Case Fans: Air Flow And Noise Level
What Does Air Flow Really Mean?
The simple answer is that air flow corresponds to the volume of air moved through the fan in a set interval. Thus, it is a very important parameter for us. Unfortunately, it's often conveyed using different units, making it potentially confusing for the beginner. In English (and in most data sheets), the typical parameter is CFM, or cubic feet per minute, while in Europe, m³/h (cubic meters per hour) is more common. Yes, it'd be easy to mix the two up, but here's a handy conversion chart to keep your comparisons straight.
| Input Value | Conversion Factor | Conversion Result |
|---|---|---|
| 1 m³/h | x 0.58867 | 0.58867 cfm |
| 1 cfm | / 0.58867 | 1.69874 m³/h |
Example Conversion:
At an end-of-summer clearance, we bought a desk fan sporting 840 m³/h at an appliance store in Germany. What would be the corresponding CFM value for the same fan purchased in the U.S.? We apply the conversion factor and get:
840 x 0.58867 = 494.4828 CFM
See? That wasn’t too hard, after all.
What Is Fan Noise, And How Does It Come About?
Typically, noise is a mix of several frequencies, which makes it hard to define and compare. Looking at the data sheets of fans, noise levels are either specified in dB (decibel), dB(A), or in Sone (loudness). The drawback of merely looking at a specified acoustic spec is that it's hard to imagine how unbearable that number is in the real world.
Whether noise is considered annoying is a multi-faceted issue affected by several factors. Is the disturbance slight, is it a humming motor, or a squeaky bearing? Unfortunately, a fan that boasts a low noise level on its data sheet can be more annoying than one with a less attractive spec as a simple consequence of its tone.
This looks like it could potentially be pretty noisy. First and foremost, a fan's blades impact its acoustic profile through more or less audible flow separation, which depends on build quality and rotational speed. To that end, surface quality, the blade's angle, and the number of blades all directly influence whether or not a fan is noisy. Many manufacturers claim to minimize acoustics with curved blades and golf ball dimples. But more than anything, you can help mitigate noise by not placing fans too close to case openings and meshes, which can result in the dreaded siren effect.
Sleeve, Ball Bearing, Or Something Entirely Different?
A fan's motor can contribute to noise output as well. Many times, motor noise is most annoying at low speeds, and is manifested as humming. Grinding or clanking sounds from the motor bearings are tough to quantify in a specification, but those are super annoying as well. Ball bearings endure less friction than sleeve bearings, and they typically last longer as well (depending on lubrication and design). However, some cheap ball-bearing fans only employ a single bearing, and the rotational forces (plus air pressure) may cause torsion, leading to premature bearing failure. Fans with double ball bearings do not exhibit this problem.
Sleeve bearings are typically quieter than ball bearings, and a ceramic sleeve bearing has the longest life span of all bearing types, since ceramics with a high-quality surface finish offer the least amount of friction. In general, however, double-ball-bearing fans present the best compromise between noise and longevity. Other design features fall into the category of personal preferences; in general, most high-quality fans perform equally well.
Keep in mind that even experts have a tough time predicting how a given fan will perform in the real world.
Interim Conclusion
Regardless of marketing proclamations about innovations like liquid bearings, the best product isn't always the most expensive one in the shiniest box. The true quality of a fan is determined by the ratio of its air flow to measured and perceived noise level. The lower the noise level for a given (high) air flow, the better we consider it for our purposes.
