Silencing

Silencing is the practice of making something operate quietly. In the context of Computers, it mainly involves drives, coolers and other components with fans. 
== The dilemma ==
A direct way to almost silence a computer is to remove all fans.
Unfortunately, computer components convert electrical energy into heat, some of which needs to be actively "removed" (really just moved). Moving heat is often done using fans, which make noise. Thus, silent operation and cool operation are conflicting requirements and need to be balanced against each other.
As always, the benefits should be balanced against the costs, in terms of time, energy, and money. Fortunately, less energy spent often means there is less noise, too (but there are exceptions).
== Noise sources ==
=== Fans ===
Fan or air noise is caused by turbulence, eddies in the air flow.
These form
* by the operation of the fan itself, in particular at the fan blades,
* by interaction of the air flow with the fan's mounting and
* by interaction of the air flow with cooled components, in particular heat sinks. This in particular is discussed in the Heat sinks section.
Fans can be optimized for various parameters such as low noise, low energy consumption, high differential pressure and high air flow. These requirements are at least partially conflicting.
====Use "silent" fans====
Fans are more silent when
*the fans spin slower. A larger fan can spin more slowly while moving the same amount of air as a smaller fan at a higher speed. 
*have less obstructing elements in the exhaust direction. On the intake side, air usually flows slow and smoothly, making less noise when passing over or around obstacles such as the fan mounting. It follows that it is a good idea to mount the fan on the intake side.
*their blades softly "let go of the air". This means that the blades should be curved such that the tip of a blade is behind its base, relative to the direction of spin. Examples are fans from manufacturers such as Arctic Cooling (the ones used in their active graphics card coolers) and Verax. The silent variant of the stock cooler of the Leadtek PX6600GT-TDH is another example. There have been at least two variants. The one with the fan that had the blades curved "backwards" was more silent.
====Use less fans====
Prefer larger fans.
====Use fans in places where they make less noise====
The farther away a fan is from an obstacle such as a grille or a heat sink, the less noise will be caused by the fan's turbulence at the obstacle.
A fan will make less noise if it blows away from an obstacle instead of onto the obstacle. For example, with some heatpipe tower CPU coolers, fans can be placed "before" or "after" the fin tower (or both). A fan that sucks air through the fins will make less noise than a fan that blows onto the fins.
====Use fans in places where their noise is less audible====
Away from exhausts and intakes.
=== Heat sinks ===
The design of a heat sink influences the amount and kind of noise that the cooling medium produces by passing through the heat sink.
Heat sinks can be designed to meet requirements on parameters such as cost, air flow resistance, looks, weight, size or heat transfer area. These requirements are at least partially conflicting.
As with fans, noise is caused by air turbulence, eddies in the air flow. These can form
* on the input side of the sink, where air hits the heat sink's fins, and is "channelled" in between the fins. Whichever direction the air was moving before it entered the heat sink, it needs to at least slightly change direction.
* inside the heat sink, when air is forced to change direction. For example, this happens when air is blown onto the top of upright fins and needs to exit on the sides, or when air needs to go around heat pipes that cross fins.
* on the output side of the sink, where air flows into free space.
==== To reduce noise generation at heat sinks, ... ====
===== Prefer still input air or laminar input currents =====
Mount fans on the exhaust side of sinks.
===== Prefer still or laminar output currents =====
Mount fans at a distance, the more the better.
===== Reduce internal air flow resistance =====
Prefer larger gaps between fins.
===== Prefer air flow along fins =====
In particular, if edges are involved, prefer air flow along the edges, avoid transversal air flow perpendicular to the fin plane. In other words, orient fin edges along the air flow. 
This means that edges lying close to a rotating fan should be circular. Some silent CPU coolers indeed use radial fans that blow from the inside of fins with circular holes.
=== Case ===
A good case...
* Emits warm air as directly as possible.
*Delivers cool air to the components as directly as possible.
=== Power supply ===
Most power supplies (PSUs) use one or more fans to remove the heat from the active components.
Back when there were only 8 cm exhaust fans, power supplies were always negative-pressure systems. Any slits on the front or sides would suck in case air. Any heat generated by the PSU itself was expelled by the exhaust at the back.
Nowadays, many power supplies have only one 12 cm intake fan and are positive-pressure systems. Some of these still have slits in the front and sides through which they blow some of the warmed air (this is done to cool the PSU components close to its walls). This moves some of the heat generated by the PSU into the case instead of out of it, which may come as a surprise to those who are used to negative-pressure power supplies. A possible solution is to provide dedicated exhaust ducts over the PSU's exhaust slits.
=== Graphics cards ===
*Passively cooled versions produce no fan noise themselves. Mid-range (medium performance) or budget (low performance) cards are often available in a passively cooled version. However, passive cooling requires good case ventilation.
*The preferred solution are dual-slot designs, whose cooler ejects the warm air out of the next lower expansion slot opening to the back of the case. Modern high performance cards usually use a dual-slot cooler.
=== Hard disks ===
* Spinning noise
** Preferred Solution: Buy a silent drive in the first place.
* Seeking noise
** Preferred Solution: Buy a silent drive in the first place.
** Solution: Use noise reduction software
=== Optical drives ===
Not really a problem when they do not run all the time.
Something to consider if the computer is used to watch movies.
== Methods ==
Some measures can be easily taken, such as moving the computer out of direct sunlight or giving the computer air to breathe.