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2/19/2004
09:30 PM
Fred Langa
Fred Langa
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Langa Letter: Cool And Quiet, Part 1

Fred Langa finds ways to silence his PC's noisy fans while still keeping the PC well cooled.

Copper By The Pound
Price-pressured PC vendors prefer to keep components as small as possible. For example, the heat sink on a high-end CPU may require a pound (call it 500g) or more of pure copper that's either precision-machined from a solid block or assembled from many delicate fins. It's a fairly expensive part to make, so the manufacturer will try to specify the smallest mass and surface area for the heat sink it can get away with.

To ensure that enough air is pumped through the heat sink for good cooling, the manufacturer will normally then specify a small, high-speed fan. This may provide all the cooling the CPU needs, but at the cost of high noise from the fan's mechanism, its blade tips, and from turbulence as the air is blown through the heat sink.

If the manufacturer wanted a quieter option, it'd mean one of two things: If he used the same-sized, but slower-spinning fan, then the lower speed would move much less air through the heat sink. To make up for the lower air volume, he'd need a much larger and more expensive heat sink to keep the CPU cool. Because heat sinks are expensive, that would raise the price of the system.

Or, he could use a large, slow fan to quietly move as much air as the small, noisy, fast fan. But larger fans cost more, and might require some kind of ducting or special mounting to mate the large fan to the small heat sink. This is an engineering challenge, and also raises the system cost, and can affect component layout and case design.

So, PC vendors try to walk a fine line that lets them keep the PC within safe temperature limits while using the smallest, least-expensive cooling components they can get away with. The usual result: Systems with whiny, whooshy, noisy fans sounding more like vacuum cleaners than computing devices.

Software Assists
PC vendors do have a couple other tricks they can use to help reduce the cooling load in a PC. One is the choice of system software.

For example, Linux and Windows NT/2K/XP all use a simple and effective method to help reduce the heat generated by a CPU: When these operating systems are idle, they effectively stop the CPU via a HLT ("halt") instruction. In contrast, a Win9x-based system continues to churn at full power, even when it's doing nothing. This means that Linux and Windows NT/2K/XP systems are easier to keep cool than Win98 systems; this is one of the advantages of moving to a newer operating system. In fact, moving to a newer operating system even on older hardware can help make that system run cooler, possibly opening the door to easy noise reductions through use of a slow fan. Or, if an operating-system change is out of the question, you can add special third-party software to Win98-equipped PCs: That software will enforce a HLT during idle cycles.

BIOS makers also can help, working in concert with motherboard vendors. Many newer systems not only monitor their own temperatures at several locations, but also can actively control fan speed, using a low--and theoretically quiet--fan speed when the system isn't working too hard, and a faster--noisier--speed only when needed.

That, in fact, is what my newest system is supposed to do. Its Intel motherboard has three built-in heat sensors which are monitored by the BIOS, one sensor for the CPU itself, and two at other points inside the system case. When the system is working very hard--calculating large spreadsheets, compressing many files, building a long video composition, etc.--and the overall system temp rises above about 90 degrees F/32 degrees C, the case fans switch to higher speed to fully ventilate the system and get rid of the growing heat. But most of the time when the system isn't working flat out, the system temp stays below about 90 degrees F/32 degrees C, and the BIOS slows down the case fans to reduce noise. (See the Intel specs here.)

It's a great idea, but it appears that the low fan speed is achieved by interrupting power to the fans several times each second. This does slow the fans, but the choppy power means the fans produce a click-click-click noise that, frankly, drove me nuts.

Plus, the CPU fan on my system always spun at a set, high speed. It was highly effective--the stock Intel fan-and-heat-sink combination provided excellent CPU cooling--but it was very noisy.

So, I went looking for ways to quiet the inherently noisy CPU fan, and to find a quieter alternative for the stock case fans. But I needed to work carefully because I didn't want to risk frying my system in a cooling error.

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