## 2 Materials and Methods
### Applicability of Point Source Model and Statistical Analysis  
The point source model consists of assuming that sound radiates equally and symmetrically in all directions from the emitting source; as the underlying hypothesis, the source is far smaller than the wavelength associated with the emitted sound [22]. This spherical symmetry implies that sound intensity \(I=W/4\pi r^{2}\), where \(W\) is the power in the wave and \(r\) is the radius of the generic sphere, decreases according to the classic inverse square law (i.e., \(\sim\)1/\(r^{2}\)) as distance from the source increases. Sound pressure follows an inversely proportional law (i.e., \(\sim\)1/\(r\)) instead; SPL expresses sound pressure relative to a reference value through a logarithmic function (i.e., \(SPL=20\log(p/p_{0})\), where \(p\) is sound pressure, 0 refers to the reference value, and the unit for SPL is dB). Combining the previously reviewed relationships, a quantitative result from the point source model consists of SPL decreasing by about 6 dB each time the distance from the emitting source is doubled. The applicability of the point source model to the present case was challenged by a preliminary set of tests (i.e., the first set described in Section 2.1 and reported in Table 3). The scope was to evaluate the agreement between theory and experimental results, mainly by assessing the distance from the nozzle to the sound level meter at which sound absorption and reflection from walls make the approximation inevitably fail, and the degree of directionality. This latter parameter was investigated by varying the angle of inclination of the sound level meter with respect to the nozzle (i.e., the source) over several values in the 0-90 \({}^{\circ}\)C range, thus following the approach by Koushik et al. [15]. As reported in Table 3, the distance between source and instrument was also varied at each testing location over the angular coordinate. Figure 5 shows the SPL dataset acquired at various angular locations at the selected values of the distance (1, 2, 4 and 8 m from the nozzle). Notably, the data points represent the mean SPL value over the dataset collected at various angular locations at the same distance, while the error bar represents the doubled standard deviation calculated over the same population [23]. The SPL trend predicted by the point source model is also included. As demonstrated by the comparison between predicted and experimental profiles, the applicability of the model to the proposed experiment appears firmly supported within a 2 m diameter sphere centered at the nozzle as the source, for both standard (Figure 5a) and silent (Figure 5b) nozzles. On the other hand, as the distance is further increased, reverberation and absorption by walls appear to make the assumption of point source increasingly fail. As previously noted, point source theory relies on the complete absence of directionality, which is in fact somewhat present for both the standard (Figure 5a) and the silent nozzle (Figure 5b). However, the variability over the investigated angular locations at 1 and 2 m distance from the nozzle (i.e., those relatively free from the wall effect) is limited to \(\pm\)2.5 dB: even though quantitatively not negligible, it was deemed small enough to allow the point source model to be reasonably applicable, even in light of a threshold distinguishing harmless from harmful noise to HDDs with a 20 dB span (110-130 dB, Section 1). Thus, the proposed experimental approach and setup (Section 2.1, Figure 1), where the sound pressure was evaluated at 1 m distance from the nozzle and hard drives were placed at the same distance to assess the impact of noise on their performance, appears overall supported for the nozzles tested in the present study.