## 3 Results and Discussion
### Overview of the Relationship between Flow Rate and Noise  
Aiming at a deeper understanding of the effect of released flow rate on the noise emitted by nozzles used in gas-based fire suppression, the SPL dataset was related to mass flow rate calculated by the approach proposed in Section 2.2. The results are presented in Figure 12 for both standard and silent nozzles, those designed for low ceiling applications are included with regard to FK-5-1-12 discharge. As already apparent from the list of tested configurations (Table 3), most of the data are available for standard and silent nozzles with nitrogen IG100 discharge, whereas datasets for the other investigated conditions (i.e.,  
Figure 11: SPL as a function of time and OTO (One-Third Octave bands) frequency throughout FK-5-1-12 discharge by (**a**) standard and (**b**) silent nozzle for low ceiling applications with 6 mm orifice diameter. Figure 10: SPL and disk performance index as a function of time throughout FK-5-1-12 discharge by (**a**) standard and (**b**) silent nozzle for low ceiling applications with 6 mm orifice diameter. all those involving FK-5-1-12 discharge) consist of a smaller population. Therefore, the degree of statistical significance varies between datasets. Data are plotted in Figure 12 with reference to the employed orifice diameter; the best-fitting curve was also generated as a power law (\(y=ax^{b}\)) and is also included in the plots. While formulating the fitting curve, SPL values below 60 dB were discarded, as they were considered almost white noise, and so were flow rate values below 0.15 kg/s, since measurements of the various involved parameters (e.g., pressure, temperature, mass) were deemed unreliable at the onset of the discharge as a result of the steep transient growth. The values of scaling factor \(a\) and exponent \(b\) are presented in Table 4 for each analyzed condition. The noise-damping effect of silent nozzles is evident in the diagrams of Figure 7. Moreover, it is worth observing that the larger the released mass flow rate, the higher the emitted noise, a result that holds for all the tested conditions and previously discussed in Section 3.1. However, and rather interestingly, the profiles suggest that the emitted noise has an ever-decreasing growth, evidence of which is also shown by the best-fitting curves and their decreasing slope as mass flow rate increases. This observation also indirectly arises from SPL trends shown in Figure 7: the peaks exhibit similar values as orifice  
Figure 12: SPL as a function of released mass flow rate, with data points (referred to orifice diameter) and best-fitting curve: (**a**) standard nozzle and nitrogen IG100 discharge; (**b**) silent nozzle and nitrogen IG100 discharge; (**c**) standard nozzle and FK-5-1-12 discharge; (**d**) silent nozzle for low ceiling applications and FK-5-1-12 discharge. diameter shifts towards the larger sizes, regardless of whether standard (Figure 7g,i) or silent (Figure 7h,j) nozzles are employed. This behavior is consistent with the discussion proposed in Sections 3.1 and 3.2 with regard to the relationship between noise and Reynolds number, which hints at turbulence intensity of the issued jet: as fully developed turbulence is approached, the emitted noise tends to increase under a milder slope. The comparison of Figure 12c and Figure 12b,d suggests that FK-5-1-12 generally yields lower SPL than nitrogen IG100, which is consistent with the findings described in Section 3.2: higher mass flow rate of FK-5-1-12 generates approximately the same noise as a lower mass flow rate of nitrogen IG100, since FK-5-1-12 density is higher than nitrogen density.