# Experimental Investigation on Effective Distances of Acoustic Emission in Concrete Structures
## 4 Discussion  
When damage occurs in structures, the main AE signals, as well as other noise such as reflected acoustic waves and vibration, are generated [32]. Most of acoustic signals induced by cracks or damage appear to be of high frequency, whereas the other noise has relatively low frequency. Attenuation increases with frequency; therefore, at large distances from a source, only the low-frequency components of the event will be observed. It is important to analyze the maximum propagation distance of the main AE signals because the aim of the monitoring system is to detect the damage in the structure by measuring these signals. Considering this fact, we newly defined an effective AE distance for yielding the maximum propagation distance of the main AE signal. That is, if the AE signal is measured at a distance longer than the effective distance, it can be determined as a noise signal. Among the AE parameters, amplitude was chosen as an indicator for suggesting the effective distance. The following amplitude variations according to the propagation distance were investigated for representative experimental cases: very small (E = 0.16 J), small (E = 0.33 J), large (E = 2.2 J), and very large (E = 5.1 J) impact energy (Figure 14). In small energy cases (E = 0.16 J, 0.33 J), the amplitude was 100 dB of the measurement limit until the propagation distance reached 2 m; then, it decreased according to the distance. However, when the impact energy was large (E = 2.2 J, 5.1 J), amplitude maintained a constant saturated value until the distance of 8 m was reached, and then it decreased. Contrary to general expectations, the amplitude increased as the propagation distance increased from 16 m to 17 m. This uncertain result is attributed to the noise and weak main signal caused by the attenuation effect with a long propagation distance. Figure 14: Amplitude according to distance. The experimental results also showed that the propagation distance at which the amplitude of AE begins to decrease from the maximum value varies depending on the impact energy. Thus, the AE signal can be propagated at long distances if the signal intensity is strong. Here, the experimental results of the amplitude at impact energy of 2.2 J were used as a representative case for showing effective distance. The signal reception levels were divided into four steps according to the amplitude values: perfect (above 100 dB), good (80-100 dB), normal (60-80 dB), and weak (40-60 dB). The propagation distances matched with each level, as shown in Figure 15.  
With the propagation distance range at perfect and good levels, the measured AE signal can be regarded as the main AE signal from the material damage. However, those at the normal and weak levels are less reliable because the intensity of the signal is weakened by the attenuation effect. Thus, in the representative case, the effective distance was 11 m.  
The effective distance differed depending on the impact energy. Figure 16 plots the propagation distances at each level according to the impact energy, where the effective distance is a propagation distance below the good level line. The effective distance increased as the impact energy increased, which indicates that a greater impact to the structure results in longer propagation of the effective AE signal. However, although the signal intensity increased, the increase in the effective distance was limited because the damage signal is of high frequency, and the AE wave is finally attenuated at a particular distance regardless of its intensity. Figure 16: Effective distance according to impact energy. Figure 15: Four levels of signal reception based on AE signal intensity. In the field, AE signals are obtained by installing a suitable AE sensor, usually numerous sensors in an array, in locations to facilitate detection of any activity that might originate in the structure under study.