# Acoustic Emission Monitoring in Prestressed Concrete:
## 4 Results
### Rebound Hammer Impacts on Girders  
In Figures 8 and 9, the experiments utilizing the rebound hammer on the girders are illustrated. These tests involved generating signals on the concrete surface orthogonal to the tendon axis and at the prestressing steel at locations where the cross-sections of the tendons were accessible. The figures provide a side-by-side comparison of these two signal input methods for the girders \(B\) of each structure. Additionally, Table 3 aggregates the statistical parameters derived from these measurements. The signal profiles resulting from the rebound hammer impulses share general characteristics with those of wire break events, demonstrating similar behaviour in terms of attenuation and signal propagation. Despite these similarities, closer examination also reveals differences in the attenuation coefficients between the two types of sources. Notably for the frequency bands, in some cases signals from rebound hammer impulses tend to exhibit slightly higher attenuation compared to those from wire breaks, e.g., for the Witzenhausen girder. This observation, however, is not confirmed by the analysis of time-domain features, which suggests the opposite for Stolpe and Witzenhausen. This discrepancy might originate from the methodological differences in data processing between the two analyses. Specifically, for the frequency band analysis only the initial part of the signal was considered, focusing on the signal onset for extraction and evaluation. In contrast, the time-domain feature analysis took the entire signal into account. Moreover, the comparison between the two methodologies is inherently constrained due to the composite nature of time-domain features, which superimpose all frequency components without delineating their distinct contributions, notably in the context of peak amplitude. Given the practical importance of time-domain features in assessing attenuation, this feature should be considered as decisive in the analysis. In the comparison of excitation types with the rebound hammer, the time-domain features for Stolpe and Witzenhausen exhibit higher attenuation coefficients when the excitation is applied to the tendon, thus emphasizing the analogy to wire breaks. Yet the time-domain features reveal the reverse pattern, except for the Roding girder, where the opposite can be observed, and the inversion represents a \"safer\" scenario for extrapolating results. In this case, the attenuation coefficients obtained with the rebound hammer are higher than those for wire breaks. However, the quality of correlations for this case are the weakest, as illustrated in Figure 9.  
These findings highlight that both wire break and rebound hammer excitations exhibit very similar attenuation behaviour, independent of the method of excitation. Nonetheless, it also underscores the complexity in interpreting individual measurements. Consequently, a statistical examination of the data will be undertaken in Section 5 to facilitate a better  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline
**Partial Spectra** & \multicolumn{6}{c}{**Attenuation Coefficient (dB/m)**} & \multicolumn{6}{c}{**Residual Standard Deviation (dB)**} \\ \cline{2-13}
**(kHz)** & \multicolumn{3}{c}{**Stolpe (BT500)**} & \multicolumn{3}{c}{**Roding**} & \multicolumn{3}{c}{**Witzenhausen**} & \multicolumn{3}{c}{**Stolpe (BT500)**} & \multicolumn{3}{c}{**Roding**} & \multicolumn{3}{c}{**Witzenhausen**} \\
**and Features** & **A** & **B** & **A** & **B** & **A** & **B** & **A** & **B** & **A** & **B** & **A** & **B** \\ \hline
0–10 & \(-4.7\) & \(-4.5\) & \(-5.8\) & \(-6.9\) & \(-4.2\) & \(-5.3\)  
understanding of how results can be transferred between the two sources based on a broader perspective. This analysis will prioritize time-domain features, reflecting their widespread adoption in both academic and practical applications.