## 3 Results  
### Phase 1 Short-Term Experiments and Results  
For phase 1 tests, an intrinsically safe version of the existing laboratory acoustic instrument was developed. This system consisted of four microphone arrays and a speaker, which were fastened on a wood panel, an IBM Lenovo Thinkpad tablet, a PCMCIA sound card (model: Digigram VXPocket 440) and a box of batteries with an amplifier circuit. This system and the method for its installation in phase 1 are shown in Figure 4. The system was initially tested in the laboratory prior to field trial. The acoustic instrument was then installed temporarily in an existing live CSO chamber. Figure 5 presents photographs of one of the two CSO sites and arrangements for the static screens. Four screen conditions were tested in these experiments: original unclean, first level of cleaning, second level of cleaning and fully clean. The unclean condition corresponded to a heavily blocked screen, which was found when the CSO chamber was opened. The first and second levels of cleaning conditions were simulated by using high pressure water jetting to gradually reduce the blocked condition in controlled portions of the CSO screen. Effectively, this means that in the field trial, we progressively cleaned the amount of debris blocking the screen perforations. This enabled us to simulate in the field four screen conditions from fully blocked condition to partially blocked condition and, finally, to fully cleaned condition. In every condition, the screen was photographed and an acoustic measurement was taken until all the debris was removed to a desired level. This would enable the acoustic attenuation data for a known percentage of screen contamination to be obtained to validate the proposed technology and calibrate the acoustic response of the CSO chamber against visual images of the condition of the CSO screen. The first field trial site was located under a public highway located in West Yorkshire, as shown in Figure 5. This CSO chamber contained a 820 \(\times\) 3380 mm\({}^{2}\) screen. As can be seen from Figure 5, this CSO chamber can be accessed through three manhole lids and the sensor was placed on the top of the middle access as a major part of the screen was underneath it. The four screen conditions for this site are illustrated in Figure 6.  
Figure 4: Installation diagram of the acoustic sensor in the chamber. which was twice the size of the previous screen. Figure 7 depicts the site topology, interior and exterior of the CSO chamber, and the location where the sensor was installed. The four conditions of the CSO screen are shown in Figure 8 to give the indication of level of the cleanliness of the CSO screen: from unclean condition to fully clean condition. Figure 8: Phase 1: CSO screen conditions on site 2 in West Yorkshire. (**a**) Unclean condition; (**b**) first level of cleaning; (**c**) second level of cleaning; (**d**) fully clean. Figure 7: Phase 1: Field trial site 2 in West Yorkshire. In both field trials, each of the test conditions was recorded five times for validation purposes. The average of the sound pressure level data was calculated and curved, fitted using the LMS method (Equations (3)-(5)). The coefficients \(b_{0}\) and \(a_{0}\) were estimated and plotted for the four CSO screen conditions, as shown in Figure 9. For site 1, the value of \(b_{0}\) gradually increased from \(-6.5\) for blinded condition to \(-6.0\) for fully cleaned condition in the first field trial. This corresponds to an \(8\%\) increase in the value of \(b_{0}\) from the case of the chamber with a fully blinded screen to that with a fully clean screen. For site 2 with a larger screen size, the values of \(b_{0}\) are higher compared to those measured on site 1 with the smaller screen size. For site 2 experiments, the value of \(b_{0}\) increases from \(-3.9\) for the case with a fully blinded screen to \(-2.8\) for cases with a fully cleaned screen, which constituted a \(39\%\) change.