Hardware & Infrastructure
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3/3/2004
11:46 AM
Fred Langa
Fred Langa
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Langa Letter: Cool and Quiet, Part II

Fred Langa continues his exploration of the causes--and cures--of noisy PCs. His solutions include some measures that you may consider counterintuitive but he proves why they'll work with your PC.

Making Up For Lost Cooling
There are two ways around the loss of cooling power that comes from using a slower fan. One way is this: Instead of replacing a fast, noisy fan with a quiet, slower-spinning fan of the same size, you may be able to replace it with a larger, slow-spinning fan that will move more air per revolution. Even though the larger fan spins slowly and quietly, it may be able to move just as much, or more, air when compared with a smaller, faster, noisier fan.

This is, in fact, what I ended up doing in my system: I replaced the original high-speed 60mm CPU fan with a low-speed 90mm fan. (I also changed the heat sink, replacing the original small unit with one specifically designed for slow airflow.) The end result is a CPU fan that's nearly inaudible, and CPU temperatures that are even lower than they were with the factory fan and heat sink.

But in some instances, it's not possible to change the fan size. In those cases, you can exploit a non-obvious quirk of the decibel scale: As a rule of thumb, two identical sound sources will generate a combined output that's only about 3dB greater than either one alone. This means that, say, two 20dB fans will generate a combined noise of only about 23dB.

It's nonintuitive, but this can give you enormous latitude in changing your system's cooling. For example, if you replace one 30 decibel, 25 CFM fan with two 20dB, 15 CFM fans, together they'll generate 23dB of noise and move 30 CFM of air. This means the two-fan system can end up about 7dB quieter, and yet achieve 20% more airflow than the one-fan system! Cooler, yet quieter--what a deal!

I also used this approach on my system: My case openings were sized for 80mm fans, with no easy way to accommodate a larger fan. So I replaced the single original case exhaust fan with two quieter units. The two fans combined (one blowing cool air into the case, the other blowing hot air out) yield greater total cooling than the original single fan, and yet are less noisy overall.

How Quiet Is "Quiet?"
In the example above, we cited a 7dB reduction in noise, going from 30dB to 23dB. What does that sound like? Well, a very quiet whisper from someone about three feet (one meter) away usually rates about 30 dB; barely rustling leaves outdoors might register at 20 dB. So, reducing a PC's fan noise from 30-ish dB to something closer to 20-ish dB would reduce the noise down to a level similar to one of nature's quietest, least-offensive sounds.

Of course, the above examples are simplified. Most PCs today actually have several fans: a case fan, a CPU fan, a power-supply fan, and a video-card fan. Plus, there are other sources of system noise, such as hard- and CD-drives. Real-world PC noise reduction is a bit more complicated than the simple replace-one-fan examples we've been using!

But the same principal applies, regardless of how many fans your system has. It's actually not that hard to achieve total system noise levels in the range of 30 dB--truly "whisper quiet." Compared with the noise levels of most ordinary PCs, which can be in the 40-50dB range (the range of normal conversation or even suburban street noise), getting down to the range of library whispers is a blessed relief!

There's a lot more to PC noise and fan acoustics, of course; we've barely scratched the surface. Human ears aren't equally responsive to all sounds and pressures, for example. And sounds can constructively or destructively interfere with each other, increasing or decreasing the perceived sound levels beyond what you might otherwise expect; or even producing a new third tone (a Tartini Tone) from two other tones. All this (and more) is way, way beyond the scope of this article; please check the references at the end for additional information.

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