## 2 Materials and Methods
### Experimental Setup and Procedure
The nozzle was placed at the center of the base of the chamber, as far away from the walls as possible. This configuration--applied within a large enclosure--was aimed at reducing the effect of reverberation by walls, which was assessed as potentially significant [15], since the tests were not conducted in an anechoic chamber. Figure 1: Schematic of the realized setup to evaluate acoustic performance by supplying (a) nitrogen IG100 or (b) FK-5-1-12. The nozzle was also placed at 2 m height from the floor (Figure 1), a common value in installations within data centers [16] and suitable for accommodating all the components surrounding it. As for nozzle types, the tested ones are all manufactured by Bettati Antincendio S.r.l. : Figure 3 presents photos of both the employed standard nozzle (Figure 3a) and silent ones designed for each category of clean agents (Figure 3b,c). Interestingly, some preliminary tests made select a silent nozzle for low ceiling installations (Figure 3b) when the discharge of halocarbon compounds (i.e., the FK-5-1-12) was involved, since it proved more effective, especially if combined with the chosen valve. On the other hand, silent nozzles were employed in the discharge of nitrogen IG100. It is worth clarifying that the tested silent nozzles present a sound-absorbent layer included within their frame, which makes them similar to those investigated in the works by Kim et al. [3; 13]. Figure 4 presents simplified technical sketches of the employed nozzles, highlighting the orifice diameter. As shown in Figure 4b,c, silent nozzles are endowed with the mentioned sound-absorbing shell. Orifice diameter varied over the following values in the whole series of tests and for both standard and silent nozzles: 5, 6, 10, 14, 17 and 23 mm. It is worth clarifying that the experimental setup proposed in the present work is aimed at comparing standard with silent nozzles in a single-nozzle configuration. Therefore, the combined action of the  
Figure 3: Photos of the nozzles employed in the experiments: (**a**) standard; (**b**) silent nozzle for the discharge of inert gases; (**c**) silent nozzle for the discharge of halocarbon compounds (low ceiling height). Figure 2: Photo of the experimental setup: (**1**) storage cylinder; (**2**) pressure transducer and thermocouple (downstream of the valve); (**3**) scale; (**4**) pipeline; (**5**) pressure transducer and thermocouple (upstream of the nozzle, only for inert gas discharge system); (**6**) nozzle; (**7**) servers, including one SSD and two HDD; (**8**) sound level meter. discharge by more nozzles at the same time, and its effect on noise, is not included; notably, superposition of waves and interference of sound may make the generated noise vary from that produced by a single nozzle throughout its discharge. A server rack containing an SSD and two HDDs with different characteristics was located at 1 m distance from the nozzle outlet (Figure 1) to assess the impact of the emitted noise on electronic equipment typical of data centers. The set distance is consistent with that applied in previous works [12; 15] for evaluating SPL from nozzles used in gas-based fire suppression. Both HDD and SSD performance was evaluated by recording their speed over time using a free software (_HD Speed_ version 1.7, released by SteelBytes). The acoustic emissions from the tested nozzles were measured by a sound level meter, with its microphone positioned at 1 m distance from the nozzle exit and at 1.5 m from the floor (Figure 1); the instrument (specifically, its axis of symmetry) was inclined by about 60 \({}^{\circ}\)C with respect to the vertical axis, coincident with the axis of symmetry of the nozzle. The distance between the sound level meter and the nozzle is consistent with the value set for the server rack, and also supported by previous studies [12; 15] and recommendations from test reports [17; 18; 19; 20], since it is representative of the typical minimum distance at which obstructions (e.g., a cabinet containing electronic equipment) are located.