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ALUMINUM
The Aluminum Myth - Some favor aluminum cases, citing an ability to better cool components mounted within. This is a myth. No heat producing component benefit in any significant way from being inside an aluminum case. The only heat producing devices that are normally mounted in direct contact with a case are the drives, particularly the hard drives. The difference between aluminum and steel in this cooling function is insignificant.
This does not mean aluminum cases cannot be used to make a silent computer, just that there are disadvantages with them when compared to similarly constructed steel cases. Regardless, many aluminum cases certainly look nice.
The Aluminum Drawback - One consistent acoustic property seems unavoidable: Aluminum cases tend to pick up hard drive and fan vibrations more readily than steel cases, and make a higher pitched, more audible humming or buzzing sound. This quality is directly related to the density of aluminum: It has only about 30% of the density of the cheaper, more commonly used steel. Internally applied panel damping materials (especially the heavier kinds) appear to damp the resonance down fairly effectively, but it can be difficult and expensive to eliminate entirely. Internal supporting cross braces that effectively divide the large panels into smaller ones also help quite a lot because smaller panels are more rigid, stiffer, and less prone to lower frequency vibration than larger ones.
Some case makers have sought to combine the desireable look of aluminum with the sturdiness of steel by using a front facia or bezel made of aluminum on a steel chassis. The Silverstone LC-11 media PC case and the Coolermaster Centurion 5 midtower are good examples of this type of hybrid case.
Silverstone LC-11: This well-built aluminum / steel HTPC case shows no signs of the dreaded Aluminum Hum.
MORE ABOUT VIBRATION
Vibration-induced Case Noise cannot be eliminated with heavy panels and solid construction alone. Hard drives are normally tightly coupled to the case with steel screws. The vibration of a hard drive occurs at the primary frequency determined by spin rate, as shown in the table below, and harmonics (multiples) of the primary frequency.
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Frequency of HDD Vibrations
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RPM
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Primary (Hz)
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Harmonics (Hz)
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4,200
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70
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140, 210, 280...
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5,400
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90
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180, 270, 360...
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7,200
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120
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240, 360, 480...
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10,000
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167
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333, 500, 667...
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15,000
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250
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500, 750, 1K...
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Such vibrations usually cause the entire case to vibrate -- you can easily feel it when touching any part of a normal PC case. They also cause low frequency acoustic noise - the humming, thrumming and growling types of sounds that are lower in level than typical fan noise or hard drive whine but certainly there contributing to the overall noise. The harmonics can cause noise in the mid-band and higher frequencies where human hearing is most sensitive. Buzzing and whining are apt descriptions of the kinds of noise HDD harmonics can cause.
The main solution against vibration-induced panel noise is to stop the vibrations from getting to the panels in the first place. The normal mounting method for fans and hard drives (and power supplies with fans) is to screw them down to the chassis. This creates direct conduction paths for vibration to go into the case. An acoustically inert and mechancially solid case can help to keep such vibration from turning into a major source of noise, but cannot eliminate it completely. The best solution is to use low vibration components in the first place, and to use effective mechanical decoupling of the noise making components. This is best done by using soft mounting techniques for fans and hard drives. Rubber bushings and grommets that insulate the fan or hard drive from the chassis can be used, as well as various forms of elastic "string".
The article Hard Drive Silencing: Sandwiches & Suspensions covers one example of elastically decoupled mounting for HDDs. There are many more in the forums: HDD vibration & noise reducing methods - ranked and HDD Suspension... Show your pics!
A few cases have offered rubber grommets as standard equipment for some time, but most have been only marginally effective, as the rubber used is usually much too hard, and the amount of decoupling achieved is minimal. True elastic suspension for hard drives can now be found on the new Antec P150 and in the Arctic Cooling Silentium T2. The Antec P180 also offers excellent soft silicone rubber grommets for hard drives in very sturdy cages.
CAVITY RESONANCE
One poorly understood aspect of PC noise is cavity resonance. The air in any mostly enclosed box or space exhibits one or more resonances centered at certain frequency points. Any noise that falls close to these resonance points become accentuated and amplified. Note that this does not have to do with any panel vibration at all, but the vibration of the air in the cavity — hence the term cavity resonance. A good explanation of resonance concepts, including cavity resonance, is available from this page at the Sound and Hearing section of the HyperPhysics web site.
Cavity resonance occurs regardless of the construction materials or methods used to make a case. Using non-parallel panels could help by reducing standing waves, but this is an impractical solution for a commercially manufactured case. The common effect of cavity resonance in a typical mid-tower computer is a <200 Hz boom or hum accentuated by hard drives and fans. It can be fairly low in level, often not noticed until the components have been quieted or when only low noise components are used. Then, if there are any hard drives firmly mounted to the chassis, this resonance can seem impossible to isolate or eliminate.
The application of acoustic damping materials on the inner surfaces of the case should be seen as the final step to achieving minimal noise. Such damping materials can reduce standing waves, and reduce the level of higher frequency noises. Below ~200Hz, very little can be done to limit sound transmission in the context of a PC case; there's simply not enough room for the mass, density and thickness of walls needed to block the lower frequency noise.
There are some acoustic damping materials developed especially for use in computers. They definitely help to reduce cavity resonance. We've done reviews of some of these materials here at SPCR; the reviews can be found in the Cases and Damping Section. In general, they are most useful with computers that have already been optimized for low noise, including soft mounting of all the major noise making components. Good acoustic damping can reduce the noise by several decibels, and more at higher frequencies. But the need for airflow leaves at least a couple of open fan holes through which fan and HDD noise always come through. The best approach is to get the noise of those components to a minimum and then consider whether acoustic damping might help further.
A Few Words about Cosmetics: Our preference is for clean, lean minimalist lines dictated by function. But... if a case "looks ugly" and works quietly and efficiently, we see it as beautiful: Form follows function. Matters of taste are really personal. Choose one that does not annoy you every time you look at it. Cases meant for use as Home Theater PC generally need to look nicer because most Audio / Video gear looks nicer, and you may want the PC to match cosmetically with other AV gear. But again, a HPTC case that looks nice without providing essential performance is not a good case, in our view.
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