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A device that moves air around a computer, either circulating within the case, extracting hot air from the case, or drawing cooler air in. The main types of fan seen in PCs are conventional bladed fans, and centrifugal 'blower' fans. Centrifugal fans are almost exclusively used to cool overclocked components.

Far from being indicative of 'poor design', cooling fans are a de facto feature in today's computers. The ommission of a cooling fan sacrifices performance for the sake of quiet. The vast majority of computers (any off the shelf PC, the later versions of the g4 cube), consoles (Playstation 2, XBox), laptops, and even set top boxes (IIRC, both the tivo and sky+) have cooling fans.

At this time, low noise levels are one of the last things an average buyer considers when buying a computer. They want high apparent performance, good looks, and a low price. The G4 cube is a good example of this - despite being good looking and near silent, it was far more expensive than a comparable 'noisy' computer, and not as good at games. Apple released a newer version with a better graphics card, adding a fan to cool it. It was still too expensive, sales were still poor, and the cube met with early retirement.

Not all fans are noisy though. If you have the money to spend, quiet power supplies with a variable speed fan, heatsinks with a high surface area and large, slow, quiet fan can be inaudible in a quiet room. Cheap fans tend to be poorly balanced (leading to vibration noise), have badly designed blades (turbulence noise), and have cheaper sleeve bearings (friction noise). If you're really lucky, a cheap fan will resonate with the heatsink or case. Expensive fans have better motors, with less stress put on them by a badly machined fan, and so last longer, and are less likely to fail catastrophicaly.

To be competitive, OEMs must cut as many corners as they can on 'peripheral' components, so that they can claim as high numbers as possible. A fan that will make a machine run costs ten pounds. The fans, enclosures, power supplies, additional heatsinks, etc. needed to make the machine silent can cost up to two hundred. For that price they could double the clockspeed, RAM, or hard disk. Knowing this, manufacturers are loathe to produce well performing, quiet, but expensive parts. Every time a major manufacturer announces a die shrink, they use it to produce faster chips, not cooler ones. IDT's winchip sold appallingly despite its low power consumption, as its poor performance stigmatized it as a 'budget' processor. AMD hardly made any k6-III+ chips - Although they performed well, and could be passively cooled, the noisier, cheaper, better performing athlon was much more popular.

Additionally, noise-reduction components are more prone to failure - Large heatsinks make less noise, but can snap off when the system is moved, destroying the processor (at the very least). Mail order OEMs would have serious bother getting the system to the door in a usable state. Hard drives with a fan will run cooler, and last longer than those without. A processor at 60 degrees will pack up a few years before one at 40. A system with delicately tuned airflow is more difficult to upgrade (large cards and badly placed ribbon cables can prevent air from reaching other components). A large noisy fan gives manufacturers a lot more leeway. While most machines can detect failed fans, and prevent them from causing damage, a detached heatsink is much more difficult to detect (and in the case of the athlon, the processor catches fire before it can react to it). A mass market OEM can't afford to support unreliable machines.

Hopefully at some point the numbers will become as irrelevent for games and multimedia as they already are for office applications1, and OEMs will be able to produce a fanless computer without commiting financial suicide. But I'm not holding my breath.

1 - Who cares if Word is spending 99 or 99.999 percent of its time in an idle loop?

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