## 3 Results and Discussion
### Discharge of Halocarbon Compounds: Effect of Generated Noise  
As reported in Sections 1 and 2, one of the scopes of the present work was comparing the discharge of the two types of clean agents in terms of emitted noise, which prompted us to test FK-5-1-12 as a representative of halocarbon compounds. The higher cost associated with these agents--also noted as a drawback by DiNenno [2]--led us to run experiments only against one configuration (Table 3), obviously through both standard and silent nozzles (for low ceiling applications). The trends of SPL and disk performance index are shown for representative tests in Figure 10. The overall consistency of these results with the dataset for the nitrogen IG100 discharge through nozzles of similar orifice size (5 mm, Figure 7a,b) seems to somewhat substantiate a statement provided in a well-recognized white paper [4]: _\"The agent used in an extinguishing solution doesn't define per-se the noise level of the system overall\"_. However, it is also worth noting that FK-5-1-12 mass flow rate released through a 6 mm diameter nozzle is remarkably larger than that of nitrogen IG100 through a 5 mm diameter nozzle; in fact, the former is close to nitrogen IG100 mass flow rate issued by a 23 mm diameter nozzle (Figure 7i,j), due to its higher density. Even though the released volumetric flow rate can be assumed to be very similar, it appears that almost the same noise is generated by a larger mass flow rate of FK-5-1-12 as that generated by a smaller mass flow rate of nitrogen IG100, when the discharge is operated at the same pressure.  
The ability of silent nozzles to reduce noise down to harmless levels is also proven by both the profile of disk performance index (Figure 10) and by SPL presented as a function of frequency, as in the maps of Figure 11. Solid-state drives are obviously not affected, and neither are HDDs with high rates of rotation (i.e., HDD2) if orifice diameter is smaller than 10 mm, yet HDDs with lower speed (i.e., HDD1) exhibit a considerable decay in their performance if standard nozzles are used to release halocarbon compounds, as opposed to the discharge produced by silent ones. However, a remarkable effect can be observed in the results from tests with the silent nozzle (Figures 10b and 11b): SPL shows a sharp  
Figure 9: Performance index as a function of maximum SPL reached in every tested condition with nitrogen IG100.  
decrease after the expected peak upon the onset of discharge (Section 3.1), then followed by another increase up to a plateau-like condition.  
This trend is arguably typical of the discharge of liquified gases such as FK-5-1-12, where another gas is also added to superpressurize the agent (i.e., nitrogen, Section 2.1). Firstly, the liquid phase (i.e., FK-5-1-12) is released; as the agent becomes almost exhausted, the discharge becomes nitrogen-laden. Thus, in this last part of the discharge, the Reynolds number changes as a result of a change in the released chemical substance, which reflects the involved thermophysical properties. The tested silent nozzles demonstrated a more effective damping action on the noise emitted through the actual FK-5-1-12 discharge (first phase), whereas the trend already observed in tests with nitrogen IG100 (e.g., Figure 7d,f,h,j) occurred once the gas used to superpressurize the agent was finally released (second phase). This outcome--also shown by the high-frequency (i.e., 1-4 kHz) SPL dataset of Figure 11b--suggests a certain dependence of the sound-absorbing performance on the clean agent type.