Case Basics & Recommendations

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VIBRATION FROM HARD DRIVES

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.

Frequency of HDD Vibrations
RPM
Primary (Hz)
Harmonics (Hz)
4,200
70
140, 210, 280...
5,400
90
180, 270, 360...
5,900
98
196, 294, 393...
7,200
120
240, 360, 480...
10,000
167
333, 500, 667...
15,000
250
500, 750, 1K...

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 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 storage section, and in the forums: HDD vibration & noise reducing methods - ranked and HDD Suspension... Show your pics!

Many cases offer rubber grommets but some have been only marginally effective, as the rubber used is often much too hard, and the amount of decoupling achieved is minimal. True elastic suspension for hard drives can be found on the Antec P150 / Solo. and the new Solo II. With low vibration HDD, vibration-induced noise from the HDD can be completely eliminated in this case. Some cases also offer good soft rubber grommets for decoupling hard drives.

Since SPCR began examining cases and HDDs back in 2002, there has been much evolution in both. HDDs are generally much quieter now than they used to be, and they also tend to exhibit a lot less vibration. Still, as our HDD reviews show, typical 7200rpm 3.5" drives remain major sources of noise and vibration. New generations of 5400rpm drives by Western Digital and Samsung — and Seagate's unique 5900rpm drives — have revived sub-7200rpm drives, mostly in response to constantly growing demand for storage as all forms of media (video, music, etc) converge to digital. The new drives are slgihtly slower but usually notably quieter, with less vibration than their 7200rpm counterparts.

Solid State Drives are the obvious answer to eliminating HDD noise entirely, and they virtually elminate heat altogether. The best of them are faster than the fastest mechanical storage drives, and seemingly more reliable. They are so light, cool and silent that they can be mounted almost anywhere in a case, even with a single screw if necessary. Still, SSDs are expensive, especially when compared with HDDs, which continue increasing in capacity while plummeting in price. A good balance of price, performance and low noise can be had by combining the two: A smaller 30~120 GB high performance SSD for the operating system, along with one or more low-vibration, extremely quiet, sub-7200rpm HDD of very high capacity.


AIR RESONANCE

One poorly understood aspect of PC noise is air resonance. The air in a mostly enclosed box exhibits resonances centered at certain frequency points. Any noise that falls close to these resonance points become accentuated and amplified. Note that this is usually not panel vibration, but the vibration of the air in the enclosure. A good explanation of resonance concepts is available at the Sound and Hearing section of the HyperPhysics web site.

Air resonances occur regardless of the type of materials used in 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 air resonance in a typical mid-tower computer is a <200 Hz boom or hum accentuated by hard drives and fans. It is usually 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 acoustically absorbent 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 can help to reduce air 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, acoustic amd panel damping materials 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 first, and then consider whether acoustic damping might help further.



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