###### Abstract
In spite of the increasing popularity of new technologies (e.g., Solid-State Disks--SDDs), which are not noise-sensitive, this problem may have a significant extent: in 2021, there were about 8000 data centers in the 110 countries providing information, with a predicted 150-fold increase in the generation of new data over the 2010-2025 timespan [11]. Moreover, a similar detrimental effect of noise yielded by the same gaseous jets can likely occur in some medical devices featuring an architecture similar to that of HDDs, an issue worth considering in fire protection systems designed for the healthcare industry. As a technical solution to reduce the noise generated by clean agent discharge, silent nozzles--often also referred to as acoustic nozzles--have been developed. Since modifying the nozzle outlet does not appear to fully address the challenge, given that SPL was proven to be largely independent of nozzle shape [7], adding sound absorptive layers to the nozzle outer surface has become the most employed approach [3; 12; 13], with the insertion of horizontal plates also being recommended to make the released flow rate and pressure as balanced as possible in multiorifice nozzles [12; 14]. Currently, the open literature presents relatively few studies that focus on the design of such nozzles and on assessing their acoustic performance, especially when compared with that exhibited by standard nozzles under the same discharge conditions. In that regard, it is worth mentioning that Koushik et al. [15] proposed an experimental approach to evaluate noise emitted by standard fire suppression nozzles releasing inert gases: an array of microphones were employed to measure SPL at various azimuthal locations with a constant distance (1 m) from the nozzle. Some degree of directionality (i.e., dependence on the angular coordinate) was found, together with potential impact of walls on the acoustic path (i.e., reflection and direct path). The extensive use of numerical modeling generally permeates the works on silent nozzles, most of which [12; 14] are focused on determining the most effective nozzle selection and configuration within an enclosure towards making SPL at the HDD locations lower than the previously mentioned threshold (110-120 dB, in those studies). On the other hand, the more recent contributions by Kim et al. [3; 13] present a computational approach--remarkably validated through dedicated experiments--to optimize some design parameters of silent nozzles. Guided by DOE (Design of Experiments), their research effort led to them identifying the diameter of the external sound-absorbing shell as the most impactful variable. It is worth noting that some of the reviewed works suggest a certain dependence of the emitted noise on the orientation of the nozzle and the location at which SPL is evaluated [14; 15], whether the nozzle is a standard or a silent one; on the other hand, other studies appear to practically consider the nozzle as a point source [3; 12]. This aspect may be mostly related to the nozzle geometry. Even though some comparison between standard and silent nozzles is presented in the work by Loureiro et al. [12], it appears that a quantitative and comprehensive approach to the purpose represents a challenge still to be addressed, at least in the open literature. More specifically, an experimental methodology allowing investigators to carry out such a comparison in terms of variables of interest for fire protection system designers (e.g., type of clean agent, released flow rate, orifice diameter) is still in demand. The present research is aimed at embarking on this quest, with the experimental datasets and facilities provided and described in the studies by Kim et al. [3] and Loureiro et al. [12] serving as a foundation. Arguably an unprecedented effort, this work may set a standard methodology to quantitatively compare the acoustic performance of various standard and silent nozzles, also leading to discussion in reference to the relevant physical quantities.