###### Abstract  
Discharge through nozzles used in gas-based fire protection of data centers may generate noise that causes the performance of hard drives to decay considerably; silent nozzles are employed to limit this harmful effect. This work focuses on proposing an experimental methodology to assess the impact of sound emitted by gaseous jets by comparing various nozzles under several operating conditions, together with relating that impact to design parameters. A setup was developed and repeatability of the experiments was evaluated; standard and silent nozzles were tested regarding the discharge of inert gases and halocarbon compounds. The ability of silent nozzles to contain the emitted noise--generally below the 110 dB reference threshold--was proven effective; a relationship between Reynolds number and peak noise level is suggested to support the reported increase in noise maxima as released flow rate increases. Hard drives with lower speed were the most affected. Spectral analysis was conducted, with sound at the higher frequency range causing performance decay even if lower than the acknowledged threshold. Independence of emitted noise from the selected clean agent was also observed in terms of released volumetric flow rate, yet the denser the fluid, the lower the generated noise under the same released mass flow rate. f +
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and employed in the present work) or inert gases (e.g., argon and nitrogen, also tested in the present research). The involved systems are total flooding: the whole compartment that is on fire is filled with the released gaseous agents. So, both categories mainly rely on oxygen depletion as their main extinguishing mechanism [2]; however, the former also tends to combine chemical action (i.e., inhibition of combustion) with heat extraction and, consequently, flame cooling towards extinction. In spite of the numerous advantages exhibited by clean agents, the main form of which is characterized as being electrically non-conductive, leaving no residue after discharge and having virtually null Ozone Depletion Potential (ODP), the whole related system may be somewhat complex, since they require a storage capacity larger than that of halon-based systems as a result of poorer extinction effectiveness [2]. Most clean agent fire suppression systems include a storage tank, where the agent is superpressurized--usually by nitrogen--if in the form of a liquified gas, as is the case for most halocarbon compounds, or is simply contained at high pressure (usually in the range of 150-300 bar) in the case of an inert gas system [2; 3]; valves, piping, nozzles and controllers (e.g., flow rate and pressure) are also part of the design. While the mechanical strength of pipes and nozzles to withstand the gas pressure does not currently appear a major concern for designers, the noise generated by the agent, mostly at the nozzle exit and, to a lesser extent, through valves and channels, represents a potentially relevant cause of damage to the equipment in the compartment, as well as first responders (e.g., firefighters) if present within the compartment as the discharge occurs [4]. A well-known problem and subject of extensive research efforts in aeronautics [5], the high-speed jet noise yielded by turbulence intensity at the nozzle outlet, and often emphasized by transitioning from subsonic to supersonic flow [6; 7], has been studied for decades, mainly with the aim of reducing its intensity. In the case of clean agent gaseous jets released in a generic compartment undergoing a fire event, the sensitivity of Hard Disk Drives (HDDs) to noise can result in a remarkable performance loss [4; 8]. As a quantitative experimental result, Sound Pressure Level (SPL) in the range of 110-130 dB is commonly accepted as the threshold beyond which HDD read/write performance becomes dramatically penalized [4; 9; 10]. Interestingly, HDD performance loss due to the overpressure and the steep pressure gradient caused by rapid discharge within the compartment appears somewhat irrelevant [4].