## 4 Conclusions  
Most hard drives are subject to performance decay as noise beyond a certain threshold --usually identified as 110 dB [4; 16; 17; 18; 19; 20]--reaches them. The problem is particularly relevant when fire protection of data centers is involved, since gas-based suppression systems are widely employed for this purpose: the noise emitted by the nozzles through a discharge of the selected clean agents (i.e., inert gases or halocarbon compounds) is well-known to damage the electronic equipment within the compartment. Hence, silent nozzles are designed and often used to address this undesired effect; however, the open literature currently offers few scientific studies on the subject. Arguably an unprecedented effort, an experimental approach was devised and developed to quantitatively assess the noise generated by both standard and silent nozzles in a comparative fashion, together with the performance decrease in the hard drives throughout several discharge conditions. To this end, a preliminary evaluation of repeatability and ability to properly capture the involved phenomena was carried out against the proposed experimental setup and methodology. Notably, the employed nozzles proved reasonably suitable for applying the point source model [22], thus allowing us to neglect the directionality of the emitted noise. The developed approach may be expanded by repeating the same measurements (i.e., sound pressure level and performance decay) at various angular locations, following the guidance from a previous work [15], which applies the here-presented setup as a reference to evaluate acoustic performance of virtually any nozzle for gas-based suppression system. A series of experiments were conducted, mostly releasing nitrogen IG100, but also FK-5-1-12, which allowed a comparison between the two types of clean agents. Constant pressure was imposed throughout a large portion of each discharge event using constant flow valves; various orifice diameters were tested, which translates into variations in the released flow rate. Moreover, three hard drives were used as targets to test their performance over a discharge event: two HDDs with different rates of rotation and one SSD, which was almost unaffected by external noise. As expected, silent nozzles proved capable of limiting the generated noise down to values less harmful to hard drives. Notably, each discharge--whether produced by standard or silent nozzles--exhibited an increase in the emitted noise up to a maximum \begin{table}
\begin{tabular}{c c c} \hline \hline
**Tested Condition (Nozzle Type, Clean Agent)** & **Scaling Factor \(a\)** & **Exponent \(b\)** \\ \hline Standard, nitrogen IG100 & 124.4 & 0.06788 \\ Silent, nitrogen IG100 & 112.3 & 0.1873 \\ Standard, FK-5-1-12 & 109 & 0.05422 \\ Silent for low ceiling applications, FK-5-1-12 & 80.49 & 0.2596 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Constant scaling factor and exponent for the fitting curves generated through a power law formulation. from its onset, usually followed by a plateau-like trend and the final decay. Fluctuations occurred possibly as a result of pressure oscillations within the controlled phase (i.e., the release at virtually constant pressure). This behavior hints at the known dependence of sound emitted by gaseous jets flowing through an orifice on turbulence intensity, thus being ultimately correlated to Reynolds number. This relationship also supports the discovered increase in peak noise level as the released flow rate increased (i.e., as larger orifice diameter was employed in the tests). Nevertheless, the difference between the reached noise maxima became less remarkable as the released flow rate increased, which also suggests that once the Reynolds number becomes sufficiently high that fully developed turbulence be reached, noise intensity increases along a milder slope.