Page 2 - A Closer Look - Hardware
The Noctua NF-A12x25 G2 series is a smaller version of the NF-A14x25 G2 series, and is the successor to the Noctua NF-A12x25. The Noctua NF-A12x25 G2 series features the brand's iconic color scheme of brown and beige. Anti-vibration pads are installed on the four corners to reduce minute vibrations, while also providing a visual contrast to the beige frame. Towards the center of the frame, there is a stepped inlet design. This design helps direct airflow and reduces tonal noise. A small amount of turbulence helps the air to stay attached to the surface of the fan longer, thereby guiding it in towards the impellers. Laminar flow also creates a pure tone that is more distracting than the broadband frequency noise created by turbulent flow, which blends into background noise. The brown impellers are made of LCP, liquid crystal polymer, which has higher tensile strength and thermal expansion resistance compared to other commonly used thermoplastics for fans. Noctua has further developed the LCP they use by reinforcing it with fiberglass that further increases the fan's resistance to deformation due to environmental changes and stress. Noctua has given this material the name Sterrox.
The resistance to deformation of Sterrox is a key component in allowing Noctua to achieve the incredible tip-clearance of 0.5mm. The smaller gap between the impellers and the frame allows the fan to work more efficiently against back pressure and potentially reduce tip vortices. However, with the small clearance, impeller creep becomes a major concern. Impeller creep is where the impellers deform and expand due to both the constant outward force caused by the fan spinning and the repeated temperature changes of the environment. Having a higher tip clearance would result in longer usage before the impellers expand to touch the walls, but Sterrox has allowed Noctua to confidently shrink the NF-A12x25 G2 series' tip clearance. The properties of Sterrox also have an additional benefit of reducing minute vibration caused by turbulent air, which reduces the overall noise.
One of the most important factors for fan longevity is the bearing used. The Noctua NF-A12x25 G2 series utilizes Noctua's SSO2 bearing, which is an enhanced version of the widely used oil-based hydrodynamic bearing. It includes a magnet at the bottom of the bearing to stabilize the rod that rotates, known as the rotor, during operation. SSO2 also has better stabilization compared to the original SSO due to the magnet being closer to the rotor. Having better stabilization results in less wear and noise. The MTTF of the fans is expected to be greater than 150,000 hours. In the event that it does fail, Noctua does offer a six-year warranty.
When I first opened the Noctua NF-A12x25 G2 series, I immediately noticed the interesting shape of the impellers. Noctua describes it as a kink-like shape where the blades are slightly swept backwards near the central hub and swept forward near the end of the blades. For fans, forward and backward are understood relative to the direction of spin during operation. The blade shape intends to push more air towards the outer sections of the blade, which spin faster compared to the center sections, resulting in higher efficiency. Noctua has carefully designed this shape to ensure near uniform loading across the blades to avoid flow separation and air leaking back in applications with high back pressure. The center hub, known as the Centrifugal Turbulator Hub, is designed to add a bit of turbulence and push air out of the center, where the efficiency is very low. As mentioned earlier, a small amount of turbulence helps ensure the air does not separate from the surface, which creates relatively large turbulence.
At the outer half of the blades, there are three grooves, which Noctua calls flow acceleration channels. These channels alter the flow distribution and speed up airflow at the outer blade regions to reduce boundary layer separation. During ideal operation, air tightly hugs the fan surface of the blades and exits out the backside. Boundary layer separation happens when the air detaches from the surface and creates turbulence. Turbulence is messy airflow, which reduces efficiency and increases noise. Moving to the end of the blades, there are small winglets to reduce tip vortices. Tip vortices are created due to the pressure difference between the top and bottom of the blades during operation, where the top part has lower pressure and the bottom part has higher pressure. Air moves from lower to higher pressure at the tip, resulting in a circular shape like a vortex. This reduces airflow efficiency, since it is flowing in a circle rather than the intended direction. Normally, we expect large winglets like those on airplanes, but the Noctua NF-A12x25 G2 series has really small ones. This was intentional, since large winglets displace more fluid and reduce overall efficiency.
On the back side, in the center, there is the Noctua logo and some technical information. On the inside part of the frame, where the blades sweep by, there are triangle-shaped surface microstructures. Like the stepped inlet design and the Centrifugal Turbulator Hub, these microstructures also create turbulence to reduce flow separation and noise while increasing efficiency. For additional insights into sound levels, airflow, and static pressure, I have referenced my colleague Jonathan Kwan’s comprehensive review of the Noctua NF-F12 PWM and NF-P12 PWM fans.
Noise and CFM, or cubic feet per minute, are closely related challenges when designing fans, as designers intend to achieve the best airflow-to-noise ratio. One would want the best amount of airflow while keeping it as quiet as possible. Even with the best ratio, it is quite difficult to measure objectively at all times. The most common unit of objective measurement is CFM for airflow and dB for noise. We will go over how application and CFM is related with regards to its standard measurements, but let us discuss perceived noise first.
dB, or Decibels, is a logarithmic unit of sound intensity. While it provides what appears to be an objective measurement for the most part, it should be noted that perceived noise levels to the human ear and actual sound intensity could result in very different perceptions. Human ears are more sensitive to particular frequencies, and when those particular frequencies are emitted from their source, it may appear louder than its numerical value suggests. The same can be said vice-versa -- frequencies that human ears are less sensitive to can actually have higher dB measurements from a sound meter, yet the human ears do not perceive it to be as loud as the numbers suggest. Other factors such as turbulence noise are often not measured correctly, therefore, while it usually provides a good reference, it does not necessarily reflect real life performance all the time. As such, a scaled unit of dBA, or Decibels A-weighted, is used to measure sound scaled to the human hearing threshold.
With considering the application and CFM of a fan, it is generally optimal to prioritize a high air volume flow rate. However, pure CFM values have limitations in indicating fan performance. It is not solely about how much air in can move per minute quantitatively, but equally important is how effectively it performs in real-world scenarios. The airflow-to-noise ratio plays a crucial role, as mentioned earlier. Additionally, static pressure is a key consideration depending on the application. High-resistance scenarios, such as dense fins on a large heatsink, require high static pressure, whereas case fans need less static pressure and faster airflow. There are instances where case fans may also benefit from higher static pressure, such as the front intake fans behind a mesh grille. Some fans are simply designed for different purposes, so choose one appropriate for your needs.
The Noctua NF-A12x25 G2 PWM and NF-A12x25 G2 PWM Sx2-PP are nearly the exact same. The only difference is the Noctua NF-A12x25 G2 PWM Sx2-PP comes with two fans with speed offests. The Noctua NF-A12x25 G2 PWM has a maximum RPM of 1800, while the Noctua NF-A12x25 G2 PWM Sx2-PP package has one fan with a maximum RPM of 1850 and the second 1750. The offset is meant to reduce harmonic noise during push-pull configurations, such as CPU coolers, or side-by-side operation on applications like water cooling radiators.
The Noctua NF-A12x25 G2 PWM is rated to have a maximum airflow of 63.15CFM and maximum static pressure of 3.14mmH₂O. The mean time to failure is over 150,000 hours. At maximum speed, the noise level is rated to be at 22.5dBA, which equates to about the noise level of leaves rustling. With the low noise adapter on, the noise level reduces to 18.1dBA at the cost of lower maximum airflow at 51.32CFM, static pressure at 1.71mmH₂O, and rotational speed at 1500RPM. On Noctua's site, the technical specification of the Noctua NF-A12x25 G2 PWM Sx2-PP are the exact same despite the offset in RPM.
The Noctua NF-A12x25 G2 Series comes with a set of accessories for each fan included. Each set has a low-noise adapter, 30cm extension cable, 4-pin splitter cable, anti-vibration mounts, anti-vibration gasket, and fan screws as mentioned on the first page.
Page Index
1. Introduction, Packaging, Specifications
2. A Closer Look - Hardware
3. Performance Tests
4. Conclusion
