Analysis: RGB LED implementation changes fan noise

Different material, different resonant frequencies

The headline may sound strange, but after reading the article it will make sense. After modifying a fan from unlit to lit, its noise level can change for a very simple reason. This is true even if the shape of the rotor or the operating speed has been maintained. As far as noise is concerned, a change by integrating ARGB LEDs does not always lead to a worse result. Still, it is important to know why any differences occur in the first place.

Earlier, we published an analysis comparing the Corsair AF120 fans in black and white. The conclusion was that the different colour variants may also differ to some extent in terms of noise. Different materials are used to make the black rotor (it is not just a case of coating the same core) than the white one, while the stiffness of the material used may differ to some extent. The same is often true for fans from the same model ranges that meet in geometry and the only difference is whether or not they have built-in LEDs. If it weren’t for the different motors and also different maximum speeds, this would also be true for the DeepCool FK120 and FC120 fans that we have already tested as well.

But now we will take a look at different variants of the Endorfy Fluctus 140 PWM (ARGB) fans, whose paper parameters do not differ. At least the ones that are available – maximum speed and motor power. Airflow and static pressure values are missing from the specifications of these fans. However, the rotor is one like the other, the shape chosen is the same, only the material composition is different.

At first, we were considering whether to incorporate the following knowledge into our standard Endorfy Fluctus 140 PWM fan tests. Doing so would have either saved space for other things, or would have bloated the text beyond what you’re used to. Therefore, we will treat the details around the various mechanical characteristics of the 140mm Endorfy fan blades (of the PWM/EY4A003 and PWM ARGB/EY4A004 models) this way, separately.

The Endorfy Fluctus 140 PWM is notable on our test timeline in that it will follow the Endorfy Fluctus 140 PWM ARGB test. So these fans have geometrically identical rotors, but while one (Endorfy Fluctus 140 PWM) has a black rotor, the other (Endorfy Fluctus 140 PWM ARGB) has a “milky”, opaque rotor to distribute the light from the RGB LED array well. The latter is traditionally located on the PCB at the root of the fan hub.

The aim of this article is mainly to clarify why the results of the mentioned Endorphy fans do not scale perfectly. To get a better idea, we will use a real-world example. Let’s replace the fan blades with plates made of different materials. But their dimensions are the same, e.g. half a metre by a metre at 5 mm thickness. Let’s say one blade is made of steel, one of laminate, one of wood, one of rubber, and so on. If you were to grab these plates on one side and move each one at the same speed while exerting the same force, then depending on the material there will be different undulations, different vibrations, different noises.

It cannot be said that a plate with higher vibrations will always be noisier. It depends on how close or far it is from the “appropriate” (noisiest) resonant frequency. For example, the Arctic P12 Slim fan has critical speeds in the lower half of the speed spectrum and runs quieter towards the higher speeds. While aerodynamic noise increases at higher speeds at surrounding sound frequencies, unpleasant peaks (rumbling or humming) will be less pronounced and fan operation more tolerable.

In short, while with one material (steel sheet, but also the “black” PBT that the fan is made of) you can effectively hit resonant frequencies at a lower speed, with another you need to increase it. With the Fluctus 140 PWM (ARGB) fans, while the mechanical properties of the materials (PBT/PC – Polycarbonate of the lighted variant) do not differ significantly, they do not have the same stiffness either. However, even very small differences are enough for a different sound characteristic, which then also determines the order of efficiency. This is then particularly evident in tests whose methodologies are based on standardised noise levels. The lower the resonant frequencies in a speed band, the higher the set fan speed will be, which naturally results in higher airflow.

You will soon find out how big the differences between the Endorfy fans mentioned above can be. The important thing at the moment was to let you know why this is happening and that it is definitely not a measurement error, although theoretically those differences can fit within it. But that’s only theoretically, practically we have extremely accurate measurements with high degree of repeatability, and we know that we can point out even such small design details.

Note: For some fans, differences at this level between lighted and unlighted do not occur. These typically include some fans from Fractal Design (Aspect or Prisma), which have the same white rotor regardless of the (non-)use of RGB LEDs.

Still, a different blade material structure does not automatically mean different characteristics, as long as the manufacturer has taken care to ensure that at least in the speed operating range there are no differences. The consistency of performance across brown and black NF-A12x25 fans has been touted by Noctua, for example. Whether or not this is the case will be discussed in detail, but probably not until the next generation of 140 mm fans, which will also be made of “sterrox” (LCP). Due to their potentially longer blades, keeping vibrations at the blade tips down is more of a challenge than with 120mm models.

What we know from Noctua, however, is that to suppress vibration as much as possible, in addition to a stiff material they also work with optimal thickness and curvature of the blade profile. So let’s see, they’re hopefully around the corner. We still have a few 140mm fans to collect into our database before their release, so that the evaluation of the new fans is then as relevant as possible. Thus, our 140 mm fan tests will be shortly added to with the Endorfy Fluctus 140 PWM.

English translation and edit by Jozef Dudáš


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