SPCR's Fan Testing Methodology [2006]

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November 3, 2006 by Devon Cooke

Fans are by far the most common and problematic sources of noise in computers. Once quiet fans are found, other noise sources like hard drives, whiny electronic components, and rattling case panels can be addressed, but a quiet system always starts with quiet fans. It is therefore a bit surprising that Silent PC Review has never reviewed fans. There is plenty of good information to be found in our forums (Felger Carbon get special mention here), and we do have a Recommended Fans page, but, believe it or not, SPCR has never actually reviewed a fan. Examined, listened to, pulled apart, measured, and recommended — many a time — but never reviewed.

Why not? There are many reasons, but here's a big one: Fans aren't really all that interesting. The technical details that differentiate fan designs are complex and difficult to understand, and, when it comes right down to it, they often don't have much impact anyway. Besides, most of SPCR's visitors really only want to know one thing when it comes to fans: Which one is quietest?

Answering that question is certainly worthwhile, but it's not a simple question by any means, and it can't be answered by looking at fans in isolation. A large roundup is required, and SPCR took the first baby steps in this direction in early 2004, when we announced a project called Calling All Good Fans, with the intent of having a published piece by September 2004.

Two years later, we've amassed hundreds of fans for the project, but we still have yet to publish anything, while our list of recommended fans has languished for almost as long. That's about to change, as we have finally amassed the proper tools, experimented with various measurement techniques, and set aside a large block of time so that we finish this project properly. This article explains the hows and whys of fan noise — how we measure fan noise, and why we measure it the way we do. The actual test data from our collection of fans will appear periodically over the next few months in batches of twelve or so at a time.


Our Calling All Good Fans project brought in hundreds of fans from all over the world.

WHAT MAKES A QUIET FAN QUIET?

Just about any fan can be made quiet if its speed is reduced enough. All other things being equal, a slower fan is a quieter fan. Keeping this in mind, it makes sense to begin the search for a quiet fan by examining the various low-speed fans that are available. But things aren't quite so simple. Most obviously: Slower, quieter fans don't cool as well as faster, noisier ones. On top of that, nobody with an interest in noise is likely to run all of their fans at full speed, so the rated speed is not important. What matters is the actual speed, and that is up to the end user.

In real life, every system requires some minimum amount of airflow to be cooled adequately, so there is a practical limit to how slowly the system's fans can spin. How slowly? That depends on the fan: Some fans can generate more airflow at a given speed than others. Given what we know about noise and rotation speed, the quietest fans should be the ones with the highest airflow to speed ratio.

As a general rule, this might be true, but a search for the fan with the best airflow to speed ratio won't always find the quietest fan. Remember, a slower fan is a quieter fan all other things being equal. It doesn't take much time playing with fans to learn that all other things are not equal. What we really want is the fan with the highest airflow to noise ratio. When choosing a quiet fan, it turns out that speed is irrelevant. Speed is a decent guideline for judging how noisy a fan is compared to other fans at different speeds or for judging how much airflow it produces, but it doesn't tell us what we really want to know: How much noise does a fan make while still keeping my system cool?

Airflow to noise ratio is certainly the most important criterion for choosing a quiet fan, but it is by no means the only one. Once the fan with the best airflow to noise ratio is found, its speed still needs to be adjusted to provide the right amount of airflow. If this speed can't be attained, the fan is useless because it can't provide the correct amount of airflow. Not all fans will start reliably at low speeds and some fans require very low voltages to run slowly. A fan that pushes the right amount of airflow at 4V is useless to most users because very few fan controllers supply less than 5V.

Concerns other than quiet are also relevant, the main ones being reliability, sample variation, price, and ready availability. Technical features such as bearing type can affect reliability, and odd design quirks like closed flanges (which can be incompatible with clip-based mounting systems on heatsinks) or unusual plugs also need to be taken into account.



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