The NA-IS1-12 is a 5mm thick intake side spacer frames for Noctua 120x25mm fans that can help to improve both acoustics and performance in suction or “pull” type applications where the fan draws air through obstacles such as grills, perforated plates, mesh panels, filters or radiators. By offsetting the fan from the obstacle, the NA-IS1 inlet spacer reduces influx turbulences, which can, depending on the application and fan model, not only improve airflow but also significantly reduce overall noise levels and make the fan sound more pleasant to the ear.

https://noctua.at/en/na-is1-12-sx2

https://noctua.at/en/na-is1-performance-guidelines