## 2 Materials and Methods
### Experimental Setup and Procedure
The measuring instruments employed in the experiments and their characteristics (e.g., acquisition frequency, accuracy) are summarized in Table 1; the characteristics of the used hard drives--both HDD and SDD--are reported in Table 2, with specific focus on rate of rotation (HDD only) and nominal speed. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Measured** & **Acquisition** & **Instrument/** & **Data Acquisition** \\
\begin{tabular}{c} _NI 9208_ board by \\ National Instruments, \\ 16 channels (current) \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Employed measuring instruments and relevant characteristics. Figure 4: Technical sketch of the employed nozzles, with values provided referring to the orifice diameter: (**a**) standard; (**b**) silent; (**c**) silent for low ceiling applications; the marked components are (**1**) nozzle body; (**2**) spring; (**3**) orifice; (**4**) holes; (**5**) sound-absorbing layer. The tested configurations are reported in Table 3. Notably, the performed experiments can be subdivided into three types, all of which were conducted on both standard and silent nozzles:  
* The first set was aimed at assessing the reliability of the experimental setup and measurements--mostly through statistical analysis--together with evaluating the potential directionality of SPL; these tests were carried out employing an inert gas mixture (i.e., IG55, 50% argon and 50% nitrogen) with a fixed nozzle diameter (5 mm) and pressure at the nozzle outlet; measurements were taken at various values of the distance between the nozzle and the sound level meter, also varying the angle of inclination of the sound level meter with respect to the nozzle. * The second set was aimed at investigating the discharge of inert gases (nitrogen IG100, in the present case); these tests were carried out at varied orifice diameters, between 5 and 23 mm, while keeping the distance between the nozzle and the sound level meter constant and equal to 1 m (Figure 1). * The third set was aimed at investigating the discharge of halocarbon compounds (FK-5-1-12, in the present case); these tests were carried out only using nozzles with 6 mm orifice diameter, given the higher cost of the involved substance; the distance between the nozzle and the sound level meter was also kept constant and equal to 1 m (Figure 1). At least three repeated tests were conducted for each configuration listed in Table 3 to acquire a statistically significant dataset. \begin{table}
\begin{tabular}{c c c c c} \hline
**Measured** & **Acquisition** & **Instrument/** & **Data Acquisition** \\
**Parameter** & **Frequency (Hz)** & **Sensor** & **Board** \\ \hline Temperature & 10 & \
\begin{tabular}{c} Thermocouple T type, \\ 1.0 mm bead diameter, \\ accuracy in accordance with standard IEC 60584 \\ Sound level meter \\ _HD2010 UIC/A_ by \\ Deltaohm, with spectral \\ analysis by octave bands \\ from 31.5 Hz to 8 kHz \\ Hard drive \\ performance & 2 & – \\ \end{tabular} &
\begin{tabular}{c} _NI 9212_ board by \\ National Instruments, \\ 8 channels \\ (thermoeuple) \\ _HD2010 UIC/A_ by \\ Deltaohm, with spectral \\ analysis by octave bands \\ from 31.5 Hz to 8 kHz \\ Data recorded by _HD_ \\ _Speed_ software, \\ version 1.7 \\ \end{tabular} \\ \hline \end{tabular}
\end{table} 
Table 1: _Cont._ \begin{table}
\begin{tabular}{c c c c c} \hline
**Reference** & **Manufacturer** & **Model** & **Type** &
\begin{tabular}{c} **Rate of** \\ **Rotation (rpm)** \\ \end{tabular} & **Nominal Speed** \\
**Name** & **Toshiba** & \
\begin{tabular}{c} _OCZ-VERT_ \\ _EX3 MI SCSI_ \\ _DeskStar_ \\ _7K160_ \\ \end{tabular} & SSD & – & 229 \\ HDD1 & Hitachi &
\begin{tabular}{c} _DeskStar_ \\ _7K160_ \\ \end{tabular} & HDD & 7200 & 77 \\ HDD2 & Western & _WD36 ADFD_ & HDD & 10,000 & 89 \\ \hline \end{tabular}
\end{table} 
Table 2: Employed hard drives and relevant characteristics (*, assessed in a quiet room using _HDSpeed_ software).