## 3 Experimental Results
### Analysis of Precise Time Alignment Using DTW  
As discussed, this study focuses on elucidating the utilization of low-cost sensors within a wireless SHM framework to accurately distinguish various structural parameters, such as natural frequencies. This is particularly relevant when the practicality of using high-grade reference accelerometers is limited. To validate the research objectives, two types of 3 DOF structures, namely full side panel (denoted as Structure 1) and partition side panel (denoted as Structure 2), were selected for experimental analysis. The proposed scheme, illustrated in Figure 6, was utilized to conduct the experimentation and analyze the collected data. The acceleration time history responses of the two structures for the initial 10-min duration are presented in Figure 11. Figure 11a displays the preprocessed acceleration time history of the full side panel (Structure 1), while Figure 11b exhibits the preprocessed  
Figure 10: Comparison of preprocessed acceleration and aligned acceleration using DTW: (**a**) the preprocessed acceleration time history of \(x\)-axis. (**b**) aligned acceleration time history of \(x\)-axis using DTW. Figure 9: Comparison of the time and frequency domains for three sensors over a duration of 60 s. (**a**) the time history of \(x\)-axis accelerations corresponding to the first five peaks. (**b**) the linear magnitude spectrum of the \(x\)-axis accelerations computed for a 60-s record. (**c**) the spectrogram of the \(x\)-axis accelerations for the same 60-s record. acceleration time history of the partition side panel (Structure 2). Upon analyzing the response of the structures, it is evident that Sensor S03, positioned on the top storey, exhibits the highest magnitude of vibration. Conversely, Sensor S01, located on the first storey, demonstrates relatively lower levels of vibration. Consequently, a meticulous segmentation process was conducted to isolate and analyze three prominent peaks within the preprocessed response data obtained from the two structures. This segmentation process is illustrated in Figure 12. Specifically, for Structure 1, the response data within the time interval of 25 s to 65 s was selected, as depicted in Figure 12a. Similarly, for Structure 2, the response data within the time interval of 35 s to 75 s was chosen, as shown in Figure 12b. It is important to note that the labels S01, S02, and S03 assigned to the sensors in the figures represent their respective positions on the 3 DOF structure, indicating the first, second, and third storeys, respectively. The windowed vibration signals obtained from the aforementioned process were further evaluated by calculating the magnitude spectrum to assess their frequency content. The analysis of Structure 1, conducted on the data collected from the three sensor nodes in each storey, revealed a dominant frequency range of approximately 1.40-1.50 Hz. In contrast, the analysis of Structure 2, based on the data obtained from the three sensor nodes in each storey, indicated a dominant frequency of approximately 1.25 Hz. This discrepancy in dominant frequencies can be attributed to the variation in stiffness between Structure 1 and Structure 2, with Structure 2 exhibiting lower stiffness compared to Structure 1. Furthermore, the utilization of the DTW algorithm played a crucial role in computing the optimal warping path within the aforementioned 40-s duration. This process was instrumental in achieving precise synchronization among the segmented and preprocessed response data from the two distinct structures, resulting in an enhanced spectrum of the windowed vibration signal. Importantly, this synchronization process ensured the accurate alignment of the windowed acceleration data and their corresponding magnitude spectra, leading to a significant improvement in coherence. The synchronized signals are clearly depicted in Figure 13. Figure 13a showcases the resampled acceleration data alongside its corresponding magnitude spectrum for Structure 1, highlighting the effectiveness of the alignment.