## 2 Materials and Methods
### Data Processing and Analysis  
The analysis of the acquired experimental dataset pursued two main purposes through dedicated approaches:  
* Determining and comparing the acoustic performance of standard and silent nozzles; to this end, SPL is presented as a function of released flow rate and time for both tested clean agents and for the whole set of investigated orifice diameters; the results from spectral analysis by octave bands are also included. * Evaluating the effect of SPL on the different tested hard drives and highlighting quantitatively the performance loss in the same configuration towards a comparison between standard and silent nozzles; to this end, an index of the disk behavior and performance through the discharge is proposed as inspired by a recent report [20], which produces measurements of read/write speed and includes that under the SPL due to white noise (i.e., baseline speed). \[Disk\;performance\;index=\frac{Disk\;speed}{Disk\;baseline\;speed}.\] (1)  
Moreover, the reliability of the acquired dataset and the overall experimental approach was challenged by the following:  
* Checking the applicability of the point source model [22], which supports the proposed design of the experimental setup, where SPL was measured at the location described in Section 2.1 and shown in Figure 1, without resorting to an array of microphones, as in [15]; this preliminary evaluation is included in Section 2.3. * Evaluating standard deviation [23] of the acquired data--mainly the SPL dataset as a function of time or released flow rate over repeated tests under the same configuration--in order to assess repeatability. As already mentioned in Section 2.1, the released flow rate was calculated for both the inert gas and the halocarbon compound discharge. Notably, a classic model was employed \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Set**} & \multirow{2}{*}{**Nozzle Type**} & \multirow{2}{*}{**Clean Agent**} & \multicolumn{2}{c}{**Nozzle-to-Sound**} & \multirow{2}{*}{**Nozzle**} \\  & & & & **Distance (m)** & \\ \hline \multirow{4}{*}{I} & Silent & IG55 & 2.5 & 5 \\  & Silent & IG55 & 1.25 & 5 \\  & Silent & IG55 & 5 & 5 \\  & Silent & IG55 & 1–2–4–8 * & 5 \\  & Standard & IG55 & 1–2–4–8 * & 5 \\  & Silent & IG100 & 1 & 5 \\  & Standard & IG100 & 1 & 5 \\  & Silent & IG100 & 1 & 10 \\  & Standard & IG100 & 1 & 10 \\  & Silent & IG100 & 1 & 14 \\   II & Standard & IG100 & 1 & 14 \\  & Silent & IG100 & 1 & 17 \\  & Standard & IG100 & 1 & 17 \\  & Silent (low & & & \\  & ceiling & IG100 & 1 & 17 \\  & applications) & & & \\  & Silent & IG100 & 1 & 23 \\  & Standard & IG100 & 1 & 23 \\  & Standard & FK-5-1-12 & 1 & 6 \\  & Silent (low & & & \\  & ceiling & FK-5-1-12 & 1 & 6 \\  & applications) & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Detailed list of the investigated configurations (*, multiple data series recorded for each tested distance at various angular locations). to calculate the former, with the flow rate of a jet (compressible flow, assuming an ideal gas) issued through an orifice [24] being expressed by Equation (2):  
\[\dot{Q}_{m}=\gamma\cdot A_{m}\cdotp_{m,CR}\cdotw_{m,CR},\text{ if }\beta_{ noz}\leq\beta_{CR}, \tag{2}\] \[\dot{Q}_{m}=\gamma\cdot A_{m}\cdot\sqrt{\frac{2k}{k-1}\cdotp_{0} \cdotp_{0}\left(\beta_{noz}\frac{2}{k}-\beta_{noz}\frac{k+1}{k}\right)},\text{ if }\beta_{ noz}>\beta_{CR},\]  
where \(\dot{Q}_{m}\) is mass flow rate, \(\gamma\) is the discharge coefficient of the nozzle, \(A_{m}\) is the surface area of the outlet section, \(\rho\) is density, \(w\) is the average jet velocity, \(\beta\) is defined as the ratio between pressure downstream of the nozzle (i.e., atmospheric pressure) and upstream of the nozzle, \(w_{m,CR}\) refers to the gas velocity at nozzle orifice, \(CR\) refers to critical value of air at 20 \({}^{\circ}\)C temperature and \(noz\) refers to nozzle.