If you're not prepared to replace your case, there are other things you can do to improve the cooling efficiency and reduce the noise level.
The most important thing you can do to improve cooling efficiency is to keep the case clean, particularly the air intake grills and the fans themselves. Test your system periodically to determine its cooling efficiency. Use an ordinary thermometer to determine ambient room temperature and then measure the temperature of the air being exhausted by the rear case fan (not the power supply fan). If the exhaust air is 5 C (9 F) or more warmer than ambient room temperature, you need more or better fans.
One quick, easy, cheap thing you can do to improve air flow and cooling is to dress the cables. Before you declare your upgrade or repair complete, tie off all of the internal cables into neat bundles and tuck them out of the way. Use cableties or those yellow plastic thingies that come with garbage bags to secure the cables to the case frame or otherwise out of the way. Dressing the cables neatly does more than just improve air flow and cooling. It also prevents loose cables from fouling fans.
But you can't just add fans willy-nilly and expect your case to be cooled optimally. You have to consider the air flow pattern inside the case, and install fans to direct that air flow for optimum cooling. Ideally, you want cool room air to enter through the front of the case, be directed over the drives, expansion cards, memory, processor, and other heat-producing components, and then be exhausted from the rear of the case. The power supply fan provides some general system cooling, but its primary purpose is to cool the power supply itself. For general cooling, you need supplemental case fans, which are readily available from online vendors and local stores.
Your goal should be to use a combination of intake and exhaust fans to provide air movement through the case. If all of your fans blow in (intake), you pressurize the case and limit air flow to whatever can escape through the vents and other gaps in the case. If all of your fans blow out (exhaust), you create a vacuum, and again limit air flow to whatever can enter through the various gaps in the case. An ideal fan pattern puts one or two intake fans at the front of the case, and one or more exhaust fans at the back of the case. (The power supply fan is also usually an exhaust fan.)
I'D LIKE ONE INTAKE FAN AND ONE EXHAUST FAN, PLEASE
You can't buy an intake fan or an exhaust fan. These aren't different types of fans; a fan functions as an intake or exhaust fan depending on which direction you mount it. Look for an arrow or arrows on the body of the fan. If there is only one arrow, it indicates the direction of air flow. Some fans have two arrows, one for air flow direction and one for blade rotation direction. Which is which will be obvious when you look at the fan.
Supplemental case fans have these characteristics:
Supplemental case fans are available in various standard sizes, including 80 mm, 90 mm, 92 mm, and 120 mm. Unless you are willing to do surgery on your case, select the size or sizes of fans that fit your existing mounting positions. If you have a choice of mounting positions that accept different-size fans, always choose the larger size. If they have the same design and rotation speed, a larger fan moves more air than a smaller fan, or, alternatively, moves the same amount of air at a lower rotation speed (and noise level).
The rotation speed of a fan, specified in revolutions per minute (RPM), is stated for the nominal voltage (usually +12V) that the fan is designed to use and when it is running in free air. All other things being equal, a faster-running fan moves more air and produces more noise.
Some fans have variable speeds, set either via a multiposition switch usually high, medium, and low or a knob that sets fan speed over a continuously variable range. Even single-speed fans can be made variable, however, by altering the feed voltage. A fan designed to run on +12V, for example, can be run instead at +7V, which slows its speed significantly. The only limit to voltage adjustments is the required startup voltage, below which the fan may run if started spinning manually, but will not start spinning on its own. For most +12V fans, that limit is +7V.
Voltage can be adjusted in several ways. You can add a fixed resistor pack, available from online sources such as http://www.endpcnoise.com and http://www.frozencpu.com, that drops the +12V supply voltage to +7V or so. There are also fan-speed control consoles available, which fit an unused external 5.25" drive bay and provide a knob for adjusting fan speeds by increasing or decreasing the voltage. Finally, some power supplies provided dedicated "fan-only" power connectors that vary the voltage to the fan, under the control of the power supply.
The air flow rate of a fan is specified in cubic feet per minute (CFM), and depends on the size, speed, and design of the fan. Variable-speed fans specify a range of flow rates from their minimum speed setting to the maximum, such as 10 CFM to 25 CFM. Nominal flow rates are invariably optimistic, as they assume an unobstructed fan. Actual flow rates are normally about half nominal.
Noise level is specified in A-weighted decibels or dB(A), with lower numbers meaning a quieter fan. Once again, the noise level of variable-speed fans is specified as a range from lowest to highest speeds (and air flow rates), such as 20 dB(A) to 28 dB(A). If you are interested in reducing the noise level of your system, look for fans that are rated for 30 dB(A) or less at maximum speed. If your goal is a "silent PC," look for fans rated at 20 dB(A) or less. Nominal noise level ratings are also invariably optimistic, as they assume no contribution from grill noise, which may be substantial.
Most case fans are designed to be connected to a standard Molex (hard drive) power connector. Some are instead designed to connect to a 3-pin motherboard power header, which has the advantage of putting fan voltage/speed under motherboard controller. Before you buy a fan of the latter type, however, make sure you have an available motherboard power header to provide power to it. Alternatively, you can buy an adapter to convert a 4-pin Molex power cable to a 3-pin fan connector.
Some fans are designed to be visually attractive for example, by using transparent blades and body or to attract attention with LED illumination, fluorescent dyes, and similar gew-gaws. We have no general objection to these fancy fans, but some transparent fans use brittle plastic, which seems to resonate and increase fan noise. If you choose such a fan, try to get one with flexible blades.
Installing a supplemental case fan isn't difficult. Fans are ordinarily supplied with mounting screws or bolts, rubber isolation pads or a foam sound-deadening surround, and similar accessories. To install the fan, orient it inside the case, with its mounting hole locations aligned with the holes in the case. (Make sure it's oriented to blow in the right direction.)
Some fans secure with screws, which are driven through the mounting holes in the case and into the body of the fan. Others use bolts, which are inserted from outside the case, through holes in the body of the fan, and are secured by nuts on the inner surface of the fan. Neither of these methods isolates the fan from the case, so fan noise and vibration can be transferred to the case. We prefer to use flexible pull-through fan mounting connectors, as shown in Figure 15-7. To use these, pull the flexible connector through the body of the fan and case mounting hole until it snaps into place and then trim off the excess, leaving perhaps a quarter of an inch protruding to retain the fan. Trim them flush only if you want the fan to fall inward onto the motherboard. In conjunction with rubber isolation blocks or a foam surround, these flexible connectors reduce fan noise, sometimes noticeably. You can buy flexible fan mounts from companies that specialize in quiet PC components, such as EndPCNoise.com (http://www.endpcnoise.com) or FrozenCPU (http://www.frozencpu.com).
Figure 15-7: Using flexible pull-through fan mounting connectors
Fan grills protect fingers from spinning fan blades, but they reduce air flow and increase noise significantly. Depending on the fan and grill, it's not unusual for noise level to be doubled.
You can have safety and efficiency with a little effort. Remove the fan grill you may have to cut it out and substitute a 4" to 6" piece of PVC tubing of the appropriate diameter, secured with screws or adhesive. The PVC tubing presents very little resistance to air flow, so the fan can operate at maximum efficiency with minimal noise. (Make sure to increase the clearance between the back of the system and the wall to provide adequate room for the air to be exhausted.)
Although adding supplemental case fans is an easy and inexpensive way to improve cooling, there are other steps you can take:
TAC-in or TAC-out?
TAC systems are ordinarily designed to draw in cool outside air through the TAC duct and direct it onto the top of the CPU cooler. Thinking about that, we wondered why we'd want to exhaust CPU waste heat inside the case. Wouldn't it be better to exhaust the warm air from the CPU cooler directly to the outside of the case? After all, in a properly cooled system, the air temperature inside the case isn't much higher than ambient room temperature.
So we did some testing by reversing the usual direction of the air flow, exhausting hot air through the TAC duct rather than drawing in cool air. For most systems, we found little difference in CPU temperature either way, but for some systems, exhausting the warm air through the TAC duct allowed the CPU to run noticeably cooler. Only in systems with a "hot" video card did running TAC in standard intake mode yield lower CPU temperatures.
If you decide to run your TAC system "backwards," make sure the CPU fan is set to push air into the TAC shroud rather than drawing it in (you can do this simply by reinstalling the CPU cooler fan "upside-down"). Also, if you use a supplemental fan in the TAC housing, make sure that fan is set to exhaust air from the case rather than pushing it into the case. The last thing you want is two fans pushing in opposite directions.
If you're upgrading an older system with a hot new processor, check to see whether there's a replacement side panel available for your case that adds a TAC shroud and duct. Removing CPU heat directly can reduce interior case temperatures by several degrees. With that excess heat disposed of by the TAC vent, case fans can run slower and quieter.
If your current case is otherwise suitable and there's no replacement TAC side panel available for it, consider making your own. To do so, mark the location of the CPU on the side panel, and use a hole saw to cut a hole of appropriate size in the side panel. Measure the depth from the inside of the side panel to the top of the CPU cooler, and cut a tube of appropriate length from PVC pipe or other material of a diameter large enough to slide down over the top of the CPU cooler, being careful to allow clearance for air to be drawn in around the CPU cooler. (The material you use for the tube is not critical; even a cardboard tube works fine.)
Mount the tube to the side panel using screws or adhesive. If you want a finished appearance, you can buy some grill material at the hardware store. For even better cooling, you can install a case fan of the appropriate size between the side panel and the tube, making sure that this fan works with the CPU cooler fan, not against it.
You can build your own chassis air duct, similar to the duct shown with the Sonata II case, using cardboard, foamboard, or a similar material. Your goal should be a duct that covers the processor and CPU cooler, memory, and video card. Make sure to allow sufficient clearance between the duct and the motherboard for air to be drawn in. Design the duct to route that air over the CPU and other heat-producing components and then to be exhausted by the rear case fan.
Although case design has a limited impact on noise level, there are some changes you can make to your current case to reduce noise level:
One of the easiest, cheapest ways to reduce system noise is to isolate the case from the floor or desk surface. Most cases have rubber or plastic feet, but these are often too hard to absorb the noise and vibration that the system transfers to the surface it sits upon. Use mouse pads or similar soft spongy material between the case feet and the floor or desktop to eliminate this source of noise. The improvement is usually minor, but sometimes it is quite noticeable.
The power supply is one of the largest noise sources in most computers (the CPU cooler fan is the other). You can reduce the noise level of most systems significantly by replacing the original power supply with a quiet or silent model. Quiet power supplies such as various models made by Antec, Nexus, PC Power & Cooling, Seasonic, Zalman, and others are much quieter than standard power supplies, although they are audible if you are near the system and in a quiet room. Silent power supplies have no fans or other moving parts, and are completely silent.
The stock case fans in most systems were chosen for their low cost rather than for their noise level or cooling efficiency. Replacing the stock fans with better models can reduce noise level, sometimes dramatically.
Air moving through a grill or holes produces noise. If your case air intakes are small and/or obstructed by grills, that not only harms cooling efficiency by restricting air flow, it also adds to the noise level. By opening up these intakes, you can improve cooling and reduce noise at the same time. (We have on occasion taken a hacksaw to a case that had inadequate air intakes.)
Online sources such as http://endpcnoise.com sell sound-absorbing insulation that can be applied to case panels and other interior case surfaces. We're of two minds about this stuff. While it's undoubtedly true that adding insulation can reduce noise levels significantly, it's also true that sound-deadening material also functions as thermal insulation, increasing interior case temperatures. If you choose to install sound-deadening material, don't overdo it. Make sure not to block air intakes, and keep a careful eye on system temperature.