3.2.2. Analysis of Noise Parameters

The first experiment consisted in the recording and analysis of noise at various sections of the bridge. As a data collection system, a 4-channel external USB I/O module "E20-10", manufactured by "L-Card" company (Moscow, Russia), was used. This system provides the consistently recording data with the sample rate 2.5 MHz for every channel. Two sensors of GT200 type (130–200 kHz) were connected to the module, which were installed on the bridge. The signal from the sensor went to the preamplifier with a gain of 26 dB and a frequency filter of 30–500 kHz.

The AE data were taken in 3 different zones of the bridge structure (Figure 5).

Zone I: In the area of the expansion joint near the shore support (Figure 5a), sensors were installed inside the beam structure near the bolted joints. The distance between the sensor and the expansion joint is 5 m. Zone II: In the area of attachment of the beam and the support (Figure 5b), AE sensors were installed on the lower girder and on the support. Zone III: In area of the exit hatch section of the box beam (Figure 5c), AE sensors were installed inside the beam structure at a distance of 3 m from each other symmetrically with respect to the hatch.

For all signals from passing vehicles, the energy is concentrated in the frequency region up to 60 kHz.

**Figure 5.** Experiment scheme: (**a**) Zone I; (**b**) Zone II; (**c**) Zone III. AE sensors are marked as yellow-green circles.

Vibration of the bridge from passing vehicles was the most common type of noise (Figure 6). Some characteristic features of these signals are a long duration (1–3 s) and relatively high amplitudes of about 56–90 dBAE. The primary reason for the appearance of these signals is the fact that the passage of the cars leads to vibration and friction of poorly fixed structural elements.

**Figure 6.** Acoustic emission (AE) signals from passing vehicles: (**a**) Zone I; (**b**) Zone II; (**c**) Zone III.

Signals caused by impacts of structural elements have amplitudes similar to signals from passing vehicles, of the order of 56–90 dBAE but they have 1–2 times shorter duration, not exceeding 50,000 μs (Figure 7). The energy of the signals, as a rule, lies in the frequency range up to 200 kHz. The primary cause of these signals is the vibration of weakly fixed structural components of the bridge, as a result of which their collision occurs. An example may be the impact of a ladder or hatch on the bridge girder or the impact of wheel sets on the expansion joint [14].

**Figure 7.** AE signals caused by impacts of structural elements: (**a**) Zone I; (**b**) Zone II; (**c**) Zone III.

## 3.2.3. Analysis of AE Signals Propagation

Hsu-Nielsen source was used to estimate signal propagation properties. The measurements in this experiment were carried out using the A-Line 32D system (Interunis-IT) with threshold data acquisition. The ADC sampling frequency was 2 MHz. The following values were obtained: AE wave propagation speed is 2838 m/s, attenuation coefficient in the far field zone is 1.65 dB/m. Based on the obtained values of the attenuation coefficient, noise level and amplitudes of signals from the AE signal simulator, the maximum distance between the AE sensors for linear or planar location was determined. This value lies in the range from 11 to 16 m.

An acoustic contact quality was also estimated in zone II (Figure 5b). In the course of the experiment it was revealed that the signals simulated in the girder region are not detected by the AE sensor mounted on the support, which indicates that there is no acoustic contact. Thus, for a full monitoring of the bridge, separate AE testing of both the supports and the main upper part of the bridge is necessary.
