No two filters are the same
The vast majority of computers use two types of dust filters – a fine nylon one and a coarser plastic one. The fact that the plastic filter lets more through and is less efficient in this respect is already apparent from the eye, as it has a coarser mesh. But do you know which one degrades the performance of the fan more and by how much? One even “wheezes”, which is nicely indicated by a detailed noise analysis at 240 sound frequencies.
Before we release the first fan tests, let’s take a look at how dust filters affect airflow, pressure and noise levels. For our first test (a three-part series), we’ve selected the two most commonly used types of filters in computer cases: nylon and plastic.
While nylon is clearly more effective when it comes to capturing dust, plastic is considered a sort of cheaper alternative. Cases from lower price ranges have it under the power supply (in more expensive cases there is usually a finer nylon filter in this place), but it is often found in higher end cases as well. Although it’s typically in the ceiling position, through which air is only sucked in atypical situations and tends to be pushed through by the exhaust fans. A coarser mesh therefore doesn’t matter that much.
The image below is a close-up photo of the structure of both filters. The grid of the nylon one is much finer than the plastic one. The latter has holes with a diameter of 0.8 mm. Although its mesh is relatively coarse in this comparison, thicker filters are also used (sometimes with holes exceeding a millimetre), but for those the name “dust filter” is quite questionable.
We test the filters’ impact on airflow and also static pressure. For the measurements we used Noctua Fan NF-A12x25 PWM. Firstly, because it is a fairly popular and widely used fan. Moreover, it is from a higher price category and one can assume that its owners will be interested in how it handles dust filters. Although we will be using not only filters but also other obstacles in our standard fan tests, a separate article with a slightly modified methodology is also appropriate.
We measure both airflow and pressure in the wind tunnel as described in the fan test methodology, but we measure noise a little differently. We don’t equate the latter to the same level in these filter tests, but we have fixed levels of operating voltages of the test fan. This is so that the filter (and later grille) tests can show the increase in noise that is caused by the greater mechanical resistance if an obstacle is placed in front of or behind the rotor. Thus, we use the NF-A12x25 PWM with 9 V (~1650 rpm), 7 V (~1330 rpm) and 5 V (~980 rpm). We didn’t test at maximum power because few will be blasting through dust filters at around 2100 rpm, where the Noctua is as well, naturally, noisy.
Don’t be alarmed by higher noise levels in the results. They are so because we increase the sensitivity of the sensing to a high enough resolution even for very low speeds by a parabolic collar around the noise meter. We also use the same fixture for in-depth sound analysis, where we measure noise levels in the 20-20,000 Hz range at 240 frequencies. For more on this topic, see Get familiar with measuring the frequency response of sound. If you are interested in fan tests and don’t know how to read spectrograms yet, we recommend you to complete the overview, it will come in handy. But these filter tests also made a good basis for a practical crash course. But we’ll come back to that in the final chapter. For now, it is better to go through all the test results in the second and third chapters.
- No two filters are the same
- Results: impact on airflow, pressure and noise
- Results: frequency response of sound
- So which type is the bigger brake?