## 3 Results
### Phase 2 Longer Term Experiments and Results
The microphone array electronics were sealed in an airtight plastic enclosure. The acoustically sensitive parts of the microphones were vented to the atmosphere. All the connections between the parts of the system were made waterproof in order to prevent the water vapours present in the CSO chamber from condensing and short circuiting the electronics, causing a spark. Figure 12 also shows the location of the field trial site. As can be seen, it was located within a grassed recreation ground in West Yorkshire. The CSO chamber on this site was equipped with a static peak screen with dimensions of 505 \(\times\) 1480 mm\({}^{2}\) and a Hawkeye telemetry system [29; 30] to measure the water level. This chamber could be accessed via two manholes, as shown in Figure 12. An ATEX-approved enclosure, which contained the data acquisition electronic devices, was installed in chamber 1, while the speaker and microphone arrays sensor bar were mounted in the centre position of the CSO screen within chamber 2. The sensor was programmed to carry out measurements and store data  
Figure 11: Schematic diagram of the acoustic sensor in the chamber. Figure 12: Practical implementation of the proposed acoustic system used in phase 2 field experiments. in the form of audio files at two-hour intervals. The CSO chamber was visited once every two weeks. During these visits, the conditions of the equipment were thoroughly checked and the SD memory card was replaced. The analysis of the data collected on the SD card was carried out back in the laboratory to study the consistency in the sensor operation and changes in the chamber over the measurement period. By examining of the conditions of the screen taken over a six-month period, it can be noticed that the CSO screen condition did not change significantly. This is reflected in a relatively small variation in the absolute values of the coefficients \(a_{0}\) and \(b_{0}\), except the times when the water level in the chamber changed considerably, affecting the screen condition. Some interesting observations from the change in the values of the coefficients \(a_{0}\) and \(b_{0}\) can be made. This change is illustrated in Figure 13, which shows examples of the time histories of the coefficients \(a_{0}\) and \(b_{0}\) measured over the six-month period. This figure also presents the water depth data, which were recorded at 15-min intervals with the Hawkeye telemetry system. The water level data are helpful to interpret the observed behaviour of the coefficients \(a_{0}\) and \(b_{0}\). During a 14-day measurement period which started on the 18 th of January, there was not much rain around the instrumented site. Therefore, the ambient (dry weather flow) water level varied little between 70 mm and 100 mm (see Figure 13a). This is reflected in relatively constant values of the coefficients \(a_{0}\) and \(b_{0}\), which did not change by more than \(\pm\)2.5% from their averages. This level of variation in the value of the coefficients \(a_{0}\) and \(b_{0}\) was a typical measure of the reproducibility of the proposed acoustic sensor. On the 23 rd of March, there was a storm event which caused a substantial rise in the water level in the chamber, up to approximately 700 mm for a period of 12 h (see Figure 13b). Figure 13: Examples of data collected in phase 2 over a six-month period: (**a**) ambient dry weather flow; (**b**) a longer term storm event which does not result in a change in screen condition; (**c**) a short-term storm event which resulted in a change in screen condition. This level variation was clearly detected with the corresponding changes in the values of coefficients \(a_{0}\) and \(b_{0}\). When the water level in the chamber increased, the volume of the chamber was reduced. This resulted in a 5 dB decrease in the sound pressure level, which was reflected in the 5 dB decrease in the value of the coefficient \(a_{0}\) (see Figure 13b). As the level of water in the chamber increased, its volume reduced.