## 2 Intrinsic Characteristics of AE Method Controlling AE Sensor Design  
The AE method is a well-known nondestructive evaluation (NDE) method for (a) detecting the initiation of damage, (b) pinpointing the location of damage site, (c) qualitatively assessing the severity of damage and (d) classifying the damage mode using pattern recognition tools. Typically, resonant type AE sensor with narrowband frequency range is selected for (a), (b) and (c). While there are numerous examples using resonant AE sensors for the classification of damage modes, the proper source classification requires wideband or multifrequency AE sensors. The AE source (related to energy release) and the structural medium carrying AE source to AE sensor (related to attenuation) control tuning the AE sensor into the right frequency, bandwidth, directional dependence and sensitivity. Once the target AE source and energy release are identified, the AE sensor characteristics can be defined. The AE sources depend on the materials' composition. For instance, crack growth is a common AE source in metals while fiber breakage, matrix cracking and delamination are common AE sources in composites. Table 1 shows examples of AE sources in solids. As the structural medium and the defect mode are related, AE sensor should be designed by taking their interaction into consideration. As shown in Table 1, when a material has porous, anisotropic and viscoelastic properties, a low-frequency AE sensor is preferred to reduce the influence of attenuation and increase signal-to-noise ratio [5]. \begin{table}
\begin{tabular}{c l l l} \hline \hline
**Material** & \multicolumn{1}{c}{**Common AE Sources**} & \multicolumn{1}{c}{\begin{tabular}{c} **Common AE Frequencies** \\ **and Sensor Types** \\ \end{tabular} } & **Reference** \\ \hline Metals & \begin{tabular}{l} Plastic deformation, internal friction, tensile crack, \\ shear crack \\ \end{tabular} & \begin{tabular}{l} Wideband, 100–900 kHz \\ High frequency resonant \\ \end{tabular} & [6; 7; 8] \\ \hline Composites & \begin{tabular}{l} Matrix cracking, matrix splitting, matrix crazing, \\ matrix lacerations, delamination, disbonding, \\ fiber fracture, fiber pull-out and fiber/matrix \\ interfacial failure \\ \end{tabular} & \begin{tabular}{l} Wideband to differentiate \\ damage modes \\ \end{tabular} & [9; 10] \\ \hline Concrete & \begin{tabular}{l} Shear crack, tensile crack, corrosion \\ \end{tabular} & \begin{tabular}{l} Resonant, 40–100 kHz \\ frequency range \\ \end{tabular} & [11] \\ \hline Wood & \begin{tabular}{l} Longitudinal crack, fracture of fibers, inter-annual \\ growth-ring crack \\ \end{tabular} & Less than 100 kHz & [12] \\ \hline \begin{tabular}{l} Granular \\ materials \\ \end{tabular} & \
\begin{tabular}{l} Grain-to-grain impact, fracture, solid and liquid \\ bond rupture, grain friction \\ \end{tabular} &
\begin{tabular}{l} Low frequency resonant \\ near 20 kHz \\ \end{tabular} & [13] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Common AE sources in solids and typical AE sensors selected. Different defects generate different strain energy releases, which influence the selection of AE sensor sensitivity. Most literature selects the AE sensor based on previous literature, space limitation that controls the size of the specimen and available off-the-shelf sensors. A more physics-based approach is needed for selecting the proper AE sensor for sensing a particular defect in a material and energy release rate. In that aspect, state-of-the-art numerical models are combined with fracture mechanics in recent studies [14; 15; 16]. AE sensors are designed as resonant or broadband type. Resonant AE sensors, typically in the range of 20-500 kHz, have high quality (Q) factor (low damping), leading to narrow frequency bandwidth and high sensitivity. As they modify the input signal, they have limited applications for AE source characterization.