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Speed and sound go hand in hand; from Ferraris to space shuttles, anything that's capable of moving with real velocity generates noise. This holds true for computers. The flow of binary digits inside a PC's case might seem abstract but all that maths has a palpable side effect. As silicon chips run faster, they consume more power and generate more heat. High temperatures stop the chips working, so the heat needs to be dissipated. Initially, hardware manufacturers looked to passive cooling, but as processorss increased in speed, fans were needed. Fast PCs may not yet emit sonic booms, but the shriek of their fans can be incredibly irritating.

Fortunately, PC manufacturers are waking up to this problem. Small systems such as those made by Shuttle (www.shuttle.com) can be very quiet, and Zalman (www.zalmanusa.com) has made its name by coming up with novel approaches to cooling that are far quieter than the average fan. 25 of the quieter cooling products available are reviewed in our Labs test.

If you don't want the expense of buying a new PC but are desperate for a little peace and quiet, there are plenty of tricks you can try.

KNOW YOUR ENEMY

Unless you resort to extreme measures, you have to accept that modern PCs make noise. However, you can easily turn your PC's roar into a quiet purr.

The first step is to find out which parts of your PC make the most noise. Typically, the case is cooled by between one and four 80mm fans. One or two are placed

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at the back as exhausts, and some manufacturers fit a couple more at the front as intakes. Both the processor and graphics card have heat sinks, and these usually have fans. The power supply unit (PSU) also has integrated fans.

The smaller a fan, the faster it must spin to move air and cool the components. Small, fast fans make the most noise, so the first fan to check is the one on the graphics card. While the PC is running, press your finger against the middle of the fan to slow it down. Be careful not to catch your fingers, and don't do it for more than 10 seconds or use too much force. You just want to slow the fan so that it stops contributing to the sound the system makes. Check how much the noise level decreases. For instructions on fitting a quieter cooling system to your graphics card, see the box on the left.

If the PC is still noisy, the system fans may be to blame. Turn off the PC and unplug the fans. They'll either be connected to the motherboard by small three-pin plugs, or directly to the power supply using four-pin 'molex' connectors. Switch the PC back on and see how the noise level has changed. Then turn the PC off and reconnect all the fans. If the system is still noisy without the system fans, you know that either the processor's cooler or the power supply is responsible, and you can check these by listening to them closely.

A PLAN OF ACTION

Once you've discovered what's making the racket, there are a number of things you can do to tackle the problem. As mentioned earlier, a smaller, faster fan creates more noise. Larger and slower are the order of the day - and all fans inside a PC, except those in the PSU, can be altered or replaced.

If you find that your PSU is making all the noise, you have to replace it, as changing the fan isn't practical or safe. Look for a PSU that uses temperature-controlled fans. Both Akasa's PaxPower and Tagan's TG480-U01 (reviewed in the Labs test) provide lots of power and run quietly. Unless your PC uses very little power, and you don't use it for anything strenuous, don't be tempted by fanless power supplies. They're frequently underpowered and have been found to fail under high loads.