Heat & Noise

A general discussion of the factors concerning heat and noise so as to better understand the problems and the possible solutions.

What is the problem with HEAT?

FACT 1

According to the US Military, 55% of electronic equipment failures are due to OVERHEATING. That could include your computer. As equipment becomes more miniaturised and operating speeds increase, temperatures rise. That waste heat must be removed. For example, if you run a modern processor, for a few seconds, without a heat sink, it will melt. So cooling is important.

FACT 2

It can be shown scientifically that for every 10 degC drop in a component's operating temperature, its working life is DOUBLED. If you drop 20 degC, then the working life is quadrupled. So cooling is a good investment.

FACT 3

Many unresolved operating problems can be caused by excessive component temperatures - random reboots, software and file errors, random error messages, faulty video and a full crash, amongst others.

Why should we worry about NOISE?

FACT 1

There are no recognised international standards for computer noise. General computers run between 36 and 59 dBA (see below) and this can be equivalent, in a large open office area, to working on the pavement beside a busy road!

FACT 2

Noise is best defined as unwanted sound and it can reduce productivity by up to 60% in an open workplace. Even a continuous noise, that you might not find obtrusive, can have harmful long term effects. Noise is stress and stress is illness. We should not have to live with it.

Opposing Forces

Because of the need for cooling in tight places as computers get hotter and smaller, fans are required to provide forced air convection cooling. And fans can be noisy - very noisy. So we have a clash between the need for low operating temperatures and the noise of fans to achieve that.

The golden rules for noise are first to eliminate it, and if that can't be done, to reduce it. Finally, muffle it.

Sound and Sound Measurement

Sound consists of airborne pressure waves, generated by a mechanical vibration at its source, which is then perceived by the human ear. A pure sound is termed a tone and has a frequency or pitch and an amplitude or volume. The Hertz or Hz is the measure of sound frequency and the decibel or dB is the measure of the sound pressure level (SPL). Most sounds are a complex mix of tones involving various frequencies and amplitudes.

SPLs are measured in decibels on the A weighted scale with the unit, dBA. A weighted scale is used as we naturally hear different frequencies at different intensities. (We hear the voice range of frequencies between 400 and 4000 Hz most easily). So this bias must be removed. Typical SPLs are as follows;

Threshold of Hearing 0 dBA (by definition)
Normal Breathing 10 dBA
Whispering at 5 feet 20 dBA
Soft Whisper 30 dBA
Quiet Office/Library 35-40 dBA
Large Office 50 dBA

Continuous SPLs above 85 dBA will cause long term damage. SPLs above 140 dBA can cause permanent damage in minutes. So aim ideally to fit equipment with an SPL below 20 dBA, at worst, below 25 dBA. The Swedes recommend an SPL below 25 dBA in a bedroom at night.

This scale is not linear, it is logarithmic, so a drop in the SPL by 10 dB will cut the perceived sound level by half. Ditto a rise of 10 dB will double the sound level. We can just notice a change of 1dB and a change of 3dB is clearly noticeable. Fitting more than one fan, of the same type, will increase the SPL. Two fans add 3 dBA, three fans 4.7 dBA and four fans 6 dBA. Every doubling of identical sound sources results in a 3dB increase in the SPL.

Sound Power

We have so far referred to the sound pressure level or SPL. This is measured, by general agreement, at a point 1 metre from the source, in a special chamber. This basically gives a 2D snapshot. Move the measuring device to another point and you get another 2D reading. But there is another approach and this is to measure the Sound Power. This is the TOTAL acoustic energy transmitted by the source so it would be akin to a 3D view. To differentiate between SPL and Sound Power, the latter is measured in bels and, again, uses a weighted scale.

Noise

Noise can best be described as UNWANTED sound. Living under an aircraft flight path, a neighbour's continuous loud music and a road drill all produce unwanted noise. We call something 'noisy' if we don't like it but we say, "That sounds nice", if it is pleasant. Noise can take many forms - it can be a pure tone. It can be broadband and consist of a wide range of frequencies. If this occurs in nature, such as a rain shower, it is termed 'pink' noise. If it is machine generated, it is termed 'white' noise and is more intrusive than pink noise. The turbulent airflow from a fan may read louder than the whine of the fan blades, but we find the whine more intrusive. So the quality of sound matters as well as its SPL.

Sound is directional, it cannot bend round corners. So we hear a noise at different levels as we move around at the same distance. Reflected sound is another problem and depends on the walls and furnishings in the room. Distance also causes sounds to diminish. Double the distance and the sound level drops by 6 dB in an open area. In an enclosed space, reflections reduce this to 3 dB. The mounting of a sound source can markedly alter the sound level. Flexibly mount it and noise will not be transmitted into a PC case, for example. Mount it solidly into a flat panel, and mechanical noises can be amplified as the panel resonates. As mentioned earlier, room furnishings are critical. Heavy drapes and thick carpets will dampen sound. Hard walls, solid floors and bare windows will cause relections and the noise level will be higher.

Computer Noise

Cooling fans on the CPU heatsink, northbridge, graphics card, in the power supply and on the PC case itself, all emit noise. Hard drives and optical drives are also other noise sources. We discuss these items below with steps to be taken to improve matters.

CASE AIRFLOW

The hot components in a PC case have their waste heat removed by cool air flowing through the body of the case itself. This cooling air is usually room air at ambient temperature. The basic requirement for the air flow is horizontally in at the front, up the centre body of the case and horizontally out at the top rear. In most PCs, the sole means of airflow is through the exhaust fan in the power supply. Many cases have stylised fronts with no air passages. They rely on a slot in the base, at the front, for airflow. Sit the case on a carpet and airflow effectively ceases.

There are two key issues with case cooling.

Firstly the waste heat must be extracted from the components,
then this waste heat must be expelled from the case.

These two features must work together. There is no point in having a superb and expensive CPU cooler if the case is effectively sealed. This is why it is impossible to say at what temperature a specific CPU cooler will run. It depends entirely on the case airflow.

So what are the options? For most systems, a 120 mm fanned power supply will provide all the required airflow PROVIDED there is a suitable aperture for incoming air. This should consist of an open hole in the front of the case, or if it has feet, in the base. Most of these existing holes are perforated and can be up to 65% blocked, so cut them out. This hole should be adequately filtered.

If the temperatures are too high, add an inlet fan at the front or base to complement the exhaust fan. This should be of the highest quality as you are most likely to hear a front mounted fan. This fan should have a slightly higher flow rate than the exhaust fan, so the case will be slightly pressurised. This ensures that no air enters the case other than through the front filter.

We regard multiple case fans as a gross confusion as they are noisy, create waste heat and often reduce the cooling effect by creating strange internal flow paths.

FANS - general

Voltage

Fans come in various voltages as well requiring AC (alternating current) or DC (direct current). For the purposes of computers we will only consider 12 volt DC axial fans. Although rated nominally at 12 volts, these fans can generally operate in the 6/7 - 14 volt range, according to the manufacturer. If the voltage is too low, the fan may not start.

Many fans have reverse polarity protection. If you want to change the airflow direction through the fan, refit the fan the other way round. Do NOT reverse the power leads. Most fans have markings on the outer edge showing the direction of the airflow and fan rotation. As a general rule (but not infallible) if you are looking at the label in the centre of the fan, the air will be blowing into your face.

Bearings and Noise

Fan motors come in two styles, with ball bearings and sintered or sleeve bearings. In general, ball bearings fans are dearer, last longer and are noisier. They are ideally suited to working in aggressive conditions. Sleeve bearings are cheaper, have a shorter service life but are quieter. However, at the low temperatures found in computers, sleeve bearings are ideal and will last much longer than quoted.

Ball bearings are generally noisier than sleeve bearings by about 1 - 3 dBA. If they are damaged or shocked, they can become very noisy and the noise is more insistent to the ear than that of a sleeve bearing. Sleeve bearings can be equally shocked but their noise level is unaffected and will remain so over the life of the fan. Ball bearings can get quite noisy in the short term and become potentially noisier with age. So the actual life of a fan may be dictated by its noise level.

Fan Life

The manufacturer's data gives a fan service life in hours with a 90% survival rate at 60 degC and for continuous operation under ideal conditions. In normal circumstances, computer temperatures are lower and the fans are used intermittently, so their service is prolonged. For example, at 40 degC, one manufacturer quotes double the service life. A cheap sleeve fan with a quoted service life of 30,000 hours will run for over 14 years if used for 40 hours a week. Some fans have a quoted service life of 200,000 hours (nearly 23 years continuous running!) So any quality fan should outlive your equipment.

Fan Specifications

Fans are tested under ideal conditions to get their performance figures, so in the real world of back pressure and obstructions, expect to get, at best, 80% of the quoted airflow.

Fan Noise

Fans produce two types of noise - aerodynamic and mechanical. Aerodynamic noise results from the fan blades producing turbulent air. This results in a whine that depends on fan speed, the number of fan blades and how many struts the fan blade passes. If you multiply the fan speed in rpm/60 (revs per sec) times the number of fan blades times how many struts the fan blade passes in one revolution, then you have the frequency of the whine in cycles per second or hertz, hz. If the resulting frequency falls in the 400 - 6000 Hz range it will be readily audible as this is the most sensitive range for the human ear - the voice range.

Suppose we have a fan running at 3000 rpm with 7 blades and 4 struts. This will generate a frequency of

3000/60 x 7 x 4 = 1400 hz

The mechanical noise can be produced by blade imbalance and bearings. This can be amplified by the enclosure itself so the way the fan is physically mounted is important.

To Blow or to Suck?

There are several approaches we can adopt with case fans. We can have a fan just blowing air into the case. We can have a fan just sucking air from the case or we can have a mixture of the two. Many commentators and quite prestigious companies differ as to which approach they favour.

To Suck

If air is sucked from the case, it will reduce fan efficiency as the fan will be working against a slight negative pressure in the case. This can result in a dramatic drop in the fan speed and so lower airflow. But it will promote smoother airflow internally. If external holes are suitably placed, it will ensure that cool ambient air is drawn in over critical components. It will also suck in dirty, unfiltered air from every nook and cranny. That includes dust and dirt being sucked into your hard drive and floppy drive.

This is the most common situation in most PCs as the sole fan moving air through the case is the exhaust fan in the power supply.

To Blow

If air is just blown into the case, the airflow will be turbulent and internal resistance to flow can be very high. But the air can be adequately filtered.

We favour the twin approach - for every inlet fan, fit an exhaust fan. Both will relieve the pressure on each other and so increase the airflow. If more air is blown into the case, so that the case is positively pressurised, no unfiltered air can leak in. And more airflow surely means the greater the opportunity to remove more waste heat.

Static Pressure

The resistance in the case will give rise to an internal static pressure. The actual figure for your case can only be measured by experimental purposes in an airflow chamber. This is obviously impractical. In general, most enclosures have a static pressure of between 0.05 and 0.15 inches of water. The aim then is to match the case pressure curve against the fan characteristic curve. This is again too scientific for the average user. But there are some rules which will minimise losses;

fit finger guards on the inlet side of the fan; they are noisier on the exhaust,
obstructions near the intake raise the noise level more than on the exhaust side of the fan.

The other obstructions in the case include flat ribbon cables, the various drives, the CPU heat sink and fan and AGP and PCI cards. All impede flow. A really cluttered case can reduce airflow by as much as 60%. Even an empty case can reduce flow between 5 and 20%. The greatest improvement will be achieved by fitting round cables and neatly tie wrapping them out of the way.

Selecting a Fan

We must decide what fans we need, with what spec etc. etc. According to NMB and others, the required airflow can be obtained from the equation below;

Airflow, Q, in cfm = 1.76W/delta T

where W is the waste heat in watts and delta T is the maximum permitted temperature rise in the case, above ambient, in degC.

AMD recommend that delta T should not exceed 7 degC.

This then gives a minimum airflow,

Q = 0.25W cfm

So for every 100 watts of waste heat we require an airflow of 25 cfm. But to allow for static pressure, multiply the resultant figure by 1.5 to 2 times. So the actual fan to be used would need an airflow of between 37 and 50 cfm.

Be aware that components may still overheat, in spite of the above calculation, if insufficient local airflow is getting to them. This may be due to their position or shrouding by local cables, for example.

Variable Speed Fans

We may wish to vary the speed of the fan to increase or decrease the airflow. And by lowering its speed, we reduce the noise level. A 20% drop in speed equates to a 5dB drop in sound level. This can be only done in one way - by changing the applied voltage. But there are several ways by which this can be achieved;

Fixed Resistor

We can put a resistor in the circuit to reduce the voltage. This is easiest done by fitting a Noiseless Connector or a 5v or 7v Converter Cable.

Variable Resistor

We can put in a variable resistor - a potentiometer - so that we can adjust the voltage at will. The Fan Mate does this job in a more sophisticated, electronic manner.

Temperature Control

But what are we using the fan for? It is to provide cooling, so the best fan speed control should be by the temperature level itself.

A fan is controlled by a thermistor - a thermal resistor - the temperature sensing device. It comes attached to the fan and can be located where you wish. If you attach it to the fan body, it will measure the temperature of the air passing through itself. So with an inlet fan, the thermistor will respond to the ambient air temperature. It will speed up as the weather gets warmer. As an exhaust fan, it will respond to the internal case air temperature.

If you wish to use a standard fan, the Temperature Control Kit provides all the parts necessary to thermally operate it.

The beauty of a temperature controlled fan is simply that. It works according to thermal demand and should provide a continuous thermal balance for the whole system, whatever the load.

A second approach is to have your fan speeds controlled by the computer itself. There is software available and most modern motherboards have the relevant chips.

Cables

You can connect the fan with a low voltage converter cable. At 5 volts the fan will run at 42% of its nominal speed and airflow. With the 7 volt cable, the figure rises to 58%. Alternatively use a 'noiseless connector' that has an inbuilt resistor of 56 or 100 ohms.

Heatsink

A heatsink is a device for taking the heat from a source and transferring it to a cooling medium, usually air. Heat travels through the heatsink by conduction and then to the air by convection. It is made either from copper or aluminium, or a combination. The design aims to provide a large surface area, through fins or pins, to increase the heat transfer to the air. This heat transfer can further be improved by forced air convection - using a fan to blow air over it.

The performance of the heatsink is measured in degC/W and is termed its Thermal Resistance. It shows what the temperature rise will be for an input of one watt. Obviously the lower the figure the better - for a given temperature rise, it can handle more heat.

The CPU heatsink is clipped or bolted to the processor. The mating surfaces look very flat and shiny. But microscopically, they consist of peaks and troughs. So the two faces will only sit on the peaks and the troughs will be filled with air, an excellent insulator. So this interface provides a thermal resistance and reduces the heat transfer. The processor can be very hot and the heatsink much cooler. The answer is to introduce a thermal interface material (TIM) such as the market leader, Arctic Silver 5. This fills the voids and improves the heat flow, in some cases, quite dramatically.

The cooling fan can either blow or suck air through the heatsink. In general, it is better to blow air from the top towards the base. This is because the intial cool air will strike the coolest part of the heatsink. As it travels down it will get hotter and so will the heatsink. So there is always a temperature difference between the hot body and the cooling medium. The air temperature feeding the fan is also critical. The cooler, the better. Commonly the air used is warm internal case air but by introducing external ambient air directly, the heatsink will run that much cooler.

For smaller chipsets, a fanless, or passive, heatsink will suffice. If it cannot be fixed mechanically, then it can be bonded on using a thermal epoxy adhesive.

Heat Pipes

A recent development has been the so called heat pipe. This is a small hollow tube or pipe containing a special fluid. When one end of the pipe is heated, the trapped fluid evaporates and travels to the other end. This cooler end causes the fluid to condense and so give up its heat. So the pipe is used to transfer heat from a hot body to a cooler body. For example, the Zalman ZM-80C VGA card cooler uses heat pipes to transfer heat from one heatsink on the hot side of the card to a second cooler heatsink on the opposite side. This is an ideal application as it turns an active card (with a fan) to a passive, silent device.

Power Supply Unit - PSU

After the CPU, the PSU is the hottest item and one of the noisiest. Often overlooked, it can be a major source of problems if its power output is inadequate or unstable. Basic units - so called 'generic' items which have been built down to a price - usually have a single speed exhaust fan with slotted grilles to allow air into the casing. The more sophisticated branded units, made by major manufacturers with a reputation to maintain, can have one or two fans. With two fans, the first draws air into the PSU from the area around the CPU - so it is located in the base. The second fan exhausts air from the unit and this fan is temperature, sometimes manually, speed controlled. The latest single fanned PSUs have a 120 mm fan in the base exhausting through a honeycomb patterned rear face. They are amongst the quietest units available.

We offer the high quality Antec TruePower, Seasonic and Fortron PSUs, all being very quiet, have power factor correction and sophisticated temperature fan control systems.

Virtually all PSUs have the +3.3v and +5v lines sharing a common output circuit off the transformer. They thus interact and effect each other's performance as well as the system stability. So in spite of generously quoted individual outputs, the sum total power of these two lines can be very restricted. This is termed the "Combined" power. The Antec PSU has SEPARATE outputs for these two voltages so each can provide the quoted performance with total stability. To realise the importance of this, view the table below;

Total COMBINED Power 3.3v/5v

Q Tec 400W 150W
Enermax 300W 170W
Q Tec 450W 170W
QuietPC 300W 175W
Zalman 300W 180W
Enermax 350W 185W
Q Tec 550W 190W
QuietPC 400W 200W
Fortron 300W 200W
Seasonic 300W 200W
Enermax 460W 200W
Enermax 550W 200W
Fortron 350W 220W
Antec TP 330W 241W
Antec TP 430W 272W
Antec TP 550W 305W

It is interesting to note, for example, that the Q Tec 400W unit has one of the lowest outputs at 150W and its big brother the 550W unit provides only 190W - less than some quality 300w units! The Antec unit beats all the competitors, even ones rated much higher. All the above information is available on the manufacturer's websites or in their literature.

PFC, power factor correction, results in more efficient operation, so saving power. This, in turn, results in less waste heat, so it runs cooler. Active PFC is more efficient than passive PFC. PFC has been a statutory requirement in the EU since 2000.

So what should we do?

Noise

There are three stages to dealing with noise;

firstly, eliminate the source of the noise,
secondly, if that cannot be done, minimise the noise level,
thirdly, provide panel damping and airborne noise absorption.

Cooling - Active & Passive

There are two types of cooling - active and passive. Active cooling involves some sort of machinery, moving parts or power usage whilst passive cooling requires no external devices or power. A heat sink is a simple passive device but add a fan and it becomes an active device. So firstly, always try and fit a quiet, energy free, passive cooler, if it works adequately. But remember, a passive device relies on natural air convection or air movement from a local active device. And a passive device can never make a noise, will never wear out or break down.

Now please read the Action Plan to see possible solutions to specific problems..