NA-FG1: Improvements from offset mounting
Noctua’s NA-FG1 fan grills feature an offset design and are made from round steel struts instead of perforated sheets in order to cause as little influx turbulence as possible. However, as with any object that is close to the intake side of a fan, a certain amount of turbulence is unavoidable and will have an impact on the fan’s acoustics and performance. By introducing an extra 5mm offset, the NA-FG1’s push-pin mounting system can help to further reduce these effects.
Generally, it must be noted that the results can vary significantly between different fan models and applications, depending on factors such as fan speed, flow resistance and interaction with the airflow caused by other fans. Typically, adding NA-FG1 fan grills without extra spacing will impact the fan’s flow rate by 2-4%. However, with certain models, the impact can be as big as 6-8%. By contrast, other models may not show any significant impact at all. On average, the impact across all tested fan models was around 3%. When using the offset push-pin mounting, most fans showed a significantly lower loss in flow rate. In many cases, the impact of the grills could roughly be cut in half and on average, the losses could be reduced to around 2%.
As far as acoustics are concerned, the results can also vary significantly between fan models, as well as being affected by various other factors. While some fans can become several decibels louder when a grill is added, others are hardly impacted. On average, the impact on sound pressure levels (SPL) when adding NA-FG1 grills is around 1.2dB(A). Again, this impact can usually be reduced significantly by using the offset push-pin mounting, which reduces the SPL increase to around 0.6dB(A) on average. With fan models like NF-A12x25 or NF-A4x10 where adding grills resulted in more significant acoustic penalties (2.5-3dB), the offset mounting option of the NA-FG1 grills also showed the biggest benefits with a 1.2-1.5dB reduction in noise levels.
In addition to the possible reduction in total noise levels, the extra spacing from the offset mounting can have a positive influence on the fan’s frequency spectrum, making it sound more pleasant to the human ear. This can be the case even if the offset yields little to no improvement in total noise levels such as with NF-A20 or NF-A8 where the frequency spectrums generally show lower tonal frequency spikes as well as a shift of spikes towards the lower, less distracting frequency ranges:
As a rule of thumb, the extra offset will shift the fan’s sound signature towards the lower side of the frequency spectrum and, in many cases, the offset can also help to reduce tonal frequency spikes. Since most people perceive lower tones as more agreeable than higher pitched ones and generally, broadband noise blends into the background noise much more easily than tonal frequency spikes, these effects will make the fan sound more pleasant to the ear even if the reduction in total sound pressure levels may not seem dramatic. While the offset mounting usually offers both acoustic and performance benefits, it must be noted that the results can vary due to multiple factors and in rare cases, the extra offset can also have no effect or even slightly negative effects. In this case, the push-pins can easily be cut to remove the offset.
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