# Experimental Investigation on Effective Distances of Acoustic Emission in Concrete Structures
## 2 Experimental Program
### Effect of Propagation Distance  
As the stress wave from a source propagates outward through the surrounding media, the energy per unit area decreases owing to numerous factors such as geometric spreading, internal friction, scattering, and mode conversion. Here, this loss of signal amplitude is referred to as attenuation. Because high-frequency waves have short wavelengths irrespective of the medium or wave type, their attenuation is greater than that in low frequency waves.  
Although the frequencies generated by AE cover a wide spectral range, the typical frequency ranges of 1 to 500 kHz are used according to the material properties. The main frequency of the AE signal is often determined based on the AE sensors used for measurement; nevertheless, most AE signals induced by cracks or damage tend to be of high frequency [24; 31]. For this reason, AE waves are largely affected by the attenuation effect according to the propagation distance; therefore, it is important to analyze this effect in concrete structures.  
Thus, representative AE parameters, including signal strength for the intensity parameter and duration for the waveform characteristics parameter, were quantitatively investigated according to the wave propagation distance.  
As shown in Figure 12, the AE signal strength in all experimental cases decreased according to the propagation distance. The signal strength suddenly decreased at the 10 m point and did not show a significant value after the 16 m propagation distance. This trend was shown regardless of the impact energy. At 4 m, however, the signal strength was less affected by the propagation distance in the case of large impact energy than that of small impact energy. For example, for ball 2, the signal strength decreased by 43% when the impact energy was 2.2 J but decreased by 58% when the impact energy was 1.1 J. The duration also decreased as the propagation distance increased (Figure 13). The reason for the decreases in both parameters is that the intensity of the AE signal is weakened by the attenuation effect according to the propagation distance. The degree of duration decrease was lower in the case of large impact energy, showing decreases of 44% and 37% when the impact energy was 10.2 J and 5.1 J, respectively, at the propagation distance of 4 m. However, the duration did not decrease abruptly at 10 m and still showed a significant value after the propagation distance of 16 m in all experimental cases. Moreover, unlike the experimental results of the signal strength, the distance ranges included increases with distance, such as that at propagation distances of 4 m and 17 m for ball 2. This phenomenon indicates that the duration parameter is more sensitive to noise such as reflected waves than main AE signals, leading to uncertain experimental results. For this reason, the attenuation effect cannot be accurately estimated by checking only the duration parameter.  
According to the experimental results, the attenuation effect is more accurately estimated by the intensity parameter than the waveform characteristics parameter. In addition, the attenuation effect is also affected by the impact energy, as evidenced by the negligible attenuation of the large wave energy according to the propagation distance. Furthermore, this effect was smaller when the propagation distance was long.  
Figure 12: Signal strength according to distance.  
Figure 13: Duration according to distance.