## 2 A Cochlea-Inspired Artificial Filter Bank (CAFB) for Structural Health Monitoring
### Optimal Design for CAFB  
In Section 2.1, we described the concept and principle of the CAFB developed in the previous study. In this section, we will explain the CAFB optimization process carried out in the previous study. As previously described, CAFB is a digital filter bank which consists of n number of band-pass filters to select and compress only the interesting range of frequency signals from the entire response of a structure. This kind of band-pass filter shall be designed to be suitable to selectively acquire only the interested frequency range. Figure 4 shows the optimally-designed process of the CAFB for SHM on long and large-scale civil structures. In Step 1, to optimize the CAFB, the interested frequency should be selected as shown in Figure 4. To do that, in the previous study, the interested frequency range of the band-pass filter is set at a frequency lower than 10 Hz. In general, the natural vibration frequency of long and large-scale civil structures is lower than 10 Hz due to the flexible behavior characteristic of the structures. As a result, a lower frequency component than 10 Hz is significant for SHM on long and large-scale civil structures. Figure 4: Process of the CAFB’s optimal design. Figure 3: Concept of the central difference method. In Step 2, to optimize the CAFB, a reference signal is necessary for frequencies lower than 10 Hz, where the reference signal is used for the optimal design of CAFB. For this purpose, in the previous study, the El-Centro seismic waveform is adapted. As shown in Figure 5, since lower frequency components than 10 Hz are predominant in the El-Centro seismic waveform, it is possible to present sub-10 Hz frequency components very well by filtering in the optimal design of the CAFB. It is well known that the El-Centro, Kobe, and Northridge are the leading random seismic waveforms in the construction industry. They represent an unexpected circumstance which may be possible when designing, constructing, and managing structures. To assess the applicability of the IDAQ system for SHM into large structures, therefore, the El-Centro seismic waveform is chosen as an original signal for the optimization of the AFB among the random seismic waveforms mentioned above. Figure 5 below shows El-Centro seismic waveform (SAC name: LA02 (1940, SE)) used as an original signal, which is expressed in the time and frequency responses. According to Figure 5a, a seismic situation lasts for a short period of time (approximately one minute) in the case of El-Centro. In Figure 5b, diverse frequency factors are distributed across the frequency bands. In particular, the interested frequency is concentrated in less than 10 Hz. Here, if the seismic situation in Figure 5 is assumed, the dynamic responses of the target structure is measured as a type which includes the target structure's own frequency factors in the conventional seismic waveform's random frequency factors. For SHM on structures, after all, the target structure's original frequency factors in the random frequency factors should be fully discovered. Since large structures in the construction industry usually have relatively flexible behavioral characteristics, the range of the target mode needed for SHM can be limited to a certain range of frequencies (e.g., below 10 Hz). From this perspective, the El-Centro seismic waveform selected as an original signal could be used as a reference response which can wholly express the large structure's range of frequencies concerned (below 10 Hz). Under the assumption that an El-Centro seismic situation has these frequency characteristics, this study designs the AFB in an optimum manner based on the El-Centro seismic waveform (total time length: 50 s, delta T: 0.02 s, total frame length: 2500). Figure 5: El-Centro seismic waveform for optimizing of CAFB: (**a**) time domain; and (**b**) frequency domain. Meanwhile, when we design band-pass filters to be used in CAFB, generally, it is necessary to define three design elements for the filters, i.e., the number of filters, interval, and bandwidth.