Some PCs are noisy. Very noisy. Other PCs run almost silently. So what is the difference, and can you make a noisy PC quiet?

The noise made by a PC is almost always from the cooling and ventilation fans. Heat generated by various components in the PC must be dissipated to prevent them from overheating, which could cause irreparable damage. There are generally three areas of the PC which require substantial cooling; the CPU (Central Processing Unit), the GPU (Graphics Processing Unit) and PSU (Power Supply Unit).

CPU
The CPU is what makes the computer work. It follows program instructions resulting in what you see on your screen. Modern CPUs run at incredible speed, carrying out millions of instructions per second. It's no wonder that they get hot.

CPUs are usually fitted with a heatsink (a finned block of metal) and a fan. The most recent
CPUs have fairly large fans compared with their earlier counterparts. In general, these fans run reasonably quietly. However, that's not true of all of them, and there are companies which supply replacement fans specially designed to run silently.

The CPU's temperature and its fan speed are usually monitored by special sensors. When the CPU starts to get hot, the system speeds up the fan to cool it down. When the CPU's temperature falls, the fan is slowed down.

GPU
The GPU is the processor on your graphics card or main board. Many very high power graphics cards are already fitted with a custom-made fan. If yours is noisy, the chances are that you won't be able to replace it. The only solution is to find a new graphics card that is either fitted with a quieter fan or doesn't need one.

PSU
This is the box in your PC which converts the mains electricity supply to a clean, lower voltage that your PC can use. In most cases, the PSU has one or more built-in fans which may or may not be replaced. Some PSUs have a fan speed controller which you can adjust to get sufficient cooling with the least amount of noise.

Case fans
To improve the cooling of everything inside your PC, case fans may be fitted. Even if your case doesn't have these fans, if you look inside, you'll probably see mounting holes for them, which means you can add additional fans if you wish.

So why is one fan noisy, when another is not? There are a few factors that determine this, and they all come down to required air-flow. To cool something down, the fan pushes air over it, replacing the hot air with cooler air. The amount of air required to do this depends upon how hot the item is and how cool you'd like it to be.

If you have a fan with a small diameter, it needs to spin round very quickly to push a certain amount of air through it in a fixed amount of time. Increase the fan's diameter and it can run a lot slower to achieve the same air-flow. Make the fan even bigger and you can slow it down even more.

In their current range of PCs, Dell fit their CPUs with huge heatsinks and no fan. Instead, they fit a case fan of 12cm (4.7") diameter (that's about the same size as a CD). They can run this fan very slowly and achieve the required air-flow to cool the CPU and other components. Consequently, their PCs run almost silently. In fact, in a typical home or office, a Dell PC on the floor under the desk cannot be heard. On top of the desk, the noise from the fan is barely audible.

Typical case fans of other PCs are only 8cm (3.1") in diameter. The area of an 8cm fan is about 50 square cm. The area of a 12cm fan is 113 square cm. So even though the diameter of the 12cm is 1.5 times that of the 8cm fan, the area of the fan is 2.26 times bigger. This means that to achieve the same air-flow, the bigger fan could run at less than half the speed of the smaller fan, thus making considerably less noise.

Many computer cases have 2 places where case fans may be fitted; usually one at the front and the second at the back. If you mount a fan at both of these places, make sure that the fan on the front is blowing INTO the box, and the fan at the back is blowing OUT of the box. This ensures a nice flow of cool air through the box.

So if you want to make your PC quieter, you could try replacing the fans with larger ones running at a slower speed. If your CPU fan is the culprit, try adding case fans - as large as possible. This will help cool the inside of the computer and the CPU fan will be able to run more slowly and quietly.

A few words of caution though -
Measure your case, and check that it has the required mounting holes BEFORE buying the new fans.
Don't attempt to replace the fans on your PSU unless you know what you are doing. There are voltages inside the PSU which can kill you.