associated with the analysis of these signals is established by the international standard ASTM E610 [20]. The main parameters are summarised below:  
* **Amplitude (A):** The highest value present in the waveform of the signal, which is correlated with both the type of source generating it and the material. It is a fundamental measure linked to signal intensity [21]. * **Event Duration:** The time interval between the first and last threshold exceedance. The ratio between this quantity and amplitude provides useful information for determining the waveform shape. * **Rise Time:** the time interval between the first threshold exceedance and the moment when the maximum amplitude is reached. * **Arrival Time:** The exact moment when an Acoustic Emission (AE) signal is first detected by a sensor. It marks the start of the transient, indicating the transition from a stationary condition to a significant variation in the signal. * **Decay Time:** complementary to the Event Duration, this represents the time interval between the moment the maximum amplitude is reached and the last threshold exceedance. * **Dead Time:** The time interval after which, if no further threshold exceedance is detected, the AE event is considered to have ended. This parameter must be defined based on the material. * **Counts:** The number of times the signal amplitude exceeds the threshold level. Depending on the waveform shape, a single hit may generate either a small or large number of counts. * **Counts to Peak:** the number of counts recorded before and including the one corresponding to the maximum amplitude. * **Counts Rate:** the number of counts per unit of time. * **Signal Strength:** the elastic energy released by each AE event, conventionally. A complete list of AE properties and definitions is provided in [13; 22; 23]. [PERSON] [24] presents an interesting correlation of the described parameters, analysed in the time domain, and their interpretation in terms of information related to the source.